Principles of nutrition

Last updated: April 1, 2022

Summarytoggle arrow icon

Nutrition is the intake and metabolization of substances the body requires to grow and maintain life. These substances are referred to as “nutrients,” and they are typically ingested orally as food, although specialized nutrition support via enteral feeding or parenteral nutrition may be necessary in patients incapable of eating (e.g., in coma patients and those with severe dysphagia). Nutrients can be divided into essential nutrients, which cannot be synthesized by the body and, therefore, require intake with food (e.g., vitamins, minerals), and nonessential nutrients, which can be synthesized by the body in adequate amounts but are nonetheless a vital part of a healthy diet (e.g., carbohydrates, proteins, and fats). Dietary fiber represents a separate class of nutrients, as it provides little to no actual nutrition but nonetheless has a significant impact on health. Nutrients can be further divided into macronutrients, which humans require in relatively large amounts (fats, carbohydrates, protein), and micronutrients (vitamins and minerals), which humans require in relatively small amounts. Nutrients provide energy; the building blocks for structural growth, maintenance, and repair; and support metabolism, enable the synthesis of endogenous nutrients, and facilitate vital chemical reactions in the body. The amount of energy a body requires depends on metabolic rate, thermogenesis, physical activity, and physical composition. In order to generate energy, the body converts macronutrients into adenosine triphosphate (ATP) through aerobic, anaerobic, protein, and ketone body metabolism. When nutrition provides excess energy, the body stores the excess energy as fat and glycogen; these stores are depleted during times of energy deficiency. Certain states and disorders can lead to nutritional deficiencies (e.g., malignancy), excesses (e.g., metabolic syndrome), or changes in nutritional demand (e.g., pregnancy, high level of activity), which require nutritional adjustment or supplementation. Certain elective diets such as a vegetarian or vegan diet can provide health benefits if balanced nutrition is ensured, while others, especially fad diets focused on quick weight loss or such based on pseudoscientific principles, may not provide balanced nutrition with potentially detrimental health effects. Nutritional status is of central clinical importance, as it can greatly influence disease outcomes and provide valuable information on risk factors, especially with regard to obesity and associated conditions. Nutritional status is assessed based on presentation, history, BMI, and waist circumference.

For further information and discussion of nutritional topics not covered here, see the articles on “Carbohydrates,” “Lipids and their metabolism,” “Amino acids,” “Proteins and peptides,” “Vitamins,” “General metabolism,” “Nutrition during pregnancy,” “Infant nutrition and weaning,” “Specialized nutrition support,” “Protein-energy malnutrition,” and/or ”Water metabolism.”

Nutrientstoggle arrow icon

Essential and nonessential nutrients [1]

Essential nutrients Nonessential nutrients
  • Nutrients that the body cannot synthesize on its own in sufficient amounts and must, therefore, be taken in with food.
  • Nutrients that the body can synthesize on its own in sufficient amounts.

Macronutrients [1][2]

  • Definition: nutrients that the body requires in relatively large amounts to ensure proper function, esp. carbohydrates, fats, and protein
Overview of macronutrients
Carbohydrates Proteins Fats
Digestible carbohydrates

Dietary fiber [1][3]

