Sources and References

Primary textbook — Whitney, E., Rolfes, S.R., Crowe, T., & Walsh, A. (2023). Understanding Nutrition, 3rd Canadian Edition. Cengage Learning Canada.

Supplementary texts — Gropper, S.S., & Smith, J.L. (2022). Advanced Nutrition and Human Metabolism, 7th Edition. Cengage Learning. Stipanuk, M.H., & Caudill, M.A. (2022). Biochemical, Physiological, and Molecular Aspects of Human Nutrition, 4th Edition. Elsevier.

Online resources — Health Canada Dietary Reference Intakes (www.canada.ca/en/health-canada/services/food-nutrition/healthy-eating/dietary-reference-intakes.html); USDA FoodData Central (fdc.nal.usda.gov); National Institutes of Health Office of Dietary Supplements (ods.od.nih.gov); Dietitians of Canada (www.dietitians.ca); PubMed clinical nutrition literature (pubmed.ncbi.nlm.nih.gov).

Chapter 1: Foundations of Human Nutrition

Introduction to Nutrition Science

Nutrition is the science that examines the interactions between living organisms and the food substances they consume. It encompasses the processes by which nutrients are ingested, digested, absorbed, transported to cells, metabolized, and ultimately excreted, as well as the consequences of inadequate or excessive intake for health and disease. The field sits at the intersection of biochemistry, physiology, epidemiology, and public health, and it draws on molecular biology to understand how dietary components regulate gene expression — a field now known as nutritional genomics or nutrigenomics.

The history of nutrition as a formal science is surprisingly recent. Although physicians and philosophers had speculated for millennia about the relationship between diet and health, systematic experimentation began only in the eighteenth century. Antoine Lavoisier demonstrated in the 1780s that respiration is a form of combustion, establishing that the body extracts energy from food in a chemically defined way. Through the nineteenth century, chemists identified the major organic constituents of food — proteins, fats, and carbohydrates — and measured the energy they yielded in the newly invented bomb calorimeter. The early twentieth century then witnessed a cascade of vitamin discoveries, each linked to a specific deficiency disease: scurvy and vitamin C, rickets and vitamin D, pellagra and niacin, beriberi and thiamine. These discoveries fundamentally changed medicine and public health by demonstrating that many devastating diseases were not caused by pathogens but by the absence of specific micronutrients.

Contemporary nutritional science has expanded far beyond deficiency prevention. The modern understanding recognizes that diet contributes to the risk of chronic, non-communicable diseases — cardiovascular disease, type 2 diabetes, cancer, and osteoporosis — that account for the majority of mortality in affluent societies. The dose-response relationships involved in chronic disease are far more complex and subtle than those for classical deficiency diseases, and they unfold over decades rather than weeks, making them more difficult to study rigorously. Nevertheless, large cohort studies, randomized controlled trials of dietary interventions, and mechanistic cell and animal research have together produced a substantial body of evidence guiding dietary recommendations.

Nutrition science also recognizes the enormous variability in human dietary patterns across cultures and throughout history. No single food or dietary pattern is universally optimal; context, genetics, life stage, and individual physiology all modulate the effects of diet. Canada’s Food Guide, most recently revised in 2019, reflects this complexity by moving away from prescriptive serving sizes toward broader principles — emphasizing vegetables, fruits, whole grains, and plant proteins; limiting highly processed foods; and attending to the social and cultural dimensions of eating.

Nutrients: Classification and Functions

Nutrients are conventionally divided into six classes: carbohydrates, lipids, proteins, vitamins, minerals, and water. The first three classes are macronutrients — consumed in gram quantities and capable of yielding metabolic energy. Vitamins and minerals are micronutrients — required in milligram or microgram quantities and serving primarily regulatory and structural roles. Water, though sometimes overlooked, is required in the largest absolute quantities of any nutrient and is essential for virtually every physiological process.

Within each class, nutrients serve multiple overlapping functions. Energy provision, structural support, and metabolic regulation are the three cardinal functions of nutrients in general. Carbohydrates and lipids are the primary energy substrates, providing fuel for ATP synthesis; proteins provide amino acids that serve as building blocks for structural and regulatory proteins; vitamins act as coenzymes or antioxidants; minerals contribute to the skeleton, to enzyme catalysis, and to electrical signaling; and water serves as the universal solvent in which biochemical reactions take place. The distinction between energy-yielding and non-energy-yielding nutrients is important but should not obscure the fact that all nutrients participate in regulatory networks — even carbohydrates and lipids act as signaling molecules, and even vitamins have been shown to have structural roles in certain contexts.

The concept of phytochemicals — non-nutrient bioactive compounds in plant foods — has gained prominence in recent decades. Phytochemicals include polyphenols such as flavonoids and resveratrol, carotenoids such as lycopene and lutein, glucosinolates in cruciferous vegetables, and numerous others. Although they do not meet the classical definition of essential nutrients — no specific deficiency syndrome arises from their absence — accumulating evidence suggests they confer health benefits ranging from antioxidant protection to anti-inflammatory effects to reduced cancer risk. They illustrate that the concept of what constitutes a “nutrient” is evolving and that food provides far more than its catalogued nutrient content.

Dietary Reference Intakes

The DRI framework includes four reference values. The Estimated Average Requirement (EAR) is the intake estimated to meet the requirements of half the healthy individuals in a given age and sex group. It is derived from controlled studies and serves as the basis for establishing the RDA. The Recommended Dietary Allowance (RDA) is set at two standard deviations above the EAR, meaning it meets the needs of approximately 97–98% of healthy individuals in the group. When insufficient data exist to establish an EAR and RDA, an Adequate Intake (AI) is set instead — a value based on observed or experimentally determined intake levels assumed to be adequate. The Tolerable Upper Intake Level (UL) is the highest daily intake that is unlikely to pose adverse health effects in almost all healthy individuals; intakes above the UL carry increasing risk of toxicity.

More recently, the DRI framework has incorporated two additional reference values. The Estimated Energy Requirement (EER) is the dietary energy intake predicted to maintain energy balance in a healthy adult of defined age, sex, weight, height, and physical activity level. Unlike other DRIs, the EER does not include a margin of safety because any systematic positive energy balance leads to weight gain. The Acceptable Macronutrient Distribution Ranges (AMDRs) provide ranges of intake for each macronutrient, expressed as percentages of total energy intake, associated with reduced risk of chronic disease while providing adequate essential nutrients. The AMDRs are: 45–65% energy from carbohydrates, 20–35% from fat, and 10–35% from protein.

Understanding the DRIs requires attention to the distinction between individual assessment and population planning. When assessing an individual’s diet, the EAR is used as a cutoff — if an individual’s usual intake is at or above the EAR, the probability of meeting requirements is at least 50%. The RDA is intended for use in dietary planning for individuals, serving as a daily intake goal. For groups, the proportion of individuals with usual intakes below the EAR estimates the prevalence of inadequacy. These distinctions matter practically: an intake that equals the RDA does not guarantee adequacy for every individual, and population surveys that find mean intakes below the RDA cannot be interpreted as indicating widespread deficiency without further analysis.

Canada’s Food Guide and Healthy Eating Principles

Canada’s Food Guide underwent a substantial revision in 2019, representing perhaps the most significant update in the guide’s history. The 2019 guide departed from the iconic “rainbow” format that had organized foods into four groups (grain products, vegetables and fruit, milk and alternatives, meat and alternatives) and replaced it with a plate-based visual emphasizing proportions rather than serving counts. The guide now recommends that half of every plate be vegetables and fruits, a quarter be whole grain foods, and a quarter be protein foods — with an explicit recommendation to choose protein from plant sources more often than from animal sources.

The rationale for this shift is both nutritional and environmental. Nutritionally, evidence has accumulated that dietary patterns high in plant foods and low in red and processed meats are associated with reduced risk of cardiovascular disease, type 2 diabetes, and certain cancers. Environmentally, plant-based foods generally have a lower carbon and water footprint than animal-based foods, so dietary shifts toward plant proteins also serve sustainability goals. The guide is careful to note that this does not mean all animal proteins should be avoided — it recommends fish, eggs, dairy, and lean meats as part of a healthy diet — but the shift in emphasis toward plant sources is deliberate and represents an evolution of Canadian dietary guidance informed by both health and environmental evidence.

Beyond the plate model, the 2019 Food Guide makes recommendations about food behaviors and food skills. It emphasizes cooking at home more often, eating with others, being mindful of eating habits, and limiting foods high in sodium, sugar, and saturated fat. These behavioral dimensions of the guide reflect the recognition that what and how much people eat cannot be separated from why, when, with whom, and under what economic and cultural circumstances they eat. The social determinants of diet — income, food literacy, cultural identity, physical access to food — are acknowledged as important contexts within which dietary guidance must be understood.

