Nutrition

Nutrition is the science that studies the relationship between food, the nutrients it contains, and the physiological processes of living organisms. It encompasses how organisms ingest, digest, absorb, transport, utilize, and excrete nutrients to sustain life, support growth, maintain health, and prevent disease. Nutrition integrates biochemistry, physiology, food science, epidemiology, and public health, making it both a biological science and a foundation of clinical and preventive medicine.

The Nutrient Classification System

All substances obtained from food that the body uses for energy, growth, repair, and regulation are classified as nutrients. They are organized into six major categories, further divided by the quantity required and whether the body can synthesize them independently.

Essential nutrients are those the body cannot synthesize in sufficient quantities and must therefore obtain from food. They include the essential amino acids, essential fatty acids, all vitamins, all dietary minerals, and water. Non-essential nutrients can be synthesized endogenously, though dietary sources may supplement endogenous production.

Carbohydrates

Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen, classified by their structural complexity into simple and complex forms.

Monosaccharides — glucose, fructose, and galactose — are the simplest units, absorbed directly into the bloodstream. Disaccharides — sucrose, lactose, and maltose — are hydrolyzed by intestinal enzymes into monosaccharides before absorption. Polysaccharides — starch and glycogen — are long glucose chains; starch is the primary dietary storage carbohydrate in plants, while glycogen is the form of glucose storage in the liver and muscle.

Dietary fiber consists of non-digestible polysaccharides and lignins. Soluble fiber (pectin, beta-glucan) forms a gel in the intestine, slowing glucose absorption and lowering serum cholesterol. Insoluble fiber (cellulose) adds bulk, accelerating intestinal transit and reducing colorectal cancer risk. Fiber also serves as the primary substrate for colonic microbiota, producing short-chain fatty acids (SCFAs) that nourish colonocytes and regulate immune function.

The glycaemic index (GI) ranks foods by how rapidly they elevate blood glucose relative to a reference. High-GI foods cause rapid glucose spikes followed by compensatory insulin surges; low-GI foods produce a slower, more sustained glucose release. Glycaemic load (GL) adjusts GI for portion size, providing a more practical measure of glycaemic impact.

Proteins and Amino Acids

Proteins are polymers of amino acids linked by peptide bonds. Twenty standard amino acids are used in human protein synthesis, of which nine are essential (histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, valine) — they must be obtained from food because the human body cannot synthesize them.

Proteins serve structural roles (collagen, keratin, actin, myosin), catalytic roles (enzymes), transport roles (hemoglobin, albumin), immune roles (antibodies, cytokines), and regulatory roles (peptide hormones, receptors, transcription factors).

Protein quality is determined by amino acid profile and digestibility. Animal proteins (meat, fish, eggs, dairy) are complete — they contain all essential amino acids in proportions matching human requirements. Most plant proteins are incomplete, deficient in one or more essential amino acids, requiring complementation across food sources.

Nitrogen balance is the difference between nitrogen intake (from dietary protein) and nitrogen output (primarily urinary urea). Positive nitrogen balance — more intake than excretion — characterizes growth, pregnancy, and recovery from illness. Negative nitrogen balance indicates net protein catabolism, common in critical illness, starvation, and excessive glucocorticoid activity.

Lipids

Lipids are hydrophobic molecules serving as concentrated energy stores, structural components of cell membranes, precursors of signaling molecules, and carriers of fat-soluble vitamins.

Triglycerides — three fatty acids esterified to a glycerol backbone — constitute the majority of dietary fat and the primary form of energy storage in adipose tissue. Phospholipids form the bilayer structure of all cellular membranes. Sterols, including cholesterol, are precursors of steroid hormones, bile acids, and vitamin D.

Fatty acid classification by degree of saturation has significant metabolic and health implications.

Essential fatty acids — linoleic acid (omega-6) and alpha-linolenic acid (omega-3) — cannot be synthesized by humans and must be obtained from diet. Their long-chain derivatives, particularly EPA and DHA (from omega-3s) and arachidonic acid (from omega-6), are precursors of eicosanoids — prostaglandins, thromboxanes, and leukotrienes — that regulate inflammation, platelet aggregation, and vascular tone.

Trans fatty acids, produced industrially by partial hydrogenation of vegetable oils, raise LDL cholesterol, lower HDL cholesterol, and increase cardiovascular disease risk. Most countries have now mandated their elimination from the food supply.

Vitamins

Vitamins are organic micronutrients required in trace amounts for specific biochemical reactions. They are classified by solubility, which determines their absorption, storage, and toxicity profile.

Fat-soluble vitamins (A, D, E, K) are absorbed with dietary fat via the lymphatic system, stored in the liver and adipose tissue, and require lipid-containing meals for efficient absorption. Because they accumulate in tissue, toxicity from excessive supplementation is possible.

