NUTRIENTS

NUTRIENTS. Nutrients are those organic and inorganic compounds that a living organism must acquire from the environment to support essential life processes, including basal metabolism, growth and maintenance of body tissues, activity, reproduction, and maintenance of general health. Nutrients are normally obtained by the ingestion of foods. Organic nutrients include carbohydrates, proteins or amino acids, lipids, and vitamins. Inorganic nutrients include minerals. Water is sometimes included in a listing of nutrients.

Classification of Nutrients

Nutrients often are classified as essential or nonessential. Essential nutrients are those that cannot be synthesized in the body at all or in sufficient amounts to meet needs and, thus, must be obtained preformed in the diet. These include the essential (indispensable) amino acids, the essential fatty acids, the vitamins, and the minerals. Two amino acids are classified as semi-essential because, although they can be synthesized in sufficient quantities in the body, their synthesis depends upon a supply of an essential amino acid. Other nutrients are considered conditionally essential, meaning that they are not normally required by a healthy adult but may be required in certain disease states or at certain stages of life because of increased demand or impaired synthesis. Nonessential nutrients include those that are oxidized as fuels and those that provide carbon skeletons and amino groups for endogenous synthesis of body constituents. The term "dispensable" is sometimes used to describe these nutrients, as the nutrients are not truly nonessential: an adequate amount of carbohydrate, protein, and fat must be taken in to supply the substrates required for maintenance of blood glucose, as fuel for oxidative metabolism and synthesis of ATP, and as substrate for synthesis of body components. They are "nonessential" only in the sense that carbohydrate, fat, or protein, as well as ethanol, can be used as fuels; in that either carbohydrate or protein or even the glycerol backbone of triacylglycerols (fat) can be a source of glucose; in that any fuel potentially can be used for synthesis of most lipids; and in that amino groups from most amino acids can be used for synthesis of indispensable amino acids. Also, some food components that have health benefits and are considered important parts of healthy diets, such as fiber and phytochemicals, are not required and are not considered nutrients per se.

The following table summarizes the nutrient classes, the essential compounds in each class, and the basic functions of these nutrients in the body.

Additional information about some of these nutrients can be found below. Additional information for the other nutrients can be found under separate entries in this volume.

Niacin

The term "niacin" is used to refer to either nicotinic acid (pyridine-3-carboxylic acid) or nicotinamide (pyridine-3-carboxamide). Niacin is widely distributed in foods of both plant and animal origin. Good sources of niacin include meats, poultry, fish, legumes, peanuts, some cereals (mainly in the bran), and enriched or whole grain products. Much of the niacin in cereals is not readily available because it is esterified to complex carbohydrates or peptides.

The amino acid tryptophan also is an important precursor for synthesis of pyridine nucleotide coenzymes (see below). The estimated conversion factor for adults is 60 mg of tryptophan to 1 mg of niacin. The term "niacin equivalent" (NE) is used for expression of niacin intakes and requirements, with either 1 mg of nicotinic acid, 1 mg of nicotinamide, or 60 mg of tryptophan equal to 1 NE.

The adult recommended daily (or dietary) allowance (RDA) for NEs is 14 mg per day for females and 16 mg per day for males (Institute of Medicine, 1998). Most mixed diets in the United States provide more than 5 mg of preformed niacin. However, for individuals consuming typical Western diets, most NEs are derived from tryptophan rather than from preformed niacin. The tryptophan content of proteins ranges from about 0.6 percent for corn to 1.5 percent for animal products. Assuming that the average tryptophan content of protein is about 1 percent, a diet for adults that contains 100 g or more of protein provides about 16 mg NEs and would by itself meet the RDA for niacin. One should note that food composition tables do not take into account the bioavailability of niacin (from plant foods) and do not include an estimate of the NE available from tryptophan in the food. The adult male RDA for NEs would be supplied by ¼ cup peanut butter, 3½ slices roast beef, 4½ cups green peas, or 15 slices enriched wheat bread.

Nicotinic acid and nicotinamide are actively absorbed from the small intestine as well as from the renal filtrate. Niacin metabolites are excreted in the urine. Defects in tryptophan absorption or reabsorption from the renal filtrate have been associated with cases of niacin deficiency (pellagra).

Niacin is essential for the formation of the pyridine nucleotide coenzymes, nicotinamide adenine dinucleotide (NAD), and nicotinamide adenine dinucleotide phosphate (NADP). Reduced forms of these coenzymes are indicated as NADH and NADPH. NAD and NADP function in oxidation-reduction reactions that are involved in the catabolism of glucose, fatty acids, ketone bodies, and amino acids. These coenzymes ultimately funnel electrons to electron-to-oxygen transfer systems, including the mitochondrial electron transport chain. These coenzymes also are essential for reductive biosynthetic reactions. In addition, NAD has a non-coenzyme function: NAD serves as the donor of adenosine diphosphate-ribose moieties for ADP-ribosylation reactions. Poly-ADP-ribosylated proteins appear to function in DNA repair, DNA replication, and cell differentiation.