Critically evaluating nutrition information is a skill emphasized in KIN 146, because the public information landscape is saturated with nutrition misinformation. Claims on social media and in popular books frequently misrepresent the strength of scientific evidence, cherry-pick studies, conflate correlation with causation, and ignore the complexity of dietary pattern research. Common logical fallacies in nutrition discourse include the appeal to nature (assuming “natural” foods are always better), single-study fallacy (treating one small study as definitive), and the dose neglect error (ignoring that toxicity and benefit are dose-dependent). A systematic approach to evaluating nutrition claims requires identifying the source, examining the quality and quantity of underlying evidence, and distinguishing mechanistic plausibility from clinical proof.

Chapter 2: The Digestive System

Anatomy and Physiology of Digestion

The gastrointestinal (GI) tract is an extraordinary organ system of approximately nine meters in length, responsible for the breakdown of complex dietary molecules into absorbable units and for the transfer of those units across the intestinal epithelium into the circulation. Digestion is simultaneously mechanical — involving the physical disruption of food by chewing, churning, and muscular contractions — and chemical — involving enzymatic hydrolysis of macromolecules and acid-catalyzed denaturation. The GI tract is also an immune organ and an endocrine organ, harboring much of the body’s lymphoid tissue and secreting dozens of regulatory hormones that coordinate gut function and communicate with the brain, liver, pancreas, and adipose tissue.

The oral cavity initiates both mechanical and chemical digestion. Teeth provide mechanical disruption through cutting and grinding, increasing food surface area exponentially and facilitating subsequent enzymatic access. The tongue mixes food with saliva, which is secreted by the parotid, submandibular, and sublingual glands at a rate of 1–1.5 liters per day. Saliva contains salivary amylase (also called ptyalin), which begins starch hydrolysis by cleaving internal alpha-1,4-glycosidic bonds; salivary lipase, which initiates modest triglyceride hydrolysis; mucins that lubricate the food bolus; and immunoglobulins and lysozyme that provide antimicrobial protection. Swallowing, or deglutition, is a coordinated reflex involving more than 25 pairs of muscles and is controlled by the swallowing center in the brainstem medulla.

The esophagus transports the bolus from the pharynx to the stomach by coordinated waves of peristalsis — sequential rings of smooth muscle contraction that propel content distally. The lower esophageal sphincter (LES) is a specialized zone of tonically contracted smooth muscle that prevents reflux of acidic gastric contents into the esophagus. Failure of the LES to maintain adequate pressure underlies gastroesophageal reflux disease (GERD), the most common GI complaint in clinical practice. The pyloric sphincter at the gastric outlet regulates the rate at which chyme enters the small intestine, ensuring that the duodenum is not overwhelmed with a large volume of concentrated, acidic material. Osmoreceptors and chemoreceptors in the duodenum detect the tonicity and chemical composition of arriving chyme and generate feedback signals that slow gastric emptying when necessary.

The stomach serves multiple digestive functions. It acts as a reservoir that allows the continuous output of a liquefied mass called chyme into the small intestine at a controlled rate, regardless of the pattern of food ingestion. Gastric glands in the fundus and body secrete hydrochloric acid from parietal cells, which acidifies the stomach contents to a pH of 1.5–3.5. This extreme acidity denatures dietary proteins, making them more susceptible to enzymatic hydrolysis, and activates the protease pepsinogen — secreted by chief cells — into its active form, pepsin. Pepsin cleaves proteins at peptide bonds adjacent to aromatic amino acids, beginning protein digestion. Parietal cells also secrete intrinsic factor, a glycoprotein essential for vitamin B12 absorption in the ileum. Mucus-secreting cells coat the gastric epithelium with a protective layer of alkaline mucus, preventing autodigestion. Antral G cells secrete gastrin, which stimulates acid secretion in a positive feedback loop that is terminated when luminal pH falls below 3.0.

The Small Intestine and Absorptive Mechanisms

The small intestine is the primary site of nutrient digestion and absorption. At approximately six to seven meters in length and divided into the duodenum, jejunum, and ileum, it provides an enormously amplified absorptive surface area. The mucosal lining is organized into finger-like projections called villi, each covered with epithelial cells called enterocytes. The luminal surface of each enterocyte bears thousands of microvilli — collectively called the brush border — that create the characteristic fuzzy appearance visible by electron microscopy. Together, villi and microvilli amplify the mucosal surface area approximately 600-fold compared to a flat tube of the same dimensions, reaching a total absorptive area of approximately 250 square meters — roughly the size of a tennis court.

The duodenum is the site of the most important digestive events. Chyme entering the duodenum triggers the secretion of two key hormones from enteroendocrine cells: secretin, released in response to acid, which stimulates the pancreas to secrete bicarbonate-rich juice that neutralizes the acid; and cholecystokinin (CCK), released in response to fat and protein, which stimulates the pancreas to secrete digestive enzymes and the gallbladder to contract, releasing bile. Pancreatic juice contains an impressive array of hydrolytic enzymes: pancreatic amylase (completing starch digestion), pancreatic lipase (hydrolyzing triglycerides with its cofactor colipase), proteases including trypsin, chymotrypsin, and carboxypeptidases (secreted as inactive zymogens to protect the pancreas from autodigestion), and ribonucleases and deoxyribonucleases. Brush border enzymes — disaccharidases such as lactase, sucrase, and maltase, and peptidases — complete the final stages of digestion at the mucosal surface.

Absorption occurs by several mechanisms depending on the nutrient. Simple diffusion drives the absorption of small lipid-soluble molecules along concentration gradients without requiring carrier proteins or energy. Facilitated diffusion uses carrier proteins embedded in the membrane to accelerate transport along concentration gradients without direct energy expenditure. Active transport uses ATP to move nutrients against their concentration gradients — critical for absorbing nutrients that are present in low luminal concentrations. Endocytosis allows the uptake of intact macromolecules by vesicle formation, important in neonates for immunoglobulin absorption and in adults for certain antigens. Most monosaccharides are absorbed by sodium-linked active transport (SGLT1) or facilitated diffusion (GLUT5 for fructose), most amino acids by sodium-linked transporters, and most lipids by passive diffusion after incorporation into mixed micelles.

The ileum performs specialized absorptive functions in addition to general nutrient absorption. It is the sole site of vitamin B12 absorption via the intrinsic factor-cubam receptor complex, and it is the primary site of bile acid reabsorption — the ileal sodium-bile acid cotransporter (ISBT) recovers approximately 95% of secreted bile acids in each digestive cycle, a process called the enterohepatic circulation. Loss of ileal function, whether from surgical resection (as in Crohn’s disease treatment) or mucosal disease, interrupts both vitamin B12 absorption and bile acid recycling, leading to vitamin B12 deficiency and fat malabsorption respectively. The enterohepatic circulation of bile acids is clinically exploited by bile acid sequestrant drugs (cholestyramine, colestipol) that bind bile acids in the intestinal lumen, preventing reabsorption and forcing the liver to synthesize new bile acids from cholesterol, thereby lowering plasma LDL cholesterol.

The Large Intestine and the Microbiome

The large intestine receives approximately 1.5 liters of ileal effluent per day and returns approximately 100 mL as stool by absorbing water and electrolytes. It harbors the largest and most complex microbial community in the human body — the gut microbiome — comprising approximately 100 trillion microorganisms representing hundreds of species, predominantly from the phyla Firmicutes and Bacteroidetes. This microbial ecosystem performs functions that are increasingly recognized as essential to human health, including the fermentation of dietary fiber to produce short-chain fatty acids (SCFAs), the synthesis of certain B vitamins and vitamin K, the training of the intestinal immune system, and the metabolism of drugs and xenobiotics.

Short-chain fatty acids — primarily acetate, propionate, and butyrate — produced by bacterial fermentation of non-digestible carbohydrates have received particular attention. Butyrate serves as the primary energy substrate for colonocytes (the cells lining the colon), providing approximately 60–70% of their oxidative energy. Propionate is transported to the liver and influences hepatic glucose and lipid metabolism. Acetate enters the systemic circulation and can be used by peripheral tissues. These SCFAs have regulatory effects beyond their roles as energy substrates: they act as ligands for G protein-coupled receptors on enteroendocrine cells and immune cells, influencing satiety hormone secretion, gut motility, and inflammatory tone. The recognition that dietary fiber influences colon health partly through the production of these microbial metabolites has transformed understanding of why high-fiber diets are associated with reduced colorectal cancer risk.

The gut microbiome is not static; it is profoundly shaped by diet, antibiotic exposure, mode of birth delivery, early life feeding (breastfeeding vs. formula), geography, and host genetics. Disruption of the normal microbiome composition — dysbiosis — has been associated with inflammatory bowel disease, obesity, type 2 diabetes, allergic diseases, and even mental health conditions through the gut-brain axis, a bidirectional communication network involving the vagus nerve, enteric nervous system, and circulating microbial metabolites. While much of the research on the microbiome is still observational and causality is not always established, the concept that diet shapes the microbiome and that the microbiome mediates dietary effects on health is now a central organizing principle of nutritional science, driving new interest in prebiotics (dietary fibers that selectively stimulate beneficial bacteria) and probiotics (live microorganisms that confer health benefits).