  • Biomolecules that are soluble in nonpolar solvents (e.g., ethanol) but insoluble in polar solvents (e.g., water)
    • Unsaturated lipids: a lipid with one or more double bonds in the fatty acid chain
    • Saturated lipids: a lipid with only single bonds in the fatty acid chain
  • Glucose (e.g., candy, soft drinks)
  • Sucrose (e.g., sugar refined from sugar cane or sugar beet)
  • Lactose (e.g., dairy products)
  • Fructose (e.g., fruit, honey)
  • Starch (e.g., potatoes, bread, rice, pasta, cereals)
  • Grains (e.g. rye, oats)
  • Vegetables (e.g., carrots, beets, artichokes)
  • Legumes (e.g., peas, beans)
  • Fruit (e.g., pears, raspberries, apples)
  • Animal products (e.g., lean meat, poultry, eggs, seafood)
  • Legumes (e.g., peas, beans)
  • Nuts and seeds
  • Mostly in the form of triglycerides
    • Animal products (e.g., meat, dairy products)
    • Nuts (e.g., macadamia nuts, walnuts)
    • Oils (e.g., olive oil, sunflower oil)
Recommended dietary allowances
  • 25–30 g/day
  • Amount of all nutrients
    • Age 1–3 years: 5–20%
    • Age 4–18 years: 10–30%
    • Age > 18 years: 10–35%
  • Amount of all nutrients
    • Age 1–3 years: 30–40%
    • Age 4–18 years: 15–25%
    • Age > 18 years: 20–35%
Caloric value
  • 4 kcal/g
  • None
  • 4 kcal/g
  • 9 kcal/g
  • Primary source of energy
  • Components of cell structures
  • Bulking: Absorption of water in the intestine increases stool size and regularity.
  • Viscosity: Dissolution in water forms a gel that increases stool motility and reduces sugar and lipid absorption.
  • Fermentation: Microbiota in the large intestine consume fiber, thereby promoting a healthy intestinal flora.
  • Contribute to the regulation of physiological cell activity (e.g., as hormones, enzymes, transporters, antibodies)
  • Components of cell structures
  • N/A

Relevant articles

  • Discussed here



Minerals [1]

Energytoggle arrow icon

  • Energy
    • The chemical, thermal, mechanical, and electrical power that drives and sustains all physical functions. In a biological system, energy is ingested in the form of chemical energy stored in food and translated into other forms of energy that are either stored or used to perform and maintain physical functions.
    • Energy is measured in kilocalories (kcal) or kilojoules (kJ)
  • Energy balance: the balance of energy intake, generation, and expenditure
    • Positive energy balance: more energy intake than expenditure → energy storage → weight gain
    • Negative energy balance: less energy intake than expenditure → energy store depletion → weight loss

Energy generationtoggle arrow icon

The body's energy metabolism is founded on converting nutrients to ATP, which then provides the energy necessary for all cellular processes. The synthesis of ATP is typically classified by the type of metabolic processes based on oxygen demand and triggering activity. For further information, see “Electron transport chain and oxidative phosphorylation.”

ATP synthesis [6]
Type of metabolism Starting product Characteristics Triggering activity Pathway
Aerobic metabolism [7]
  • Requires O2
  • Predominant pathway of energy metabolism
  • In every cell
  • Low-impact activity (e.g., walking)
  1. Glycolysis and/or β-oxidation
  2. TCA cycle and oxidative phosphorylation via the electron transport chain
Anaerobic metabolism [7]
  • Glucose
  • Prolonged high-impact
  1. Anaerobic glycolysis
  2. Lactic acid cycle
  • Short bursts of high-impact activity
Protein metabolism [8]

Ketone body metabolism [9]

  • Main energy source when glucose is not readily available.
  • Are constantly produced in small amounts by the liver.
  • Not stored

Proteins and ketones are only used during catabolic states.

Energy expendituretoggle arrow icon

Energy expenditure is measured in energy unit per time, e.g., kcal/day or J/day.

Total energy expenditure (TEE) [10]

  • Definition: the total amount of energy the body requires to maintain all metabolic processes
  • Composition [11]

Metabolic rate

  • Definition
    • The rate of energy consumed to perform physical functions, measured in unit time, e.g., kJ/day or kcal/day
    • The metabolic rate is affected by genes, age, sex, race, diet, exercise, and disease (e.g., hyperthyroidism, sepsis).
  • Basal metabolic rate (BMR)
    • The amount of energy required to maintain basic life-sustaining function at rest in a temperate environment during digestive inactivity
    • Typically measured in the morning, after an overnight fast and 24 hrs of no exercise [12]
  • Resting metabolic rate (RMR) [12]
    • The amount of energy required to maintain basic life-sustaining function at rest in a temperate environment during digestive activity
    • Typically measured during the day after 12 hrs of no exercise
Overview of metabolic states
Catabolism Anabolism
  • The metabolic breakdown of complex molecules into simpler ones
  • Releases energy
  • The synthesis of simple molecules into more complex ones
  • Requires energy
Signal hormones
State of body
  • Nonfasting
  • During rest
  • After physical activity