Symptoms of pellagra, or niacin deficiency, include functional changes in the gastrointestinal tract and nonspecific lesions of the central nervous system. Early symptoms include weakness, lassitude, anorexia, and indigestion. Later symptoms include various gastrointestinal and mental symptoms and a bilaterally symmetrical dermatitis that affects parts of the body exposed to sunlight, heat, or mild trauma. Pharmaceutical doses of nicotinic acid cause vasodilation, and long-term use can cause gastrointestinal irritation and possibly liver damage. The tolerable upper intake level (UL) set by the Institute of Medicine (1997) is 35 mg of niacin per day for adults.

Riboflavin

Riboflavin is the common name for 7,8-dimethyl-10-(1′-D-ribityl)isoalloxazine, which also is known as vitamin B₂. Much of the riboflavin in the American diet is supplied by dairy products. Meats, especially organ meats,

eggs, and vegetables such as broccoli, spinach, and mushrooms are also good sources. Enriched flour and enriched breakfast cereals also contribute significantly to riboflavin intakes. The RDA for riboflavin is 1.3 mg for men and 1.1 mg for women (Institute of Medicine, 1998). Some amounts of common foods that would need to be consumed to supply 1.3 mg of riboflavin (assuming they were the sole dietary source of this vitamin) are 3 cups milk, 1¼ pounds beef round, 8 large eggs, 4⅓ cups broccoli, or 65 slices whole wheat bread. Daily intakes of riboflavin in the United States average about 1.5 to 2 mg for adults (Institute of Medicine, 1998).

Following ingestion, flavin coenzymes are released from noncovalent attachment to proteins by gastric acidification and subsequent proteolysis. Nonspecific pyrophosphatases and phosphatases act on coenzyme forms to release riboflavin. Covalently bound flavin coenzymes make up about 5 percent to 10 percent of the riboflavin naturally occurring in foods, and the 8α-(amino acid)-riboflavins obtained from their digestion cannot by used for resynthesis of coenzymes. Free riboflavin is actively taken up from the small intestine. Riboflavin and small amounts of riboflavin catabolites are excreted in urine.

Riboflavin is required for synthesis of flavin mono-nucleotide (FMN), which is riboflavin 5′-phosphate, and flavin-adenine dinucleotide (FAD). Fully reduced forms of these coenzymes are indicated by FMNH₂ and FADH₂. Riboflavin coenzymes are involved in oxidationreduction reactions in which the ring portion of the coenzyme undergoes sequential addition or loss of hydrogens and electrons. Flavoproteins function in either one-or two-electron transfer reactions.

The flavin coenyzmes, FAD and FMN, function indispensably in oxidation-reduction reactions involved in the catabolism of glucose, fatty acids, ketone bodies, and amino acids, as well as in energy production via the respiratory chain and in reductive biosynthetic reactions.

Inadequate dietary intake of riboflavin can result in stunting of growth, a variety of lesions involving the skin and the epithelium of the gastrointestinal tract, anemia, and neuropathy. Riboflavin has a low toxicity, perhaps because of its low solubility or ready excretion in the urine. No tolerable upper intake level has been established because of a lack of suitable data.

Thiamin

Thiamin, also known as vitamin B₁, is 3-(2-methyl-4-aminopyrimidinyl)methyl-4-methyl-5-(β-hydroxyethyl)thiazole. Excellent sources of thiamin include unrefined cereal germs and whole grains, meats (especially pork), nuts, and legumes. Enriched flours and grain products in the United States contain thiamin, as well as niacin, riboflavin, iron, and folic acid.

The RDAs for thiamin are 1.2 mg of thiamin for men and 1.1 mg for women (Institute of Medicine, 1998). Typical intakes of thiamin in the United States average 1.2 to 2.0 mg per day for adults (Institute of Medicine, 1998). The recommended 1.2 mg of thiamin per day is provided by a 3½-ounce pork chop, 20 slices of whole wheat bread, 1⅔ cups of pecan halves, or 17 ounces of roasted peanuts.

Thiamin is released from its phosphate ester forms in which it is found in most natural foods by the action of pyrophosphatases and phosphatases in the small intestine. Free thiamin is absorbed by an active transport process that is probably carrier mediated. Trapping of thiamin as thiamin pyrophosphate in the mucosal cells appears to facilitate the uptake by metabolic trapping. Excess thiamin is excreted in the urine as various metabolites.