Chapter 3: Carbohydrates

Chemistry and Classification of Carbohydrates

Monosaccharides are the simplest carbohydrates and the units from which all more complex carbohydrates are built. They are classified by the number of carbon atoms — trioses (3C), pentoses (5C), and hexoses (6C) — and by the nature of their carbonyl group — aldoses (aldehyde) or ketoses (ketone). The nutritionally most important monosaccharides are the hexoses: glucose (an aldohexose and the universal cellular fuel), fructose (a ketohexose found in fruit and honey and produced by hydrolysis of sucrose), and galactose (an aldohexose produced by hydrolysis of the milk sugar lactose). The distinction between glucose and fructose extends well beyond chemistry: they are metabolized by different pathways, have different effects on insulin secretion and hepatic lipid synthesis, and exert different effects on satiety signaling. Fructose is metabolized largely in the liver, where high intakes can contribute to de novo lipogenesis and hypertriglyceridemia, a phenomenon of considerable public health concern given the prevalence of high-fructose corn syrup in processed foods.

Disaccharides consist of two monosaccharides joined by a glycosidic bond. The three nutritionally important disaccharides are sucrose (glucose + fructose, joined by an alpha-1,2-glycosidic bond; table sugar), lactose (galactose + glucose, joined by a beta-1,4-glycosidic bond; milk sugar), and maltose (glucose + glucose, joined by an alpha-1,4-glycosidic bond; produced during starch digestion and found in malt products). The geometry of the glycosidic bond — alpha or beta — profoundly affects digestibility. Alpha-1,4 bonds are hydrolyzed by human amylases and maltase; the beta-1,4 bond of lactose is cleaved by lactase; but the beta-1,4 bonds of cellulose cannot be hydrolyzed by human enzymes, making cellulose indigestible. Lactase persistence — the continued production of lactase into adulthood — is genetically determined and varies widely among populations. The derived allele conferring persistence is most common in Northern European and some East African pastoral populations (where dairy was historically a primary food source) and is much less common in East Asian and indigenous populations.

Polysaccharides are polymers of ten or more monosaccharide units and are further classified as digestible or indigestible (dietary fiber). Starch is the primary storage polysaccharide in plants and the most abundant digestible carbohydrate in the human diet. It consists of two polymers: amylose, a largely linear chain of glucose units connected by alpha-1,4 bonds (comprising 20–30% of most starches), and amylopectin, a highly branched polymer with alpha-1,4 bonds along chains and alpha-1,6 bonds at branch points (comprising 70–80%). The branch points of amylopectin are the sites attacked by the debranching enzyme during digestion. Glycogen is the mammalian analogue of amylopectin — a highly branched glucose polymer stored in the liver (approximately 100 g) and skeletal muscle (approximately 400 g) — with more frequent branching than amylopectin, reflecting the need for rapid glucose mobilization in mammals.

Resistant starch refers to starch that escapes digestion in the small intestine and reaches the colon, where it serves as a substrate for microbial fermentation. Resistant starch types include physically inaccessible starch (RS1, e.g., in whole grains with intact cell walls), native granular starches (RS2, e.g., raw potato and unripe banana), retrograded starches that form after cooking and cooling (RS3, e.g., cooled cooked potato), and chemically modified starches (RS4). Resistant starch produces SCFAs upon fermentation and has effects on glycemic response and colon health similar to soluble dietary fiber; it is increasingly recognized as an important component of a healthful dietary pattern.

Digestion and Absorption of Carbohydrates

Carbohydrate digestion begins in the mouth with salivary amylase, which cleaves internal alpha-1,4-glycosidic bonds of starch, producing shorter chains called dextrins and some maltose. However, food transit through the mouth and esophagus is rapid, and salivary amylase is inactivated by gastric acid, so oral digestion is quantitatively minor. The major site of starch digestion is the small intestinal lumen, where pancreatic amylase continues the hydrolysis initiated by salivary amylase. Pancreatic amylase is an endoamylase — it attacks internal glycosidic bonds rather than cleaving from the ends of chains — and produces primarily maltose, maltotriose, and alpha-limit dextrins (oligosaccharides containing alpha-1,6 branch points that cannot be cleaved by amylase alone). These products are then acted upon by brush border enzymes. Glucoamylase (maltase) cleaves glucose from the non-reducing ends of oligosaccharides. Alpha-dextrinase (isomaltase) cleaves the alpha-1,6 bonds at branch points. Sucrase hydrolyzes sucrose to glucose and fructose. Lactase hydrolyzes lactose to glucose and galactose. The result is a mixture of monosaccharides ready for absorption.

Absorption of monosaccharides occurs primarily in the duodenum and proximal jejunum. Glucose and galactose are absorbed by the sodium-glucose cotransporter 1 (SGLT1), an apical membrane transporter that uses the electrochemical gradient of sodium (maintained by the basolateral Na+/K+-ATPase) to drive uphill transport of glucose into the enterocyte — a process of secondary active transport. Fructose is absorbed by the facilitated transporter GLUT5 on the apical membrane. Once inside the enterocyte, all three monosaccharides exit via GLUT2 on the basolateral membrane into the portal circulation, which carries them to the liver for initial processing. An interesting regulatory feature is that high luminal glucose concentrations can acutely upregulate SGLT1 expression on the brush border, increasing absorptive capacity. In individuals with lactase deficiency who consume lactose, the undigested disaccharide passes to the colon where it is fermented by bacteria, producing gas (hydrogen, methane, carbon dioxide) and osmotically active organic acids — causing the bloating, cramping, and diarrhea characteristic of lactose intolerance.

The rate of carbohydrate digestion and absorption is not uniform across foods; it varies with the physical and chemical structure of the food. The glycemic index (GI) was developed to quantify the postprandial blood glucose response elicited by a food relative to a reference food (usually glucose or white bread). Foods with a high GI produce a rapid, large spike in blood glucose and subsequent insulin secretion, while low-GI foods produce a slower, more sustained glucose rise. Factors that lower GI include the presence of dietary fiber (which slows gastric emptying and forms viscous solutions that slow glucose absorption), intact cell walls in minimally processed foods, the presence of protein and fat (which slow gastric emptying), and the ratio of amylose to amylopectin (amylose-rich starches are more resistant to digestion). The glycemic load (GL) extends the GI concept by accounting for the quantity of carbohydrate consumed: GL = (GI × grams of carbohydrate per serving) / 100. Foods that provide a large glycemic load — refined grain products, sugar-sweetened beverages, large portions of high-GI foods — have been most consistently associated with increased risk of type 2 diabetes and cardiovascular disease in prospective cohort studies.

Dietary Fiber: Types, Functions, and Health Effects

Dietary fiber is classified by its physicochemical properties into soluble (or viscous) fiber and insoluble fiber. This classification reflects fundamentally different physiological effects. Soluble fibers dissolve in water to form viscous gels in the intestinal lumen, slowing gastric emptying, attenuating postprandial glucose absorption, binding bile acids to reduce cholesterol absorption, and providing fermentable substrate for the colonic microbiome. Major soluble fibers include pectin (found in fruits and vegetables), beta-glucan (found in oat bran and barley), and psyllium. Clinical trials have demonstrated that daily consumption of 3 grams of beta-glucan reduces LDL cholesterol by approximately 5–10%, an effect large enough to have regulatory approval in Canada, the United States, and the European Union for cholesterol-lowering health claims. The mechanism involves bile acid binding in the intestinal lumen, which reduces bile acid reabsorption and forces the liver to synthesize replacement bile acids from cholesterol, thereby drawing down hepatic cholesterol and upregulating LDL receptor expression to increase LDL clearance from blood.

Insoluble fiber does not dissolve in water and does not form gels. It adds bulk to the stool, accelerates intestinal transit, and dilutes potentially carcinogenic compounds in the colonic lumen. Major insoluble fibers include cellulose (found in cell walls of all plant foods), hemicellulose, and lignin. High intake of insoluble fiber is associated with reduced risk of constipation, diverticular disease, and colorectal cancer. The mechanism underlying the cancer-protective effect may involve dilution of carcinogens, reduction of transit time limiting carcinogen exposure to the mucosa, promotion of a favorable microbiome composition, and the action of SCFAs (particularly butyrate) in promoting colonocyte differentiation and inhibiting proliferation. Colorectal cancer is the third most common cancer in Canada, and the association between low dietary fiber intake and elevated risk is one of the most consistently replicated findings in cancer epidemiology.

The Adequate Intake for total fiber is set at 38 g/day for adult men and 25 g/day for adult women, values derived from epidemiological observations of intakes associated with reduced risk of cardiovascular disease. Canadian and American surveys consistently find average intakes well below these values — approximately 17 g/day in adults — reflecting the low consumption of whole grains, legumes, fruits, and vegetables that characterizes Western dietary patterns. A practical strategy for increasing fiber intake involves replacing refined grains with whole grains, legumes, and vegetables at every meal and choosing whole fruit rather than juice. Increasing fiber intake abruptly can cause temporary GI discomfort; a gradual increase over several weeks, accompanied by adequate fluid intake, is recommended.