Energy storagetoggle arrow icon

Vegetarian and vegan diettoggle arrow icon

Definitions [13]

  • Vegetarian diets
    • Lacto-vegetarian diet: a plant-based diet that includes dairy products but not eggs, meat, or seafood
    • Lacto-ovo vegetarian diet: a plant-based diet that includes eggs and dairy products without the consumption of meat, or seafood
    • Pescetarian (pesco-vegetarian) diet: a plant-based diet that includes seafood, eggs, and dairy products but not meat
  • Vegan diets: a plant-based diet that excludes all animal products (e.g., meat, fish, dairy, eggs, honey) [14]
    • Raw vegan diet: a vegan diet in which 75–100% of the food is consumed raw
    • Fruitarian diet: a vegan diet that includes raw fruits but no vegetables or grains; some fruitarians eat also seeds and nuts
  • Prevalence

Positive health effects [16][17]

Associated deficiencies [16][17][18]

  • Vitamin B12 deficiency
    • Animal sources: meat, poultry, eggs, dairy products, fish
    • Plant-based diet: supplemental vitamin B12 capsules or injections
  • Iron deficiency [19]
    • Animal source: red meat, pork, poultry
    • Plant-based diet
      • Dark green leafy vegetables (e.g. spinach, rocket), dried fruits (e.g., raisins, apricots, peaches, and prunes)
      • Supplemental iron capsules
  • Calcium deficiency: associated with low bone mineral density
    • Animal source: milk and dairy products
    • Vegan diet
      • Dark green leafy vegetables (e.g., kale, broccoli, bok choy, turnip greens, dried figs, Chinese cabbage)
      • Calcium-fortified food (e.g., soy products, oat milk, mineral water)
      • Supplemental calcium
      • Avoid products with high oxalate concentration (e.g., spinach, arugula, yams, beet greens, and Swiss chard), as they decrease calcium absorption.
  • Omega-3 fatty acid deficiency: associated with the development of several psychiatric disorders (e.g., depressive disorders, bipolar disorder, schizophrenia, and ADHD)
    • Animal source: fish, organ meat, and eggs
    • Plant-based diet
      • Walnuts, soy products, flaxseed, chia seeds, hemp seeds, sea vegetables
      • Supplemental omega-3 fatty acids capsules and/or oil
  • Vitamin D deficiency
    • Animal source: egg yolks, animal liver
    • Plant bases diet: supplemental vitamin D capsules
  • Protein deficiency (see also “Kwashiorkor”)
    • Animal sources: meat, poultry, eggs, dairy products, fish
    • Plant-based diet: legumes, grains, vegetables, seeds, nuts, soy products

Enteral feeding and parenteral nutritiontoggle arrow icon

Specialized nutrition support is required when oral intake is either limited or not possible for a prolonged period of time. For further information see “Specialized nutrition support.”

Enteral feeding Parenteral nutrition

Diagnostic tools for assessing healthy weight and body masstoggle arrow icon

Body mass index [20]

Definition: the ratio of a person's mass (weight) to their height used to classify individuals by weight class as underweight, healthy, overweight, and obese (expressed in units of kg/m2)

Body mass index
Underweight < 18.5 kg/m2
Healthy weight 18.5–24.9 kg/m2
Overweight 25–29.9 kg/m2
Obesity Class 1 30–34.9 kg/m2
Class 2 35–39.9 kg/m2
Class 3 ≥ 40 kg/m2

Waist circumference [21][22]

Definition: the circumference of the waist measured in centimeters or inches just above the hip bones

Waist circumference
Healthy weight Overweight Obesity
Women < 80 cm or < 31.5 in 80–88 cm or 31.5–35 in > 88 cm or > 35 in
Men < 94 cm or < 37 in 94–102 cm or 37–40 in > 102 cm or > 40 in