Raw fish may contain microbial thiaminases, which hydrolyze and, thus, destroy thiamin in the gastrointestinal tract. Certain thiamin antagonists that are found in coffee, tea, rice bran, and heme-containing animal products can impair thiamin uptake or utilization. Chronic alcoholism results in impaired thiamin absorption, which may be secondary to a folate deficiency. Thiamin requirements also appear to be elevated in individuals with high caloric intakes, especially when calories are derived primarily from carbohydrates, in renal patients undergoing long-term dialysis, in patients fed intravenously for long periods, and in patients with chronic febrile infections.

Thiamin is required for synthesis of thiamin pyrophosphate (TPP), which is also known as thiamin diphosphate (TDP); this may be the sole coenzyme form of thiamin. However, monophosphate and triphosphate esters occur naturally, and thiamin triphosphate has been implicated in nerve function. TPP functions in two general types of reactions in which TPP functions as a Mg²⁺-coordinated coenzyme for "active aldehyde transfers." First, TTP is a coenzyme for the oxidative decarboxylation of α-keto acids (catalyzed by the pyruvate, ketoglutarate, and branched-chain keto acid dehydrogenase complexes). Second, TPP is required as a coenzyme for transketolase, which catalyzes sugar rearrangements in the pentose phosphate pathway of glucose metabolism.

Thiamin deficiency, or beriberi, affects the nervous and cardiovascular systems. Clinical symptoms include mental confusion, anorexia, muscular weakness, ataxia, peripheral paralysis, paralysis of the motor nerves of the eye, edema, muscle wasting, tachycardia, and an enlarged heart. In Western countries, symptomatic thiamin deficiency is usually observed only in association with alcoholism.

No toxic effects of thiamin administered by mouth have been reported in humans, and thiamin is readily cleared by the kidneys. Injection of doses of thiamin that are more than 200 times those required for optimal nutrition produces a variety of pharmacological effects and can even induce death because of depression of the respiratory center. No tolerable upper intake level has been established for thiamin because of a lack of sufficient data.

Vitamin B₁₂

Vitamin B₁₂, or cobalamin, consists of a central cobalt atom coordinately linked to the four pyrrole nitrogens of a heme-like planar corrin ring structure. The 5^th coordinate bond of cobalt is to one of the nitrogens in a phosphoribo-5,6-dimethylbenzimidazolyl side group of the corrin ring structure, and the 6^th coordinate bond of cobalt can be occupied by a number of ligands. In vitamin B₁₂ preparations, this ligand is typically a cyano group that is formed by trace amounts of cyanide during purification of the vitamin from natural sources.

Vitamin B₁₂ is synthesized by some anaerobic microorganisms and by some algae, such as seaweed. Most plants and higher organisms do not use vitamin B₁₂ as a coenzyme, and they do not synthesize it. Vitamin B₁₂ is found in meat, dairy products, some seafoods, and in fortified cereals. A strictly vegetarian diet contains low levels of vitamin B₁₂, most of which come from algal sources or possibly microbial contamination associated with plant roots.

The RDA for vitamin B₁₂ is 2.4 micrograms for adults (Institute of Medicine, 1998). This amount of vitamin B₁₂ can be obtained from 1/10 ounce of beef liver, 1 egg, or 2⅔ ounces of canned tuna. Typical intake of vitamin B₁₂. in the United States averages 3.3 to 5.6 micrograms per day for adults (Institute of Medicine, 1998).

Absorption of vitamin B₁₂ is a complex process. Vitamin B₁₂ in food must be released from proteins to which it is naturally bound; this is accomplished in the stomach by the acid environment and by proteolysis of proteins by pepsin. The vitamin B₁₂ then binds to other proteins that have affinity for vitamin B₁₂, but these binding proteins are hydrolyzed by pancreatic proteases in the small intestine. The free vitamin B₁₂ then binds to an intrinsic factor, which is a high-affinity vitamin B₁₂-binding protein secreted by the gastric glands. The vitamin B₁₂-intrinsic factor complex binds to receptors located near the end of the small intestine, and the complex is taken up by endocytosis. The intrinsic factor is degraded by lysosomal enzymes, and free vitamin B₁₂ is released into the cytosol of the mucosal cells. The vitamin B₁₂ is released from the intestinal mucosal cells into the plasma as a complex with another protein, transcobalamin II. The transcobalamin II-B₁₂ complex is transported into tissues by receptor-mediated endocytosis; the complex is degraded in the lysosome, and the free vitamin B₁₂ is transported out of the lysosome into the cytosol.

Vitamin B₁₂ is excreted from the body in the urine. It is also secreted in the bile, but vitamin B₁₂ secreted in the bile is normally reabsorbed via the enterohepatic circulation. Vitamin B₁₂ is needed for synthesis of two coenzymes: methylcobalamin, which is a cofactor for cytosolic methionine synthase, and 5′-deoxyadenosylcobalamin, which is a cofactor for mitochondrial methylmalonyl CoA mutase.