Carbohydrate Metabolism and Cellular Energy Production

After absorption, glucose enters the portal vein and travels to the liver, where it undergoes several possible fates depending on the metabolic state of the organism. In the fed state, when insulin levels are elevated, glucose is taken up by hepatocytes through the high-capacity, low-affinity transporter GLUT2 and either used for immediate energy production through glycolysis, stored as glycogen via glycogenesis, or converted to fatty acids via de novo lipogenesis. In skeletal muscle, insulin stimulates glucose uptake via GLUT4 translocation to the plasma membrane, and glucose is used for glycolysis and glycogen synthesis. In adipose tissue, insulin promotes glucose uptake and conversion to glycerol-3-phosphate for triacylglycerol synthesis.

Glycolysis is the cytoplasmic metabolic pathway that converts one molecule of glucose (6 carbons) into two molecules of pyruvate (3 carbons each), with a net yield of 2 ATP and 2 NADH. The pathway has ten enzymatic steps; the irreversible committed step is the phosphorylation of fructose-6-phosphate to fructose-1,6-bisphosphate by phosphofructokinase-1 (PFK-1), allosterically activated by AMP (indicating energy deficit) and inhibited by ATP and citrate (indicating energy surplus). Under aerobic conditions in the presence of mitochondria, pyruvate is transported into the mitochondrial matrix, where pyruvate dehydrogenase complex (PDC) converts it to acetyl-CoA, releasing one molecule of CO2 and one NADH. Acetyl-CoA enters the citric acid cycle (Krebs cycle), where it is oxidized to CO2 while reducing NAD+ to NADH and FAD to FADH2. The electron carriers NADH and FADH2 then donate electrons to the mitochondrial electron transport chain, where oxidative phosphorylation generates the bulk of cellular ATP — approximately 30–32 ATP per glucose molecule under optimal conditions.

The regulation of carbohydrate metabolism is exquisitely sensitive to cellular energy status, hormonal signals, and substrate availability. AMP-activated protein kinase (AMPK) serves as the master energy sensor of the cell: when the AMP:ATP ratio rises (indicating energy deficit), AMPK is activated, stimulating glucose uptake, glycolysis, and fatty acid oxidation while inhibiting anabolic processes. This pathway is the target of the diabetes medication metformin and is activated by exercise, making it of great relevance to sports nutrition and metabolic disease management. During prolonged fasting or very low carbohydrate intake, hepatic glycogen stores are depleted and the liver shifts to producing ketone bodies (acetoacetate, beta-hydroxybutyrate, and acetone) from acetyl-CoA derived from fatty acid oxidation. Ketone bodies are exported to peripheral tissues, including the brain, which can derive up to 70% of its energy from ketones during prolonged fasting — a crucial adaptation that preserves the need for muscle protein catabolism to provide gluconeogenic substrates.

Chapter 4: Lipids

Classification and Structure of Dietary Lipids

Dietary lipids comprise four major categories: triglycerides (triacylglycerols), phospholipids, sterols (primarily cholesterol), and fat-soluble vitamins (which are technically lipids by virtue of their solubility). Triglycerides constitute approximately 95% of dietary fat and serve primarily as concentrated energy stores, providing 37 kJ (9 kcal) per gram — more than twice the energy density of carbohydrates or protein. A triglyceride consists of a glycerol backbone esterified at all three hydroxyl positions with fatty acids. It is the variation in fatty acid composition — chain length and degree of unsaturation — that determines most of the biological effects of different dietary fats.

Fatty acids are the fundamental structural and functional units of triglycerides and phospholipids. They consist of a carboxyl group (-COOH) at one end and a methyl group (-CH3) at the omega end, with a variable-length hydrocarbon chain connecting them. Fatty acids are classified by their chain length (short-chain: fewer than 6 carbons; medium-chain: 6–12 carbons; long-chain: 14–22 carbons; very long-chain: more than 22 carbons), by the number of carbon-carbon double bonds (saturated: no double bonds; monounsaturated: one double bond; polyunsaturated: two or more double bonds), and by the position and configuration (cis or trans) of double bonds. The omega nomenclature identifies the position of the double bond closest to the methyl end: omega-3 fatty acids have their first double bond at the third carbon from the methyl end, and omega-6 fatty acids at the sixth carbon. In human physiology, the two major polyunsaturated fatty acid families — n-6 and n-3 — are precursors to eicosanoids (prostaglandins, leukotrienes, thromboxanes) with often opposing biological activities: n-6 derived eicosanoids tend to be pro-inflammatory and pro-aggregatory, while n-3 derived eicosanoids and specialized pro-resolving mediators (resolvins, protectins, maresins) are anti-inflammatory and pro-resolving.

Essential fatty acids are those that humans cannot synthesize de novo and must obtain from the diet: linoleic acid (LA, 18:2 n-6) and alpha-linolenic acid (ALA, 18:3 n-3). The body can elongate and desaturate these precursors to produce longer-chain derivatives: from LA, arachidonic acid (ARA, 20:4 n-6); from ALA, eicosapentaenoic acid (EPA, 20:5 n-3) and docosahexaenoic acid (DHA, 22:6 n-3). However, the conversion efficiency is low — estimates suggest that only 5–10% of dietary ALA is converted to EPA and less than 1% to DHA in healthy adults. This limited conversion is one reason why dietary sources of preformed EPA and DHA (primarily fatty fish such as salmon, mackerel, sardines, and herring) are nutritionally important, particularly for brain and eye development in infancy and for cardiovascular protection in adults. DHA constitutes approximately 40% of the polyunsaturated fatty acids in the brain and 60% in the retinal photoreceptors — its structural role in membrane fluidity in these tissues is essential for optimal neurological and visual function.

Digestion and Absorption of Lipids

The digestion of lipids is more complex than that of carbohydrates or proteins because lipids are hydrophobic and require special handling to overcome the aqueous environment of the GI tract. The process begins in the mouth with lingual lipase and continues in the stomach with gastric lipase — together accounting for roughly 10–30% of triglyceride hydrolysis, preferentially cleaving short- and medium-chain fatty acids at the sn-3 position of the glycerol backbone. The major digestive events occur in the small intestinal lumen, where the combined action of pancreatic lipase (with its cofactor colipase), bile salts, and physical mixing creates the conditions for efficient fat digestion.

Bile is produced by hepatocytes, stored in the gallbladder, and released into the duodenum in response to CCK. Bile is an aqueous solution containing bile acids (synthesized from cholesterol), phospholipids (primarily lecithin), cholesterol, and bilirubin. Bile acids are amphipathic — they have both hydrophilic and hydrophobic regions — enabling them to function as biological detergents that emulsify dietary fat, breaking large fat globules into smaller droplets and increasing the surface area available for lipase action. Together, bile acids, monoglycerides (products of lipase action), and phospholipids spontaneously assemble into mixed micelles — nanoscale aggregates with a hydrophobic core and hydrophilic exterior that solubilize the products of fat digestion and carry them to the brush border for absorption.

At the brush border, the products of lipase action — free fatty acids and monoglycerides — dissociate from the micelle and diffuse passively across the enterocyte apical membrane, driven by the concentration gradient maintained by the subsequent re-esterification inside the cell. A membrane transporter, CD36, facilitates fatty acid uptake at the apical membrane. Within the enterocyte, free fatty acids and monoglycerides are re-esterified to triglycerides in the smooth endoplasmic reticulum. These triglycerides, along with cholesterol, phospholipids, and apoproteins (particularly apolipoprotein B-48), are packaged into chylomicrons — the largest and least dense lipoproteins — in the Golgi apparatus. Chylomicrons are too large to enter the capillaries directly and instead are secreted into intestinal lacteals (lymphatic capillaries) and transported via the thoracic duct to the subclavian vein, bypassing the portal circulation. This route is clinically significant: it means that most dietary fat bypasses hepatic first-pass metabolism and can directly reach peripheral tissues.

Lipoproteins and Cholesterol Metabolism

Because lipids are insoluble in water, they are transported in the blood within lipoprotein particles — complex assemblies of lipid and protein in which the protein (apolipoprotein) shell and a phospholipid monolayer at the surface provide the aqueous interface needed for circulation. The major lipoprotein classes are distinguished by their density, which reflects the ratio of lipid to protein content.

Chylomicrons carry dietary (exogenous) lipids from the intestine to peripheral tissues and the liver. In peripheral tissues, especially adipose tissue and muscle, lipoprotein lipase (LPL) on capillary endothelial cells hydrolyzes the chylomicron triglycerides, releasing fatty acids for uptake by adjacent cells. The remnant chylomicron particle, depleted of triglyceride but enriched in cholesterol, is taken up by the liver via apolipoprotein E receptors. Very low-density lipoprotein (VLDL) is synthesized by the liver and carries endogenous triglycerides and cholesterol to peripheral tissues. As VLDL triglycerides are hydrolyzed by LPL, VLDL is progressively converted to intermediate-density lipoprotein (IDL) and then to low-density lipoprotein (LDL), which is cholesterol-enriched and contains apolipoprotein B-100 as its sole apolipoprotein. LDL is taken up by tissues through the LDL receptor (LDLR), which binds apoB-100 and mediates endocytosis of the entire particle.

