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Overview of Nutrition


Adrienne Youdim

, MD, David Geffen School of Medicine at UCLA

Last full review/revision May 2019| Content last modified Apr 2021
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Nutrition is the science of food and its relationship to health. Nutrients are chemicals in foods that are used by the body for growth, maintenance, and energy.

Nutrients that cannot be synthesized by the body and thus must be derived from the diet are considered essential. They include

Nutrients that the body can synthesize from other compounds, although they may also be derived from the diet, are considered nonessential.

Macronutrients are required by the body in relatively large amounts; micronutrients are needed in minute amounts.


Macronutrients constitute the bulk of the diet and supply energy and many essential nutrients. Carbohydrates, proteins (including essential amino acids), fats (including essential fatty acids), macrominerals, and water are macronutrients. Carbohydrates, fats, and proteins are interchangeable as sources of energy; fats yield 9 kcal/g (37.8 kJ/g); proteins and carbohydrates yield 4 kcal/g (16.8 kJ/g).


Dietary carbohydrates are broken down into glucose and other monosaccharides. Carbohydrates increase blood glucose levels, supplying energy.

Simple carbohydrates are composed of small molecules, generally monosaccharides or disaccharides, which increase blood glucose levels rapidly.

Complex carbohydrates are composed of larger molecules, which are broken down into monosaccharides. Complex carbohydrates increase blood glucose levels more slowly but for a longer time.

Glucose and sucrose are simple carbohydrates; starches and fiber are complex carbohydrates.

The glycemic index measures how rapidly consumption of a carbohydrate increases plasma glucose levels. Values range from 1 (the slowest increase) to 100 (the fastest increase, equivalent to pure glucose—see table Glycemic Index of Some Foods Glycemic Index of Some Foods Nutrition is the science of food and its relationship to health. Nutrients are chemicals in foods that are used by the body for growth, maintenance, and energy. Nutrients that cannot be synthesized... read more ). However, the actual rate of increase also depends on what foods are consumed with the carbohydrate.


Carbohydrates with a high glycemic index may increase plasma glucose to high levels rapidly. It is hypothesized that as a result, insulin levels increase, inducing hypoglycemia and hunger, which tends to lead to consumption of excess calories and weight gain. Carbohydrates with a low glycemic index increase plasma glucose levels slowly, resulting in lower postprandial insulin levels and less hunger, which probably makes consumption of excess calories less likely. These effects are predicted to result in a more favorable lipid profile and a decreased risk of obesity Obesity Obesity is excess body weight, defined as a body mass index (BMI) of ≥ 30 kg/m2. Complications include cardiovascular disorders (particularly in people with excess abdominal fat), diabetes mellitus... read more , diabetes mellitus Diabetes Mellitus (DM) Diabetes mellitus is impaired insulin secretion and variable degrees of peripheral insulin resistance leading to hyperglycemia. Early symptoms are related to hyperglycemia and include polydipsia... read more , and complications of diabetes Complications of Diabetes Mellitus In patients with diabetes mellitus, years of poorly controlled hyperglycemia lead to multiple, primarily vascular, complications that affect small vessels (microvascular), large vessels (macrovascular)... read more Complications of Diabetes Mellitus if present.


Dietary proteins are broken down into peptides and amino acids. Proteins are required for tissue maintenance, replacement, function, and growth. However, if the body is not getting enough calories from dietary sources or tissue stores (particularly of fat), protein may be used for energy.

As the body uses dietary protein for tissue production, there is a net gain of protein (positive nitrogen balance). During catabolic states (eg, starvation, infections, burns), more protein may be used (because body tissues are broken down) than is absorbed, resulting in a net loss of protein (negative nitrogen balance). Nitrogen balance is best determined by subtracting the amount of nitrogen excreted in urine and feces from the amount of nitrogen consumed.

Of the 20 amino acids, 9 are essential amino acids (EAAs); they cannot be synthesized and must be obtained from the diet. All people require 8 EAAs; infants also require histidine.

The weight-adjusted requirement for dietary protein correlates with growth rate, which decreases from infancy until adulthood. The daily dietary protein requirement decreases from 2.2 g/kg in 3-month-old infants to 1.2 g/kg in 5-year-old children and to 0.8 g/kg in adults. Protein requirements correspond to EAA requirements (see table Essential Amino Acid Requirements Essential Amino Acid Requirements in mg/kg Body Weight Nutrition is the science of food and its relationship to health. Nutrients are chemicals in foods that are used by the body for growth, maintenance, and energy. Nutrients that cannot be synthesized... read more ). Adults trying to increase muscle mass need very little extra protein beyond the requirements in the table.

The amino acid composition of protein varies widely. Biological value (BV) reflects the similarity in amino acid composition of protein to that of animal tissues; thus, BV indicates what percentage of a dietary protein provides EAAs for the body:

  • A perfect match is egg protein, with a value of 100.