Vitamin B₁₂ deficiency seldom is caused by a dietary lack of the vitamin and most commonly is because of a defect in vitamin B₁₂ absorption. Malabsorption of vitamin B₁₂ can result from a lack of intrinsic factor secretion, decreased gastric acid production, or pancreatic enzyme insufficiency. Food vitamin B₁₂ is malabsorbed by many elderly individuals, and it is recommended that adults older than 50 years ingest adequate vitamin B₁₂ from supplements or fortified foods. Symptoms of vitamin B₁₂ deficiency include megaloblastic anemia and a severe, and often irreversible, neurological disease called subacute combined degeneration.

No toxicity of vitamin B₁₂ has been reported. Absorption is limited by the amount of intrinsic factor secreted. No tolerable upper intake level has been established for vitamin B₁₂ because of lack of suitable data.

Vitamin B₆

Vitamin B₆ refers to several 4-substituted 2-methyl-3-hydroxyl-5-hydroxymethylpyridine compounds, which include pyridoxal, pyridoxine, pyridoxamine, and their respective 5′-phosphate derivatives. Good sources of vitamin B₆ include cereals, meat, especially organ meats, poultry, fish, starchy vegetables, and noncitrus fruits and juices.

The RDA for vitamin B₆ is 1.3 mg for adults (Institute of Medicine, 1998). The median intake of vitamin B₆ from food sources (i.e., not including supplements) is about 2 mg for men and about 1.5 mg for women. Amounts of some foods that would by themselves supply the daily RDA for vitamin B₆ include 1⅓ whole chicken breasts, 2 bananas, 1⅓ cups of oatmeal, 12 cups of milk, or 22 large eggs.

Phosphate derivatives of vitamin B ₆ are hydrolyzed by phosphatase prior to uptake from the small intestine. Some plants contain pyridoxine as a glucoside derivative; these normally are deconjugated by a mucosal glucosidase before the pyridoxine is absorbed. Vitamin B₆ in a mixed diet is about 75 percent bioavailable, whereas the vitamin B₆ in supplements is about 90 percent bioavailable. Vitamin B₆ is absorbed by a nonsaturable passive diffusion mechanism with metabolic trapping of the vitamers by formation of the phosphate derivatives. Excess vitamin B₆ is excreted in the urine. The major excretory form of vitamin B₆ is the 4-carboxylate derivative 4-pyridoxic acid, but unmetabolized vitamin also is excreted and may be the major excretory form when very high doses of vitamin B₆ are ingested.

Vitamin B₆ is used to form pyridoxal phosphate (PLP), this vitamin's major coenzyme form. PLP binds to proteins and PLP-dependent enzymes via Schiff base formation with the ε-amino group of specific lysyl residues in the proteins. PLP serves as a coenzyme for many enzymes involved in amino acid metabolism, including aminotransferases, decarboxylases, aldolases, racemases, and dehydratases. Aminotransferase reactions convert the coenzyme between the PLP and pyridoxamine phosphate forms.

Vitamin B₆ deficiency can result in seborrheic dermatitis, microcytic anemia (because of decreased hemoglobin synthesis), convulsions, depression, and confusion. Low vitamin B₆, folate, or vitamin B₁₂ intakes can lead to an elevated plasma homocysteine level. Alcoholics tend to have low vitamin B₆ status.

Some subjects taking very large pharmaceutical doses of pyridoxine have developed severe sensory neuropathy. There is some evidence for toxicity at daily doses of 500 mg or more, and a safe upper level of intake is thought to be 100 mg/day. The tolerable upper intake level set for vitamin B₆ by the Institute of Medicine (1998) is 100 mg/day for adults.

Pantothenic Acid

Pantothenic acid, also known as vitamin B₅, consists of β-alanine moiety condensed with pantoic acid. Pantothenic acid is distributed widely in plant and animal sources. Meat (especially liver), fish, poultry, milk, yogurt, legumes, and whole-grain cereals are good sources of pantothenic acid. Pantothenic acid is present in foods in the free form and in various bound forms, including coenzyme A, coenzyme A esters, acyl carrier protein, and glucosides.

The Adequate Intake established for pantothenic acid by the Institute of Medicine (1998) is 5 mg per day for adults. This amount of pantothenic acid can be obtained by eating 2½ cups of peanuts, 6 eggs, 3 whole chicken breasts, 6½ cups of milk, or 19 slices of whole wheat bread. The average dietary intake of pantothenic acid in the United States is about 5 to 6 mg, with somewhat lower average intakes in the elderly and young children.