High-density lipoprotein (HDL) plays a central role in reverse cholesterol transport — the process by which cholesterol is retrieved from peripheral tissues and returned to the liver for excretion. HDL collects cholesterol from peripheral cells via the ABCA1 and ABCG1 transporters, esterifies it using the enzyme lecithin-cholesterol acyltransferase (LCAT), and delivers the mature, cholesterol-ester-rich HDL to the liver via scavenger receptor class B type I (SR-BI). Epidemiological studies have consistently shown that high HDL cholesterol concentrations are inversely associated with cardiovascular risk. The cholesterol ester transfer protein (CETP) moves cholesterol esters from HDL to VLDL and LDL in exchange for triglycerides, redistributing cholesterol among lipoprotein fractions — a process that is pharmacologically inhibited by CETP inhibitors in an attempt to raise HDL, though clinical trial results have been mixed. The liver is central to cholesterol homeostasis: it synthesizes approximately 75% of the body’s cholesterol (the other 25% comes from diet), regulates LDL receptor expression, and excretes cholesterol either directly into bile or after conversion to bile acids.

Trans Fatty Acids and Saturated Fat: Clinical Relevance

The relationship between dietary fat and cardiovascular disease has been one of the most intensely studied and debated topics in nutritional epidemiology. The macronutrient effects on serum lipoprotein concentrations are well established from controlled feeding experiments. Replacing saturated fatty acids (SFAs) with polyunsaturated fatty acids (PUFAs) reduces LDL cholesterol; replacing SFAs with monounsaturated fatty acids (MUFAs) also reduces LDL; and replacing SFAs with refined carbohydrates has a smaller or neutral effect on LDL while raising triglycerides and lowering HDL. Not all saturated fatty acids behave identically: lauric (12:0), myristic (14:0), and palmitic (16:0) acids raise LDL cholesterol, while stearic acid (18:0) is neutral, as it is rapidly desaturated to the monounsaturated oleic acid in vivo.

Trans fatty acids arise through partial hydrogenation of vegetable oils. Industrial trans fats have uniquely adverse effects on lipoprotein profiles: they raise LDL cholesterol and lower HDL cholesterol simultaneously, a combination more atherogenic than that of any other dietary fatty acid. Epidemiological data strongly link industrial trans fat consumption to cardiovascular disease risk; the Harvard Nurses’ Health Study estimated that replacing just 2% of energy from trans fat with unsaturated fat would reduce coronary heart disease risk by approximately 53%. Health Canada eliminated artificial trans fats from the Canadian food supply in 2018, and the United States FDA followed with a similar ruling. Small amounts of naturally occurring trans fats (primarily vaccenic acid and conjugated linoleic acid, CLA) are found in ruminant dairy and meat products; evidence suggests these may not have the same adverse effects as industrial trans fats, and CLA may even have anti-carcinogenic properties in animal models.

Chapter 5: Proteins and Amino Acids

Chemistry and Classification of Proteins

All proteins are assembled from a repertoire of 20 standard amino acids, each characterized by a central alpha carbon bearing an amino group (-NH2), a carboxyl group (-COOH), a hydrogen atom, and a distinctive side chain (R group) that confers the amino acid’s unique chemical properties. Amino acids are classified by the polarity and charge of their side chains: nonpolar aliphatic (glycine, alanine, valine, leucine, isoleucine, proline, methionine), aromatic (phenylalanine, tyrosine, tryptophan), polar uncharged (serine, threonine, cysteine, asparagine, glutamine), positively charged/basic (lysine, arginine, histidine), and negatively charged/acidic (aspartate, glutamate). The peptide bond that joins amino acids is formed by a condensation reaction between the alpha-carboxyl group of one amino acid and the alpha-amino group of the next, releasing water. Peptide bonds are planar and have partial double-bond character due to resonance, restricting rotation and imposing structural constraints on protein folding.

Of the 20 standard amino acids, nine are essential (indispensable) and must be obtained from the diet: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. The remaining eleven are nonessential (dispensable) — they can be synthesized endogenously from carbon skeletons and nitrogen. Two amino acids — arginine and glutamine — are sometimes classified as conditionally essential: under normal circumstances they can be synthesized in sufficient quantities, but in states of physiological stress (trauma, sepsis, rapid growth, critical illness), demand may exceed synthetic capacity. In critically ill patients, for example, glutamine supplementation has been investigated as a means of supporting gut mucosal integrity and immune function, though the evidence for benefit is context-dependent and has been revised downward after some large trials showed harm at high doses.

Protein quality refers to the degree to which a food protein meets the body’s amino acid requirements and how efficiently it is digested and absorbed. The Digestible Indispensable Amino Acid Score (DIAAS), adopted by the FAO in 2013, is the current gold standard for protein quality assessment. DIAAS uses ileal digestibility — measured at the end of the small intestine — for each indispensable amino acid, and calculates the ratio of the digestible amount of each IAAA in the food to the corresponding requirement. A DIAAS above 1.0 indicates that the protein provides all IAAAs in adequate amounts relative to requirements. Whey protein has DIAAS values around 1.25, reflecting excellent digestibility and high leucine content; whole eggs and milk protein are also high-quality. Most plant proteins have DIAAS values below 1.0 for at least one IAAA: soy protein is limiting in methionine, wheat in lysine, and corn in lysine and tryptophan. However, dietary diversification — consuming protein from multiple plant sources throughout the day — readily meets all essential amino acid requirements for most healthy adults following well-planned plant-based diets.

Protein Digestion and Absorption

Protein digestion begins in the stomach with pepsin, which is activated from its precursor pepsinogen by the low gastric pH. Pepsin is an endopeptidase with preferences for aromatic amino acids, and it reduces proteins to large polypeptide fragments. The major digestive events for protein occur in the small intestine, where pancreatic proteases perform extensive hydrolysis. Trypsin, activated by the brush border enzyme enterokinase (enteropeptidase) from its precursor trypsinogen, cleaves at the carboxyl side of lysine and arginine residues. Trypsin is also the activator of the other pancreatic zymogens — chymotrypsinogen (to chymotrypsin), proelastase (to elastase), procarboxypeptidases (to carboxypeptidases A and B) — so trypsin activation is the critical control point for the entire proteolytic cascade. Protection against pancreatic autodigestion is maintained by several mechanisms: zymogens are inactive until they reach the intestinal lumen; the pancreas synthesizes a trypsin inhibitor; and acinar cells are relatively resistant to trypsin activity. Failure of these protective mechanisms underlies acute pancreatitis.

Brush border peptidases complete hydrolysis of small peptides to free amino acids and dipeptides and tripeptides. The latter are absorbed as intact small peptides via the proton-coupled transporter PEPT1, which is driven by a transmembrane proton gradient. This transporter has a very broad substrate specificity — it accepts essentially all di- and tripeptide combinations — and transports peptides efficiently even when they are present in low concentrations. Inside the enterocyte, cytoplasmic peptidases hydrolyze absorbed peptides to free amino acids, which then exit the basolateral membrane via amino acid transporters into the portal circulation. The efficiency of protein absorption is generally high — over 90% for most animal proteins and 60–90% for plant proteins, depending on food processing and the presence of antinutritional factors.

Protein Metabolism: Nitrogen Balance and Turnover

The body’s proteins are in a state of dynamic turnover — simultaneously synthesized and degraded at all times. The rate of whole-body protein turnover is approximately 300–400 g/day in a 70 kg adult, representing a substantial fraction of total body protein content turned over daily. This turnover is energetically expensive, accounting for approximately 15–25% of whole-body energy expenditure. It also provides metabolic flexibility: damaged or misfolded proteins are rapidly degraded by the ubiquitin-proteasome system (UPS), and the released amino acids are recycled for new protein synthesis or oxidized for energy. The rate of protein synthesis in specific tissues is regulated by hormones (primarily insulin and IGF-1, which stimulate synthesis through the PI3K/Akt/mTORC1 pathway), amino acid availability (leucine is a particularly potent activator of mTORC1), and mechanical loading (which activates mTORC1 in skeletal muscle through Akt and other pathways).

Nitrogen balance is the difference between nitrogen intake (from dietary protein) and nitrogen excretion (primarily as urea in urine, but also in feces, sweat, and other routes). In a state of nitrogen equilibrium (zero balance), intake equals output. Positive nitrogen balance occurs during growth, pregnancy, recovery from illness, and resistance exercise-induced hypertrophy. Negative nitrogen balance occurs during inadequate protein intake, caloric restriction, illness, injury, and immobilization. The current RDA for protein is 0.80 g per kilogram of body weight per day for adults. This value is adequate for sedentary healthy adults but is increasingly recognized as insufficient for older adults, athletes, and those under physiological stress. Importantly, the RDA was derived predominantly from short-term nitrogen balance studies in young adults, a methodology that has limitations and may not capture the protein needs for optimal muscle function, immune competence, and healthy aging across the full lifespan.