  • Animal proteins in milk and meat have a high BV (~90).

  • Proteins in cereal and vegetables have a lower BV (~40)

  • Some derived proteins (eg, gelatin) have a BV of 0.

The extent to which dietary proteins supply each other's missing amino acids (complementarity) determines the overall BV of the diet. The recommended daily allowances (RDA) for protein assumes that the average mixed diet has a BV of 70.



Fats are broken down into fatty acids and glycerol. Fats are required for tissue growth and hormone production. Saturated fatty acids, common in animal fats, tend to be solid at room temperature. Except for palm and coconut oils, fats derived from plants tend to be liquid at room temperature; these fats contain high levels of monounsaturated fatty acids or polyunsaturated fatty acids (PUFAs).

Partial hydrogenation of unsaturated fatty acids (as occurs during food manufacturing) produces trans fatty acids, which are solid or semisolid at room temperature. In the US, the main dietary source of trans fatty acids is partially hydrogenated vegetable oils, used in manufacturing certain foods (eg, cookies, crackers, chips) to prolong shelf-life. Trans fatty acids may elevate LDL cholesterol and lower HDL; they may also independently increase the risk of coronary artery disease.

  • Linoleic acid, an omega-6 (n-6) fatty acid

  • Linolenic acid, an omega-3 (n-3) fatty acid

Other omega-6 acids (eg, arachidonic acid) and other omega-3 fatty acids (eg, eicosapentaenoic acid, docosahexaenoic acid) are required by the body but can be synthesized from EFAs.

EFAs are needed for the formation of various eicosanoids (biologically active lipids), including prostaglandins, thromboxanes, prostacyclins, and leukotrienes. Consumption of omega-3 fatty acids may decrease the risk of coronary artery disease.

Requirements for EFAs vary by age. Adults require amounts of linoleic acid equal to at least 2% of total caloric needs and linolenic acid equal to at least 0.5%. Vegetable oils provide linoleic acid and linolenic acid. Oils made from safflower, sunflower, corn, soy, primrose, pumpkin, and wheat germ provide large amounts of linoleic acid. Marine fish oils and oils made from flaxseeds, pumpkin, soy, and canola provide large amounts of linolenic acid. Marine fish oils also provide some other omega-3 fatty acids in large amounts.



Water is considered a macronutrient because it is required in amounts of 1 mL/kcal (0.24 mL/kJ) of energy expended, or about 2500 mL/day. Needs vary with fever, physical activity, and changes in climate and humidity.


Water-soluble vitamins are vitamin C (ascorbic acid) and 8 members of the vitamin B complex: biotin Biotin and Pantothenic Acid Biotin acts as a coenzyme for carboxylation reactions essential to fat and carbohydrate metabolism. Adequate intake for adults is 30 mcg/day. Pantothenic acid is widely distributed in foods... read more , folate Folate Deficiency Folate deficiency is common. It may result from inadequate intake, malabsorption, or use of various drugs. Deficiency causes megaloblastic anemia (indistinguishable from that due to vitamin... read more , niacin, pantothenic acid Biotin and Pantothenic Acid Biotin acts as a coenzyme for carboxylation reactions essential to fat and carbohydrate metabolism. Adequate intake for adults is 30 mcg/day. Pantothenic acid is widely distributed in foods... read more , riboflavin Riboflavin Deficiency Riboflavin deficiency usually occurs with other B vitamin deficiencies. Symptoms and signs include sore throat, lesions of the lips and mucosa of the mouth, glossitis, conjunctivitis, seborrheic... read more (vitamin B2), thiamin Thiamin Deficiency Thiamin deficiency (causing beriberi) is most common among people subsisting on white rice or highly refined carbohydrates in developing countries and among alcoholics. Symptoms include diffuse... read more (vitamin B1), vitamin B6 Vitamin B6 Deficiency and Dependency Because vitamin B6 is present in most foods, dietary deficiency is rare. Secondary deficiency may result from various conditions. Symptoms can include peripheral neuropathy, a pellagra-like... read more (pyridoxine), and vitamin B12 Vitamin B12 Deficiency Dietary vitamin B12 deficiency usually results from inadequate absorption, but deficiency can develop in vegans who do not take vitamin supplements. Deficiency causes megaloblastic anemia, damage... read more (cobalamin).

Only vitamins A, E, and B12 are stored to any significant extent in the body; the other vitamins must be consumed regularly to maintain tissue health.