Dietary coenzyme A, coenzyme A esters, and acyl carrier protein are degraded enzymatically in the small intestine to release free pantothenic acid. Pantothenic acid is taken up by active transport. Approximately 50 percent of dietary pantothenic acid is available. Pantothenic acid is excreted unchanged in the urine. The kidneys regulate excretion of pantothenic acid, secreting it when plasma concentrations are high and largely reabsorbing it when plasma concentrations are in the physiological range.

Cells use pantothenic acid to synthesize coenzyme A, which consists of pantothenate linked to cysteamine by a peptide bond and to a 3′-phospho-ADP moiety via a phosphoester linkage. Coenzyme A contains a reactive sulfhydryl group that is involved in the formation of thioesters with fatty acids and other carboxylic acids. Coenzyme A plays a major role in fatty acid metabolism and in the final oxidative steps in the catabolism of all fuels. Much of the metabolism of fatty acids and certain amino acid derivatives, as well as a numerous amphibolic steps in metabolism, use coenzyme A thioester substrates and produce coenzyme A thioester products. Coenzyme A also is used for the synthesis of the acyl carrier protein domain of fatty acid synthase, a multifunctional enzyme that catalyzes palmitate synthesis. Coenzyme A is involved in oxidative decarboxylation reactions catalyzed by α-keto acid dehydrogenase complexes, β-oxidation of fatty acids, ketone body synthesis, fatty acid and triacyl-glycerol synthesis, amino acid and organic acid catabolism, and in synthesis of isoprenoids, cholesterol, and steroids.

A naturally occurring deficiency of pantothenic acid has not been documented reliably and is undoubtedly rare because of the wide distribution of pantothenic acid in foods. Pantothenic acid deficiency has been produced experimentally in a small number of volunteers via a pantothenic acid-free diet; these volunteers appeared listless and complained of fatigue after nine weeks on the pantothenic acid-free diet. A "burning feet" syndrome that was observed among prisoners of war and among malnourished individuals in Asia may have been because of pantothenic acid deficiency, as symptoms appeared to be reduced by pantothenic acid.

Pantothenic acid is relatively nontoxic. Doses below 10 g of pantothenic acid per day do not seem to be associated with any toxic symptoms. No tolerable upper intake limit was set by the Institute of Medicine (1998) because of insufficient data.

Biotin

Biotin contains a ureido group attached to a tetrahydrothiophene ring and has a valeric acid side chain extending from the tetrahydrothiophene ring. Biotin is synthesized by bacteria, yeast, algae, and some plant species. Biotin is distributed widely in foods, existing both as free biotin and as biotin covalently bound to lysyl residues in biotinyl-proteins. Liver, whole-grain cereals, nuts, legumes, yeast, and egg yolks are relatively high in biotin. Biotin is synthesized by microflora in the large intestine, but biotin produced at that site appears to be excreted mainly in the feces.

The Adequate Intake for biotin, as set by the Institute of Medicine (1998), is 30 micrograms per day for adults. About 3 ounces roasted peanuts, 3 medium eggs, 5 cups of milk, ⅓ cup of peanut butter, or 1⅓ cups of oatmeal will provide 30 micrograms of biotin. The daily intake of biotin in Western countries is estimated to be about 60 micrograms per day.

Digestion of proteins releases biotinyl-lysine (biocytin) and small lysine-containing peptides with biotin attached covalently. These are hydrolyzed to release free biotin by a specific hydrolase called biotinidase that is present in the pancreatic digestive secretions. Free biotin is transported into the mucosal cells of the small intestine by a carrier-mediated, sodium-dependent process.

Biotin is excreted as such and as several degradation products. Degradation products include bisnorbiotin, in which the 5-carbon valerate side chain has been shortened by two carbons, and biotin sulfoxide, in which the thiophene ring sulfur has been oxidized to a sulfoxide.

The only known function of biotin in humans and other mammals is as a prosthetic group for four carboxylases: pyruvate carboxylase, acetyl CoA carboxylase, propionyl CoA carboxylase, and 3-methylcrotonyl CoA carboxylase (in the pathway of leucine catabolism). Holocarboxylase synthetase attaches biotin to the apocarboxylases in an ATP-requiring reaction; the biotin is attached by an amide bond to an ε-amino group of a specific lysyl residue in the enzyme protein. In the holocarboxylases, biotin serves as a CO₂ carrier and carboxyl donor to substrates.

A dietary deficiency of biotin is very rare because of the wide distribution of biotin in foods. Biotin deficiency with clinical symptoms of hair loss, dermatitis, and neurological symptoms has occurred in individuals consuming an abnormal diet that is low in biotin and high in raw egg white. Raw egg white contains avidin, a protein that binds biotin with a very high affinity and prevents its uptake from the intestine. Biotin deficiency may occur in individuals who routinely take certain anticonvulsants or in individuals with severe protein-energy malnutrition. 3-Hydroxyisovalerate is elevated in the urine of biotin-deficient subjects.