Chapter 6: Vitamins

Fat-Soluble Vitamins

Vitamin A encompasses a family of related compounds collectively called retinoids, including retinol, retinal, retinoic acid, and their esters, as well as the provitamin A carotenoids (primarily beta-carotene) found in plant foods. Retinol is the primary dietary form, found in liver (the most concentrated source), dairy products, eggs, and fish oils. Beta-carotene and other provitamin A carotenoids are found in orange, yellow, and dark green vegetables; beta-carotene is cleaved centrally by the enzyme BCO1 (beta-carotene oxygenase 1) to produce two molecules of retinal. The efficiency of this conversion is highly variable among individuals and is influenced by genetic polymorphisms in BCO1, the food matrix, and vitamin A status itself — conversion decreases as retinol status improves. The DRI for vitamin A is expressed as retinol activity equivalents (RAE): 1 mcg RAE = 1 mcg retinol = 12 mcg beta-carotene from food = 24 mcg other provitamin A carotenoids.

Retinal is the chromophore of rhodopsin in rod photoreceptors, involved in dim-light vision. The visual cycle involves the isomerization of 11-cis-retinal to all-trans-retinal upon photon absorption, triggering a conformational change in rhodopsin that initiates the phototransduction cascade. Vitamin A deficiency impairs dark adaptation (night blindness) long before total vision is affected — night blindness is one of the earliest clinical signs of deficiency. With more severe deficiency, xerophthalmia develops — a spectrum from conjunctival and corneal drying to corneal ulceration and irreversible blindness. Vitamin A deficiency is the leading preventable cause of childhood blindness globally, affecting millions of children in sub-Saharan Africa and South and Southeast Asia. Beyond vision, retinoic acid acts as a ligand-activated transcription factor (via nuclear RAR/RXR receptors) regulating genes for cell differentiation, embryogenesis, and immune function; it is essential for the normal development of the respiratory, urogenital, and intestinal epithelia, and its deficiency impairs mucosal immunity and increases susceptibility to infectious diseases. Vitamin A toxicity from excessive supplement use causes teratogenicity (particularly craniofacial malformations), liver damage, and increased bone fracture risk.

Vitamin D functions as a hormone and can be synthesized endogenously when skin is exposed to UVB radiation. Ultraviolet-B (UVB) radiation catalyzes the photochemical conversion of 7-dehydrocholesterol in the skin to previtamin D3, which isomerizes thermally to vitamin D3 (cholecalciferol). Both dietary vitamin D3 (from animal sources and supplements) and D2 (ergocalciferol, from fungi and plant sources) undergo sequential hepatic hydroxylation to 25-hydroxyvitamin D [25(OH)D, calcidiol] — the major circulating form and standard clinical measure of vitamin D status — and then renal hydroxylation to 1,25-dihydroxyvitamin D [calcitriol], the most biologically active form. Calcitriol acts through the vitamin D receptor (VDR), a nuclear receptor expressed in virtually every cell type.

The classical function of calcitriol is to maintain calcium homeostasis: it upregulates TRPV6 calcium channels and calbindin in the intestine to enhance calcium absorption (increasing absorption efficiency from approximately 10–15% without calcitriol to 30–40% with adequate vitamin D), stimulates RANKL in osteoblasts to promote bone resorption in the context of hypocalcemia, and enhances calcium reabsorption in the kidney. When dietary calcium is adequate and serum calcium is normal, calcitriol’s effects are more anabolic — supporting bone mineralization via osteocalcin and other bone matrix proteins. Beyond calcium homeostasis, vitamin D has pleiotropic effects: it modulates innate and adaptive immunity by promoting antimicrobial peptide (cathelicidin) production in macrophages and regulating T cell differentiation; it inhibits renin expression; it has antiproliferative effects in many cell types. The optimal serum 25(OH)D concentration for non-skeletal health outcomes remains debated, but the current Endocrine Society clinical guideline defines sufficiency as 75 nmol/L or above.

Vitamin E refers to alpha-tocopherol as the biologically active form preferentially retained in the body, functioning as a chain-breaking antioxidant in cell membranes. Because polyunsaturated fatty acids in membrane phospholipids are susceptible to lipid peroxidation by free radicals — a self-propagating chain reaction — alpha-tocopherol donates a hydrogen atom to peroxyl radicals, terminating the chain reaction. Vitamin C regenerates the tocopheroxyl radical back to tocopherol. Deficiency is rare in healthy adults but occurs in premature infants and individuals with fat malabsorption; clinical manifestations include hemolytic anemia, spinocerebellar ataxia, and peripheral neuropathy. Vitamin K is the cofactor for gamma-glutamyl carboxylase, adding a carboxyl group to glutamate residues in vitamin K-dependent proteins including clotting factors II, VII, IX, and X, anticoagulant proteins C and S, osteocalcin, and matrix Gla protein. The anticoagulant warfarin inhibits vitamin K epoxide reductase (VKORC1), preventing recycling of vitamin K and thereby impairing coagulation factor carboxylation; VKORC1 polymorphisms explain much of the inter-individual variability in warfarin dose requirements.

Water-Soluble Vitamins

Vitamin C (ascorbic acid) is a six-carbon lactone with two ionizable hydroxyl groups that readily donate electrons, making it an effective reducing agent and antioxidant in aqueous environments. Its most critical metabolic role is as a cofactor for two enzyme families: the 2-oxoglutarate-dependent dioxygenases, including prolyl hydroxylases (which hydroxylate proline residues in procollagen chains, essential for the triple-helix stability and cross-linking of mature collagen fibers) and lysyl hydroxylases, and dopamine beta-hydroxylase (which converts dopamine to norepinephrine). In the absence of vitamin C, collagen synthesis is impaired and existing collagen becomes unstable, leading to scurvy — characterized by perifollicular hemorrhages, bleeding gums, impaired wound healing, and eventual death. Historically, scurvy decimated sailors on long voyages and soldiers on campaigns until it was discovered that citrus fruits could prevent and cure it — a pivotal discovery in the history of nutrition science.

The B vitamins are chemically diverse but share the functional role of coenzymes in energy metabolism and biosynthetic reactions. Thiamine (B1) as thiamine pyrophosphate (TPP) is essential for the pyruvate dehydrogenase complex, alpha-ketoglutarate dehydrogenase complex, branched-chain alpha-keto acid dehydrogenase, and transketolase — all involving oxidative decarboxylation. Thiamine deficiency causes beriberi (dry: peripheral neuropathy; wet: cardiovascular failure with edema) and Wernicke-Korsakoff syndrome in alcoholics (confusion, ataxia, ophthalmoplegia, and if untreated, permanent amnesia and confabulation). Alcoholics are particularly vulnerable because alcohol impairs thiamine absorption, hepatic storage, and phosphorylation to the active form.

Riboflavin (B2) as FMN and FAD participates in oxidation-reduction reactions throughout metabolism, including in complex I (NADH dehydrogenase) and complex II (succinate dehydrogenase) of the electron transport chain, in fatty acid beta-oxidation, and in the metabolism of folate, pyridoxine, and niacin. Niacin (B3) as NAD+ and NADP+ participates in over 400 enzymatic reactions — it is arguably the most widely used electron carrier in biochemistry. Niacin can be synthesized from the essential amino acid tryptophan (approximately 60 mg tryptophan yields 1 mg niacin), which is why pellagra occurs most severely in populations depending on corn (which is low in both niacin and tryptophan) that has not been treated by nixtamalization — the traditional Mesoamerican practice of processing corn with an alkaline solution that liberates bound niacin and makes it bioavailable.

Folate in its active form tetrahydrofolate (THF) carries and transfers one-carbon units at various oxidation states, participating in the synthesis of purines, thymidylate (essential for DNA synthesis), and the amino acids methionine and serine. The most clinically critical folate function is the provision of a methyl group (from 5-methylTHF) to homocysteine to regenerate methionine, catalyzed by methionine synthase (which requires vitamin B12). Folate deficiency impairs rapidly proliferating cells, causing megaloblastic anemia and, in early pregnancy, neural tube defects. Mandatory folic acid fortification of flour in Canada (implemented 1998) has reduced NTD rates by approximately 50%, one of the great success stories of nutritional public health. Vitamin B12 is unique in being synthesized only by bacteria and archaea, requiring intrinsic factor from gastric parietal cells for absorption in the ileum, and being found almost exclusively in animal foods. The elderly are at particular risk of B12 deficiency due to atrophic gastritis, which impairs acid-mediated release of food-bound B12 from protein matrices; crystalline B12 in supplements is absorbed without requiring gastric acid and is preferred in this population.

Chapter 7: Minerals

Macrominerals: Calcium and Phosphorus

Calcium is the most abundant mineral in the body, with approximately 1,000 g in an adult, of which 99% is stored in bone and teeth as calcium phosphate crystals (predominantly hydroxyapatite, \(\text{Ca}_{10}(\text{PO}_4)_6(\text{OH})_2\)). The remaining 1% in blood, extracellular fluid, and soft tissues performs vital functions: calcium ions act as intracellular second messengers, trigger neurotransmitter release at the neuromuscular junction, initiate muscle contraction by binding troponin C, participate in blood coagulation, and maintain the resting membrane potential of excitable cells. The serum calcium concentration is maintained within a narrow range (2.2–2.6 mmol/L total calcium; 1.1–1.3 mmol/L ionized calcium) by the integrated action of PTH, calcitriol, and calcitonin — a hormonal system of such physiological priority that bone serves as an essentially unlimited reservoir to be resorbed if dietary calcium intake is inadequate to maintain serum calcium.