Clinical significancetoggle arrow icon






Trace elements

Weight changes

Referencestoggle arrow icon

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  2. Heydenreich J, Kayser B, Schutz Y, Melzer K. Total Energy Expenditure, Energy Intake, and Body Composition in Endurance Athletes Across the Training Season: A Systematic Review. Sports Medicine - Open. 2017; 3 (1).doi: 10.1186/s40798-017-0076-1 . | Open in Read by QxMD
  3. McMurray RG, Soares J, Caspersen CJ, McCurdy T. Examining Variations of Resting Metabolic Rate of Adults. Medicine & Science in Sports & Exercise. 2014; 46 (7): p.1352-1358.doi: 10.1249/mss.0000000000000232 . | Open in Read by QxMD
  4. Morris AL, Mohiuddin SS. Biochemistry, Nutrients. StatPearls. 2021.
  5. Venn BJ. Macronutrients and Human Health for the 21st Century. Nutrients. 2020; 12 (8): p.2363.doi: 10.3390/nu12082363 . | Open in Read by QxMD
  6. Lin D, Peters BA, Friedlander C, et al. Association of dietary fibre intake and gut microbiota in adults. Br J Nutr. 2018; 120 (9): p.1014-1022.doi: 10.1017/s0007114518002465 . | Open in Read by QxMD
  7. Cockell KA. Mineral Nutrients. Springer Berlin Heidelberg ; 2017: p. 2318-2322
  8. Koubová E, Sumczynski D, Šenkárová L, Orsavová J, Fišera M. Dietary Intakes of Minerals, Essential and Toxic Trace Elements for Adults from Eragrostis tef L.: A Nutritional Assessment.. Nutrients. 2018; 10 (4).doi: 10.3390/nu10040479 . | Open in Read by QxMD
  9. Cornier M-A, Després J-P, Davis N, et al. Assessing Adiposity. Circulation. 2011; 124 (18): p.1996-2019.doi: 10.1161/cir.0b013e318233bc6a . | Open in Read by QxMD
  10. Ross R, Neeland IJ, Yamashita S, et al. Waist circumference as a vital sign in clinical practice: a Consensus Statement from the IAS and ICCR Working Group on Visceral Obesity. Nature Reviews Endocrinology. 2020; 16 (3): p.177-189.doi: 10.1038/s41574-019-0310-7 . | Open in Read by QxMD
  11. Organization WH. Waist Circumference and Waist-hip Ratio. World Health Organization ; 2008
  12. Feher JJ. Quantitative Human Physiology. Academic Press ; 2017
  13. Simpson RJ. Immunosenescence. Springer Berlin Heidelberg ; 2016: p. 444-447
  14. Gurina TS, Mohiuddin SS. Biochemistry, Protein Catabolism. StatPearls. 2021.
  15. Cantrell CB, Mohiuddin SS. Biochemistry, Ketone Metabolism. StatPearls. 2021.
  16. Pawlak R. Vegetarian Diets in the Prevention and Management of Diabetes and Its Complications. Diabetes Spectrum. 2017; 30 (2): p.82-88.doi: 10.2337/ds16-0057 . | Open in Read by QxMD
  17. Appleby PN, Key TJ. The long-term health of vegetarians and vegans. Proc Nutr Soc. 2015; 75 (3): p.287-293.doi: 10.1017/s0029665115004334 . | Open in Read by QxMD
  18. Paslakis G, Richardson C, Nöhre M, et al. Prevalence and psychopathology of vegetarians and vegans – Results from a representative survey in Germany. Scientific Reports. 2020; 10 (1).doi: 10.1038/s41598-020-63910-y . | Open in Read by QxMD
  19. Craig WJ. Nutrition Concerns and Health Effects of Vegetarian Diets. Nutrition in Clinical Practice. 2010; 25 (6): p.613-620.doi: 10.1177/0884533610385707 . | Open in Read by QxMD
  20. Craig WJ. Health effects of vegan diets. Am J Clin Nutr. 2009; 89 (5): p.1627S-1633S.doi: 10.3945/ajcn.2009.26736n . | Open in Read by QxMD
  21. Vegetarian diets. Updated: September 1, 2017. Accessed: January 28, 2022.
  22. West AR, Oates PS. Mechanisms of heme iron absorption: Current questions and controversies. World Journal of Gastroenterology. 2008; 14 (26): p.4101.doi: 10.3748/wjg.14.4101 . | Open in Read by QxMD

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