Intakes of biotin up to 10 mg per day have not been reported to be associated with toxicity. No tolerable upper intake level has been set for biotin because of lack of data. Inborn errors of biotin metabolism, biotinidase deficiency and holocarboxylase synthetase deficiency, can both be treated with pharmacological doses of biotin.

Vitamin C

Ascorbic acid or vitamin C is a 6-carbon lactone synthesized from glucose by plants and many animals. Humans, as well as nonhuman primates and several other species, are unable to synthesize ascorbic acid because of a lack of gulonolactone oxidase, the terminal enzyme in the biosynthetic pathway.

Ascorbic acid is found in many fruits and vegetables. Some dietary vitamin C is present as an oxidized form, dehydroascorbic acid. Cantaloupe, kiwi, oranges, lemons, strawberries, and watermelon are especially high in vitamin C. Vegetables that are rich sources of vitamin C include broccoli, red peppers, cauliflower, brussels sprouts, asparagus, potatoes, cabbage, spinach, collard greens, green peas, and carrots. Citrus juices and tomato juice are good sources of vitamin C. Many foods, such as fruit drinks and breakfast cereals, are fortified with vitamin C.

The current RDA for vitamin C is 75 mg for women and 90 mg for men (Institute of Medicine, 2000). An additional 35 mg per day is recommended for smokers. The adult female RDA is contained in ¾ cup orange juice, 1 orange, 1 kiwi, ⅓ cantaloupe, 1 small sweet pepper, 2 cups broccoli, or 3 baked potatoes. Typical intake of vitamin C by adults is 70 to 100 micrograms per day.

Ascorbic acid absorption probably occurs by a Na⁺-dependent system in the intestine. Bioavailability is close to 100 percent for vitamin C at doses between 15 and 200 mg but declines at higher doses. Ascorbic acid and its metabolites are excreted mainly in the urine.

Ascorbic acid acts as an electron donor, or reducing agent. Two electrons are lost, probably sequentially, with formation of semidehydroascorbic acid (free radical) and dehydroascorbic acid. Dehydroascorbic acid can be enzymatically or nonenzymatically reduced back to ascorbate or hydrolyzed irreversibly to 2,3-diketogulonic acid, which is converted to other products, including oxalate.

Vitamin C acts as an electron donor for eight mammalian enzymes: three dioxygenases that are involved in collagen hydroxylation (prolyl 4-hydroxylase, prolyl 3-hydroxylase, and lysyl hydroxylase), two dioxygenases that are involved in carnitine synthesis (6-N-trimethyl-L-lysine hydroxylase and γ-butyrobetaine hydroxylase), 4-hydroxyphenylpyruvate dioxygenase, dopamine β-hydroxylase, and peptidylglycine α-amidating monooxygenase. Ascorbic acid may also function in nonenzymatic reduction reactions and thus acts as a water-soluble antioxidant.

An early sign of vitamin C deficiency is fatigue. With more severe deficiency, petechial hemorrhage, coiled hairs, ecchymoses, bleeding and tenderness of the gums, hyperkeratosis, joint pain, and shortness of breath may occur.

Vitamin C is relatively nontoxic. Excess vitamin C may promote the formation of oxalate kidney stones. The tolerable upper intake level, or maximum intake level likely to pose no risk of adverse health effects in most individuals, was set at 2000 mg ascorbic acid per day from food plus supplements (Institute of Medicine, 1998).

Vitamin K

Vitamin K refers to a group of compounds that are 2-methyl-1,4-napthoquinones with a hydrophobic substituent at the 3-position. Phylloquinone (vitamin K₁), which is synthesized by plants, has a 20-carbon phytyl substituent at the 3-position of the napthoquinone ring. Menaquinones (vitamin K₂) are synthesized by bacteria and have an unsaturated side chain, made up of four to thirteen isoprenyl units, instead of the saturated phytyl chain present in phylloquinone. Animal tissues contain both phylloquinone and menaquinones. In addition to these naturally occurring compounds with vitamin K activity, a synthetic form of vitamin K called menadione can be alkylated to an active form in the liver and is used in animal feeds. Human vitamin K supplements are phylloquinone.

Green vegetables are the major dietary source of phylloquinone: kale, spinach, broccoli, brussels sprouts, cabbage, and lettuce are rich sources. Some vegetable oils, especially soybean oil and rapeseed (canola) oil, are good sources. Menaquinones, which are obtained especially from liver, provide only a minor portion of the vitamin K needed to meet the requirement. The nutritional significance of menaquinones synthesized by bacteria in the lower bowel is uncertain.