Essential trace minerals include chromium Chromium Deficiency Only 1 to 3% of biologically active trivalent chromium (Cr) is absorbed. Normal plasma levels are 0.05 to 0.50 mcg/L (1.0 to 9.6 nmol/L). However, it is not clear whether chromium should be... read more , copper Copper Deficiency Copper is a component of many body proteins; almost all of the body’s copper is bound to copper proteins. Copper deficiency may be acquired or inherited. (See also Overview of Mineral Deficiency... read more , iodine Iodine Deficiency In the body, iodine (I) is involved primarily in the synthesis of 2 thyroid hormones, thyroxine (T4) and triiodothyronine (T3). Iodine occurs in the environment and in the diet primarily as... read more , iron Iron Deficiency Iron (Fe) is a component of hemoglobin, myoglobin, and many enzymes in the body. Heme iron is contained mainly in animal products. It is absorbed much better than nonheme iron (eg, in plants... read more , manganese Manganese Deficiency Manganese (Mn), necessary for healthy bone structure, is a component of several enzyme systems, including manganese-specific glycosyltransferases and phosphoenolpyruvate carboxykinase. Median... read more , molybdenum Molybdenum Deficiency Molybdenum (Mo) is a component of coenzymes necessary for the activity of xanthine oxidase, sulfite oxidase, and aldehyde oxidase. Genetic and nutritional deficiencies of molybdenum have been... read more , selenium Selenium Deficiency Selenium (Se) is a part of the enzyme glutathione peroxidase, which metabolizes hydroperoxides formed from polyunsaturated fatty acids. Selenium is also a part of the enzymes that deiodinate... read more , and zinc Zinc Deficiency Zinc (Zn) is contained mainly in bones, teeth, hair, skin, liver, muscle, leukocytes, and testes. Zinc is a component of several hundred enzymes, including many nicotinamide adenine dinucleotide... read more Zinc Deficiency . Except for chromium, each of these is incorporated into enzymes or hormones required in metabolism. Except for deficiencies of iron and zinc, micromineral deficiencies are uncommon in developed countries.

Other minerals (eg, aluminum, arsenic, boron, cobalt, fluoride, nickel, silicon, vanadium) have not been proved essential for people. Fluoride Fluorine Deficiency Most of the body’s fluorine (F) is contained in bones and teeth. Fluoride (the ionic form of fluorine) is widely distributed in nature. The main source of fluoride is fluoridated drinking water... read more , although not essential, helps prevent tooth decay by forming a compound with calcium (calcium fluoride [CaF2]), which stabilizes the mineral matrix in teeth.

All trace minerals are toxic at high levels, and some (arsenic, nickel, and chromium) may cause cancer.

Other Dietary Substances

The daily human diet typically contains as many as 100,000 chemicals (eg, coffee contains 1000). Of these, only 300 are nutrients, only some of which are essential. However, many nonnutrients in foods are useful. For example, food additives (eg, preservatives, emulsifiers, antioxidants, stabilizers) improve the production and stability of foods. Trace components (eg, spices, flavors, odors, colors, phytochemicals, many other natural products) improve appearance and taste.


Fiber occurs in various forms (eg, cellulose, hemicellulose, pectin, gums). It increases gastrointestinal motility, prevents constipation Constipation Constipation is difficult or infrequent passage of stool, hardness of stool, or a feeling of incomplete evacuation. (See also Constipation in Children.) No bodily function is more variable and... read more , and helps control diverticular disease Definition of Diverticular Disease Diverticula are saclike mucosal pouches that protrude from a tubular structure. True diverticula of the gastrointestinal (GI) tract contain all layers of the GI wall. Esophageal diverticula... read more . Fiber is thought to accelerate the elimination of cancer-causing substances produced by bacteria in the large intestine. Epidemiologic evidence suggests an association between colon cancer Colorectal Cancer Colorectal cancer is extremely common. Symptoms include blood in the stool and change in bowel habits. Screening using one of several methods is recommended for appropriate populations. Diagnosis... read more Colorectal Cancer and low fiber intake and a beneficial effect of fiber in patients with functional bowel disorders, Crohn disease Crohn Disease Crohn disease is a chronic transmural inflammatory bowel disease that usually affects the distal ileum and colon but may occur in any part of the gastrointestinal tract. Symptoms include diarrhea... read more Crohn Disease , obesity Obesity Obesity is excess body weight, defined as a body mass index (BMI) of ≥ 30 kg/m2. Complications include cardiovascular disorders (particularly in people with excess abdominal fat), diabetes mellitus... read more , or hemorrhoids Hemorrhoids Hemorrhoids are dilated vessels of the hemorrhoidal plexus in the anal canal. Symptoms include irritation and bleeding. Thrombosed hemorrhoids are usually painful. Diagnosis is by inspection... read more Hemorrhoids . Soluble fiber (present in fruits, vegetables, oats, barley, and legumes) reduces the postprandial increase in blood glucose and insulin and can reduce cholesterol levels.

The typical Western diet is low in fiber (about 12 g/day) because of a high intake of highly refined wheat flour and a low intake of fruits and vegetables. Increasing fiber intake to about 30 g/day by consuming more vegetables, fruits, and high-fiber cereals and grains is generally recommended. However, very high fiber intake may reduce absorption of certain minerals.

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