Water-soluble vitamins (B-complex group and vitamin C) are absorbed directly into the portal circulation, are not stored in significant quantities, and are excreted renally when intake exceeds immediate needs. Regular dietary intake is therefore essential, and toxicity is rare.

Minerals

Minerals are inorganic elements required for structural, regulatory, and electrolytic functions. They are divided into macrominerals — required in amounts greater than 100 mg/day (calcium, phosphorus, magnesium, sodium, potassium, chloride, sulfur) — and trace elements — required in milligram or microgram quantities (iron, zinc, iodine, selenium, copper, manganese, fluoride, chromium, molybdenum).

Calcium is the most abundant mineral in the body, comprising approximately 99% of bone and tooth mineral content, with the remaining 1% serving as a critical intracellular and extracellular signaling ion regulating muscle contraction, nerve transmission, and blood clotting. Its absorption is regulated by vitamin D and parathyroid hormone.

Iron is central to oxygen transport as the functional component of hemoglobin and myoglobin, and to oxidative metabolism as a component of cytochrome enzymes. Heme iron from animal sources is absorbed at 15–35% efficiency; non-heme iron from plant sources is absorbed at 2–20%, with absorption enhanced by vitamin C and inhibited by phytates, tannins, and calcium.

Iodine is essential for synthesis of thyroid hormones (T3 and T4), which regulate basal metabolic rate, growth, and development. Iodine deficiency causes hypothyroidism and, in pregnancy, irreversible fetal neurological damage.

Water and Fluid Balance

Water is the most essential nutrient — survival without it is measured in days rather than weeks. It constitutes approximately 60% of adult body weight, serving as the solvent for all biochemical reactions, the medium for nutrient and waste transport, the thermoregulatory fluid through evaporative cooling, and the structural component of cells and tissues.

Total body water is distributed between the intracellular compartment (two-thirds) and the extracellular compartment (one-third, comprising plasma and interstitial fluid). Fluid balance is maintained through the integrated action of antidiuretic hormone (ADH), the renin-angiotensin-aldosterone system, atrial natriuretic peptide, and thirst — mechanisms that collectively regulate renal water and sodium excretion in response to changes in plasma osmolality and circulating volume.

Digestion and Nutrient Absorption

Digestion converts food into absorbable molecules through mechanical and enzymatic processes spanning the gastrointestinal tract.

The small intestine, with its vast absorptive surface area amplified by villi and microvilli, is the principal site of nutrient uptake. Carbohydrates are absorbed as monosaccharides, proteins as amino acids and small peptides, and fats as fatty acids and monoglycerides that are reassembled into triglycerides and packaged into chylomicrons for lymphatic transport.

Energy Balance

Energy balance is the relationship between energy intake from food and energy expenditure through metabolism and physical activity. It determines body weight over time.

Basal Metabolic Rate (BMR) accounts for 60–70% of total energy expenditure in sedentary individuals, representing the energy required to maintain vital functions at rest — circulation, respiration, thermoregulation, and cellular maintenance. It is influenced by lean body mass, age, sex, thyroid status, and genetic factors. The thermic effect of food contributes approximately 10%, representing the metabolic cost of digestion, absorption, and assimilation. Physical activity is the most variable component, ranging from 15% in sedentary individuals to 50% or more in highly active persons.

The Gut Microbiome

The human gut harbors approximately 38 trillion microbial cells — predominantly bacteria — constituting an ecosystem of extraordinary diversity and metabolic capacity. The gut microbiome digests dietary fiber through fermentation, producing short-chain fatty acids (acetate, propionate, butyrate) that nourish colonocytes, regulate intestinal motility, and exert systemic metabolic and immunological effects. The microbiome synthesizes vitamin K and several B vitamins, modulates immune system development and tone, protects against pathogenic colonization, and communicates with the brain via the gut-brain axis. Dietary composition — particularly fiber intake, fermented food consumption, and the balance of plant versus animal foods — is the primary modifiable determinant of microbiome composition and diversity.

Nutritional Epidemiology and Dietary Patterns

Nutritional epidemiology examines the relationships between dietary exposure and health outcomes at the population level. Because diet is composed of hundreds of interacting nutrients and food components consumed in complex patterns, the analysis of isolated nutrients often produces inconsistent findings. Dietary pattern analysis — evaluating overall eating patterns rather than individual nutrients — provides more robust associations with chronic disease outcomes.

The Mediterranean dietary pattern — characterized by high intakes of vegetables, legumes, whole grains, nuts, olive oil, and fish, with moderate red wine and low red meat — is the most extensively documented dietary pattern in relation to cardiovascular disease reduction, cognitive health, and longevity. The DASH diet (Dietary Approaches to Stop Hypertension) emphasizes fruits, vegetables, low-fat dairy, and reduced sodium, with strong evidence for blood pressure reduction. These patterns share an emphasis on whole, minimally processed foods, dietary fiber, unsaturated fats, and micronutrient density that is consistent across the strongest bodies of nutritional evidence.