The RDA for vitamin K for adults who are age twenty-five years and older is 65 and 80 micrograms for women and men, respectively (National Research Council, 1989). The RDA for males is provided by ⅓ ounce of spinach or kale, ⅓ cup of broccoli, ⅔ cup of shredded cabbage, or 2¼ ounces of lettuce. Typical intake of vitamin K by adults is 70 to 100 micrograms per day.

Absorption of dietary vitamin K depends upon adequate lipid absorption. Vitamin K is incorporated into chylomicrons, along with other lipids, and ultimately is taken up by the liver as part of the chylomicron remnants. Vitamin K is stored in liver; the hepatic phylloquinone pool turns over more rapidly than that of menaquinones. Vitamin K is excreted predominantly as metabolites and glucuronides; these are excreted primarily in feces via the bile, but significant amounts are also excreted in urine.

The hydroquinone form of vitamin K is required for the posttranslational modification (γ-glutamylcarboxylation) of a group of proteins (referred to as Gla proteins or vitamin K-dependent proteins) during their synthesis. Vitamin K serves as substrate, or coenzyme, for an enzyme that converts targeted glutamyl residues to carboxyglutamyl (Gla) residues in these proteins. This posttranslational modification of glutamyl residues is essential for the normal physiological function of vitamin K-dependent proteins. Continued function of vitamin K in γ-glutamylcarboxylation reactions is dependent upon the recycling of oxidized vitamin K (vitamin K epoxide) back to the hydroquinone form (vitamin KH₂).

Vitamin K-dependent proteins include four plasma clotting proteins (prothrombin, factor VII, Factor IX, and factor X), two plasma proteins involved in thrombin-initiated inactivation of factor V (protein C and protein S), plasma protein Z of uncertain function, and two bone proteins (osteocalcin, or bone Gla protein, and matrix Gla protein). At physiological pH, both carboxyl groups of each Gla residue are negatively charged, and these anionic residues are involved in the association of Gla proteins with Ca²⁺.

Primary vitamin K deficiency is rare. Vitamin K-responsive hemorrhagic disease of the newborn can occur because of low vitamin K stores in the liver of the newborn and the low vitamin K content of human milk, along with other factors. In developed countries, commercial infant formulas are supplemented routinely with phylloquinone, and the practice of oral or intramuscular administration of phylloquinone to the newborn is almost universal. Vitamin K deficiency also has been reported in adults with low intakes of vitamin K who are receiving antibiotics and in patients subjected to long-term total parenteral nutrition without vitamin K supplementation. Vitamin K status should be of concern in disorders of lipid digestion or absorption and in persons treated with anticoagulant drugs that act by blocking reduction of oxidized vitamin K.

Toxic manifestations from ingestion of large amounts of vitamin K have not been reported. Menadione administration to infants has been associated with hemolytic anemia and liver toxicity, and should not be used for human supplements.

Vitamin E

Vitamin E is the term used for all tocopherols and tocotrienols and their derivatives that exhibit vitamin E activity. Tocopherols are more important sources of vitamin E. Both the tocopherols and tocotrienols consist of a chromanol head and a phytyl tail. The side chain of tocopherols is saturated, whereas that of tocotrienols contains double bonds at the 3′, 7′, and 11′ positions. Four tocopherols and four tocotrienols occur naturally; they differ in the number and position of the methyl groups on the chromanol ring. The naturally occurring isomer of α-tocopherol is the 2′R, 4′R, 8′R isomer, whereas synthetic tocopherols are mixtures of all eight possible stereoisomers.

Tocopherols in foods exist primarily as the free or unesterified forms. Ester forms (e.g., α-tocopheryl acetate or α-tocopheryl succinate) are less susceptible to oxidation and are used for food fortification and for supplements. The 6-hydroxyl group on the phenolic ring is the site for esterification of fatty acids.

A variety of naturally occurring RRR-α-tocopherols and tocotrienols are supplied by foods. Tocopherols differ in their antioxidant and biological activities. Currently, the biological activity of various forms of vitamin E are expressed as units of activity in relation to that of all-rac -α-tocopheryl acetate, which is a common pharmaceutical or synthetic form of vitamin E. The unit used to express vitamin E activity is the α-tocopherol equivalent (α-TE) with 1 equivalent equal to 1.49 mg of all-rac -α-tocopheryl acetate or 1.0 mg of RRR-α-tocopherol. The majority of the tocopherols consumed in the diet are not α-tocopherol, and γ-tocopherol accounts for more than half the estimated total tocopherol intake. Rich sources of vitamin E include vegetable oils, vegetable shortenings, margarines, mayonnaise, salad dressings, wheat germ, rice bran, nuts, seeds, peanut butter, eggs, potato chips, whole milk, and tomato products.