Calcium absorption in the small intestine occurs by two mechanisms: active transcellular transport (predominating at low-to-moderate intakes), which involves apical entry via TRPV6 channels, intracellular transport by calbindin, and basolateral extrusion by the plasma membrane Ca2+-ATPase (PMCA) and Na+/Ca2+ exchanger; and passive paracellular transport (predominating at high intakes), driven by concentration gradient through tight junctions. The overall efficiency of calcium absorption is approximately 25–35% in healthy adults but varies considerably by age (higher in infancy and adolescence when requirements are high, lower in elderly adults) and vitamin D status. The RDA for calcium is 1,000 mg/day for adults 19–50 years, rising to 1,200 mg/day for women over 50 and adults over 70.

Phosphorus is the second most abundant mineral in the body. In bone, it is esterified with calcium in hydroxyapatite; in soft tissues, it is a component of phospholipid membranes, nucleic acids (DNA and RNA), ATP and other nucleotides, and is critical for the phosphorylation-dependent regulation of enzyme activity and signal transduction. Phosphorus absorption is generally high (55–80%) because inorganic phosphate is absorbed by a sodium-phosphate cotransporter (NaPi-IIb) that is upregulated by calcitriol. The primary regulators of phosphorus homeostasis are the kidneys, where PTH and FGF-23 (fibroblast growth factor 23, secreted by osteocytes) regulate phosphate reabsorption. FGF-23 is the primary phosphaturic hormone and also inhibits renal 1-alpha-hydroxylase, reducing calcitriol production — a regulatory axis that is deranged in chronic kidney disease, leading to hyperphosphatemia, reduced calcitriol, and secondary hyperparathyroidism.

Trace Minerals: Iron, Zinc, and Iodine

Iron participates in oxygen transport (as a component of hemoglobin and myoglobin), cellular respiration (as a component of cytochromes in the electron transport chain and Krebs cycle enzymes), and many other enzymatic reactions including ribonucleotide reductase (essential for DNA synthesis). The body contains approximately 3–5 g of iron, of which 60–70% is in hemoglobin. Iron absorption is regulated by hepcidin — a hepatic peptide hormone that binds ferroportin and causes its degradation, reducing iron export from enterocytes and macrophages into the blood. High iron stores and inflammation both increase hepcidin, reducing absorption; iron deficiency decreases hepcidin, increasing absorption. This regulation prevents accumulation of free iron, which would participate in Fenton chemistry — reacting with hydrogen peroxide to generate highly reactive hydroxyl radicals that damage DNA, proteins, and lipids.

Heme iron (from meat, poultry, and fish) is absorbed at approximately 15–35% efficiency regardless of dietary context. Non-heme iron (from plant foods and fortified foods) is absorbed at 2–20% efficiency and is strongly influenced by dietary enhancers (vitamin C, organic acids, meat factor) and inhibitors (phytates, polyphenols, calcium). Iron deficiency is the most prevalent nutrient deficiency worldwide. It progresses through three stages: iron depletion (reduced ferritin, normal hemoglobin and function), iron-deficient erythropoiesis (reduced transferrin saturation, elevated erythrocyte protoporphyrin, normal hemoglobin), and iron deficiency anemia (low hemoglobin, microcytic hypochromic erythrocytes, impaired physical and cognitive performance). The RDA for iron is 8 mg/day for adult men and 18 mg/day for premenopausal women.

Zinc participates in over 300 enzymatic reactions and has structural roles in thousands of zinc finger transcription factors. It is essential for immune function (particularly T cell development and function), wound healing, taste and smell, antioxidant defense (as a component of Cu/Zn superoxide dismutase), and normal growth and sexual maturation. Iodine is essential for thyroid hormone synthesis. Iodine deficiency — the leading preventable cause of intellectual disability worldwide — causes goiter, hypothyroidism, and cretinism (profound intellectual disability, growth retardation, and deafness when severe deficiency occurs during fetal development). Universal salt iodization remains the primary global public health strategy. Sodium, potassium, and chloride are the major electrolytes regulating fluid balance, membrane potential, and acid-base balance. Average Canadian sodium intakes (approximately 3,400 mg/day) substantially exceed the AI of 1,500 mg/day and the chronic disease risk reduction value of 2,300 mg/day, contributing to hypertension and cardiovascular disease risk.

Chapter 8: Energy Balance, Body Composition, and Weight Management

Energy Metabolism and Total Energy Expenditure

Energy balance is the relationship between energy intake from food and beverages and energy expenditure from all physiological processes. A state of energy balance — intake equaling expenditure — maintains a stable body weight. Positive energy balance results in storage of excess energy primarily as adipose tissue triglycerides, with approximately 7,700 kcal stored per kilogram of adipose tissue gained. Negative energy balance results in mobilization of adipose stores, muscle glycogen, and (in prolonged deficits) muscle protein.

Total energy expenditure (TEE) comprises three major components. Basal metabolic rate (BMR) is the energy expended at rest in a post-absorptive state to maintain basic physiological functions — heartbeat, breathing, temperature regulation, and cellular maintenance — accounting for approximately 60–70% of TEE in sedentary individuals. BMR is primarily determined by lean body mass; the organs with the highest metabolic rates per unit mass are the brain, liver, heart, and kidney. The thermic effect of food (TEF) is the energy cost of digesting, absorbing, and storing nutrients — approximately 10% of caloric intake across mixed diets, with protein having the highest thermic effect (20–30%) due to the high ATP cost of peptide bond synthesis. Physical activity is the most variable component of TEE, ranging from less than 15% of TEE in very sedentary individuals to over 50% in elite athletes. Non-exercise activity thermogenesis (NEAT) — the energy expended in all physical activities outside structured exercise, including fidgeting, posture maintenance, and occupational activity — can vary by up to 2,000 kcal/day among individuals and is an important but often underappreciated determinant of total energy expenditure.

The Mifflin-St. Jeor equation is the most validated formula for estimating BMR in clinical practice:

where \( W \) is weight in kg, \( H \) is height in cm, \( A \) is age in years, and \( S = +5 \) for males and \( S = -161 \) for females. TEE is estimated by multiplying BMR by a physical activity factor: 1.2 (sedentary), 1.375 (light activity), 1.55 (moderate activity), 1.725 (very active), or 1.9 (extra active). These multiplication factors are necessarily approximations; doubly-labeled water studies — the gold standard for measuring free-living TEE — reveal substantial individual variability in physical activity levels that these categories cannot fully capture.

Body Composition and Obesity Pathophysiology

Body mass index (BMI), defined as weight in kg divided by height in m squared, is widely used for epidemiological classification of adiposity. The World Health Organization defines underweight as BMI below 18.5, normal weight as 18.5–24.9, overweight as 25.0–29.9, and obesity as 30.0 or above. BMI’s limitations are well recognized: it does not distinguish between fat and lean mass, and it does not capture the distribution of fat. Visceral adipose tissue (VAT) — the fat deposited within the abdominal cavity, surrounding the organs — is far more metabolically harmful than subcutaneous fat. VAT adipocytes are highly lipolytic, releasing free fatty acids directly into the portal circulation and promoting hepatic lipid accumulation, insulin resistance, and inflammation. VAT also secretes adipokines (inflammatory cytokines including TNF-alpha, IL-6, and resistin) that impair insulin signaling systemically. Waist circumference measurements (thresholds: 102 cm for men, 88 cm for women in Canadian guidelines) provide a practical clinical indicator of visceral adiposity.

The adipokine leptin is secreted by adipocytes proportional to fat mass and acts on hypothalamic arcuate nucleus neurons to reduce food intake and increase energy expenditure. In obesity, leptin levels are high but hypothalamic leptin sensitivity is impaired — leptin resistance — due to impaired transport across the blood-brain barrier and post-receptor signaling defects involving SOCS3 and PTP1B. Ghrelin, a stomach-derived peptide, rises sharply before meals and is suppressed by food intake; it stimulates appetite via the arcuate nucleus and GH secretagogue receptor (GHSR). GLP-1 and PYY are released from intestinal L cells after meals, promoting satiety through vagal afferents and direct hypothalamic actions. GLP-1 also stimulates pancreatic insulin secretion and inhibits glucagon, making GLP-1 receptor agonists (semaglutide, liraglutide) effective treatments for both type 2 diabetes and obesity. The STEP trials demonstrated that semaglutide 2.4 mg/week produces approximately 15% body weight reduction in adults with obesity — approaching the magnitude of bariatric surgery — representing a transformative advance in obesity pharmacotherapy.

Chapter 9: Sports Nutrition

Energy and Macronutrient Needs for Athletes

Sports nutrition addresses dietary practices that support athletic performance, training adaptations, and recovery. The substantially elevated energy expenditure of regular training creates increased requirements for energy, carbohydrates, protein, and some micronutrients. Carbohydrates are the primary fuel for moderate-to-high intensity exercise. Muscle glycogen stores are finite (approximately 400–500 g in a trained athlete) and become performance-limiting in events lasting more than 60–90 minutes. Carbohydrate recommendations range from 3–5 g/kg/day for low-intensity activities to 8–12 g/kg/day for very high volume training. During exercise lasting more than 60 minutes, consuming 30–60 g of carbohydrate per hour maintains blood glucose and spares muscle glycogen; consuming carbohydrates from multiple sources (glucose + fructose, which use different intestinal transporters — SGLT1 and GLUT5 respectively) can increase oxidation rates to 1.5–1.7 g/min, compared to 1.0 g/min from glucose alone.

Protein requirements for athletes are elevated to support muscle protein repair and synthesis. Current recommendations from sports dietetics organizations suggest 1.6–2.2 g/kg/day for regular resistance training, with protein distributed in doses of 20–40 g per meal to maximize muscle protein synthetic responses. The amino acid leucine is a particularly potent activator of mTORC1 and muscle protein synthesis, with a threshold of approximately 2–3 g per serving needed to maximally stimulate synthesis in young adults and 3–4 g in older adults. Consuming a leucine-rich protein source (whey, eggs, or a complete plant-protein blend) within several hours before and after resistance exercise optimizes anabolic outcomes. Hydration is also a performance-critical nutritional concern: fluid losses as small as 2% of body weight impair aerobic performance, cognitive function, and thermoregulation. Athletes should aim to begin exercise well-hydrated, consume fluid during prolonged exercise to replace losses, and rehydrate with sodium-containing fluids after exercise to restore both fluid volume and electrolyte balance.

Ergogenic Aids and Supplements

Creatine monohydrate is the most extensively studied and consistently effective performance supplement. Supplementation (3–5 g/day maintenance or 20 g/day loading for 5–7 days) increases muscle phosphocreatine stores by 20–30%, improving performance in high-intensity short-duration exercise and augmenting strength and lean mass gains during resistance training programs. Caffeine (3–6 mg/kg body weight) is an adenosine receptor antagonist that reduces perceived effort and improves performance across a wide range of exercise modalities with strong evidence of efficacy. Beta-alanine (3.2–6.4 g/day for 4–6 weeks) increases muscle carnosine content, improving performance in exercise lasting 1–10 minutes by buffering intramuscular acidosis. Dietary nitrate (from beetroot juice or sodium nitrate supplements) increases plasma nitrite and nitric oxide, reducing the oxygen cost of exercise and improving performance in endurance activities. Beyond these well-supported supplements, hundreds of products marketed to athletes have little or no evidence of efficacy; distinguishing evidence-based supplements from ineffective ones requires critical evaluation of the quality, quantity, and independence of supporting research.

Chapter 10: Nutrition Across the Life Cycle

Pregnancy and Lactation

Pregnancy substantially increases requirements for energy (approximately 340 kcal/day in the second trimester and 452 kcal/day in the third trimester above pre-pregnancy baseline), protein (to 71 g/day), folate (to 600 mcg DFE/day), iron (to 27 mg/day), iodine (to 220 mcg/day), and DHA (200 mg/day for fetal neurodevelopment). The timing of nutrient adequacy relative to gestational events is critical: the neural tube closes between gestational days 22–28 before many women know they are pregnant, making periconceptional folate adequacy essential. Iron requirements peak in the third trimester when fetal iron accretion is most rapid; the fetus preferentially takes iron at the expense of maternal stores, so maternal iron deficiency anemia can develop even without fetal anemia.

Lactation imposes the highest energy and micronutrient demands of any life stage, with human milk production requiring approximately 500 kcal/day above baseline. Breast milk provides all essential nutrients in appropriate proportions (with the exception of vitamin D, which is often insufficient, requiring infant supplementation with 400 IU/day) and also bioactive components — secretory IgA, lactoferrin, lysozyme, human milk oligosaccharides (HMOs), and growth factors — that profoundly influence infant immune development and gut microbiome colonization. HMOs are the third most abundant solid component of breast milk and serve as prebiotics for Bifidobacterium, promoting the establishment of a protective microbiome; they are also directly antimicrobial and anti-adhesive against pathogens.

Pediatric and Adolescent Nutrition

Iron deficiency is the most common nutritional deficiency in children, affecting approximately 20% of toddlers in North America due to excessive milk intake displacing iron-rich foods and the low iron content of human milk after 6 months. Calcium requirements reach their peak during adolescence (1,300 mg/day for 9–18 years) as the pubertal growth spurt drives rapid bone mineral accretion — up to 40% of peak bone mass is accumulated during this period. Eating disorders — anorexia nervosa, bulimia nervosa, and binge eating disorder — have their peak incidence in adolescence and young adulthood and represent severe nutritional and medical risks. Anorexia nervosa has the highest mortality rate of any psychiatric disorder, with nutritional rehabilitation as a cornerstone of treatment. Increasing consumption of ultra-processed foods throughout childhood and adolescence — driven by marketing, availability, and palatability — is a major public health concern, associated with displacing nutrient-dense foods, promoting energy overconsumption, and contributing to rising rates of childhood obesity.

Nutrition and Older Adults

Aging produces a constellation of physiological changes that alter nutrient requirements and metabolism: reduced lean body mass lowers BMR and energy requirements; gastric acid secretion declines (atrophic gastritis), impairing absorption of protein-bound B12, calcium (at low intakes), iron (non-heme), and zinc; renal function declines, impairing vitamin D activation and electrolyte homeostasis; taste and smell become less acute, reducing appetite and dietary diversity; and social isolation, depression, and mobility limitations impair food access and preparation. The combination of reduced energy intake and unchanged or increased micronutrient requirements demands high dietary nutrient density in older adults. Sarcopenia — the progressive loss of skeletal muscle mass and function with aging — is the major nutritional-physiological concern in gerontology, associated with falls, fractures, disability, and mortality. Dietary strategies to attenuate sarcopenia center on protein intake (1.0–1.2 g/kg/day distributed evenly throughout the day), leucine-rich protein sources, and combined resistance exercise training.

Chapter 11: Food Politics and the Nutrition Information Environment

Food Systems and Policy

The food supply is not neutral; it is shaped by agricultural policies, trade agreements, corporate interests, and regulatory frameworks that profoundly influence what foods are available, affordable, and marketed to the public. Agricultural subsidies in Canada and the United States have historically disproportionately supported commodity crops (corn, soybeans, wheat, sugar beets) used primarily for animal feed, processed food ingredients, and biofuels, rather than fruits, vegetables, and legumes — the foods most prominently recommended in dietary guidelines. This policy mismatch distorts relative food prices: whole plant foods are often more expensive per calorie than highly processed alternatives, creating structural barriers to healthy eating for lower-income populations.

The ultra-processed food category — defined by the NOVA classification system as industrial formulations containing ingredients not used in home cooking (such as isolated proteins, modified starches, hydrogenated fats, artificial colors, flavors, and emulsifiers) — now accounts for approximately 50% of caloric intake in Canada and the United States. Multiple large prospective cohort studies have found independent associations between ultra-processed food consumption and obesity, cardiovascular disease, type 2 diabetes, depression, and all-cause mortality, even after adjustment for nutrient composition — suggesting that processing characteristics themselves (energy density, palatability, texture, eating rate, disruption of food structure, and loss of bioactive components) may contribute to harm beyond simple nutrient profiles. While causality is not fully established from observational data, the evidence is sufficient to inform public health recommendations to limit ultra-processed foods in favor of minimally processed alternatives.

Food marketing — particularly to children — is a powerful environmental determinant of food preferences and intake. Children see an estimated 5,000–10,000 food advertisements annually, the vast majority for ultra-processed foods high in sugar, sodium, and saturated fat. Marketing strategies leverage emotional associations, cartoon characters, sports celebrity endorsements, and social media influencers to build brand loyalty and bypass rational decision-making. Restrictions on food marketing to children, implemented in Quebec since 1980 (which has contributed to significantly lower rates of childhood obesity and caloric intake from fast food compared to the rest of Canada), provide a natural experiment demonstrating the effectiveness of this policy tool. Evidence-based food policy approaches with demonstrated effectiveness include sugar-sweetened beverage taxes, mandatory front-of-package nutrition labeling, menu calorie labeling in restaurants, and national food guidelines that reflect scientific consensus rather than industry influence.

Nutrition misinformation is pervasive in the digital age, and developing media literacy around nutritional claims is a critical skill for health professionals and the public alike. Characteristics of misinformation include testimonial-based rather than evidence-based claims, promises of rapid dramatic effects, demonization of single foods or nutrients, promotion of supplements as cures for non-deficiency conditions, and failure to acknowledge uncertainty. Critical evaluation of nutritional claims requires asking: Is the claim supported by randomized controlled trials or only by observational data or animal studies? How large is the effect and is it clinically meaningful? Who funded the research and do they have a financial stake in the outcome? Is the claim plausible given established mechanisms? Has the finding been replicated independently? These questions form the foundation of evidence-based nutritional practice and are essential for navigating the complex and often contradictory landscape of modern nutrition communication.