The RDA for vitamin E is 15 mg α-TEs for adults. This amount of vitamin E could be provided by 4 teaspoons of soybean oil, ⅔ cup of margarine, 2 cups of whole milk, 4½ cups of green peas, or 2 pounds of salmon. The average intake of vitamin E from American diets is 11 to 13 mg α-TEs daily in adults not taking vitamin E supplements.

Tocopheryl esters are hydrolyzed to free tocopherol in the small intestinal lumen, presumably by pancreatic esterases. Vitamin E is absorbed with other lipids, and the majority of the vitamin E is incorporated into chylomicrons in the mucosal cells of the small intestine. The chylomicrons are secreted into the lymph and then enter the circulation. Vitamin E is taken up by the liver in the chylomicron remnants and is then either stored in the parenchymal cells of the liver, incorporated into nascent very low density lipoproteins (VLDL) that are secreted into the blood stream, or excreted via the bile. Both vitamin E and its metabolites are primarily excreted in the feces via biliary secretion from the liver. Some metabolites are excreted in the urine.

Vitamin E is the major lipid-soluble, chain-breaking antioxidant found in plasma, red cells, and tissues, and it plays an essential role in maintaining the integrity of biological membranes. Among the biological functions proposed for vitamin E, the reaction of α-tocopherol with lipid peroxyl radicals to prevent uncontrolled free radical-initiated lipid peroxidaion is the best understood. Whether other tocopherols have other roles is uncertain.

Patients with familial isolated vitamin E deficiency have clear signs of vitamin E deficiency (extremely low plasma vitamin E levels and neurological abnormalities—spinocerebellar dysfunction with progressive ataxia) but do not have fat malabsorption or lipoprotein abnormalities. Absence of hepatic α-tocopherol transfer protein impairs secretion of α-tocopherol into hepatic lipoproteins (VLDL) and appears to be responsible for the low plasma vitamin E status of patients with familial isolated vitamin E deficiency and the low delivery of vitamin E to tissues. In humans, low plasma levels of vitamin E are associated with shorter lifespans of red blood cells because of their increased susceptibility to hemolysis. Vitamin E deficiency is rarely associated with lipid malabsorption syndromes or lipoprotein abnormalities. Neurological symptoms occur in individuals with malabsorption syndromes as well as in individuals with familial isolated vitamin E deficiency.

Vitamin E is relatively nontoxic when taken by mouth. The upper tolerable intake level set by the Institute of Medicine (2000) is 1000 mg of α-TEs per day from vitamin E supplements in addition to dietary intake. Consumption of more than this increases risk of hemorrhagic damage because vitamin E can act as an anticoagulant.

Potassium

Potassium (K⁺) is distributed widely in the body and is the principal cation in intracellular fluids. Like sodium and chloride ions, potassium ions exist as free hydrated ions that bind only weakly to organic molecules. Potassium functions in the maintenance of electrolytic and osmotic balances or gradients. The distribution of potassium between the intracellular and extracellular fluids is the result of ion pumps and of the permeability characteristics of cell membranes. The Na⁺, K⁺-ATPase pump, which moves 3 Na⁺ out of the cell in exchange for 2 K⁺ that are moved into the cell, is of particular importance.

Potassium is widely distributed in foods, especially in fruits and vegetables. Rich sources of potassium include fruits such as avocado, banana, cantaloupe, orange juice, and watermelon; vegetables such as lima beans, potatoes, tomatoes, spinach, and winter squash; and fresh meats.

Obligatory losses of potassium, which must be replaced, average about 800 mg of potassium per day. The estimated minimum requirement for potassium established by the National Research Council (1989) is 2 g per day for adults. Two grams of potassium are provided by 4 cups of fresh orange juice, 5½ small bananas, 5 medium potatoes, or 9/10 pound of beef chuck. Typical Western diets provide about 3 g of potassium per day.

Over 90 percent of the potassium in the diet is absorbed from the gut into the circulation. However, although nearly all of the dietary K⁺ is absorbed in the small intestine, there is normally some net secretion of K⁺ in the colon that results in loss of potassium in the feces. Absorption of dietary K⁺ causes a rise in the concentration of K⁺ in the plasma, and this immediately stimulates physiological mechanisms to promote rapid entry of K⁺ into cells so that a rapid rise in the plasma K⁺ concentration is prevented. Uptake of K⁺ by cells is essential in preventing life-threatening hyperkalemia. Nevertheless, in the long-term, to maintain K⁺ balance, the excess K⁺ from the diet must be excreted by the kidneys.

At typical potassium intakes, renal tubular secretion of K⁺ is required to maintain potassium balance. Renal secretion of K⁺ is under the control of various homeostatic regulatory mechanisms. The most important hormone regulating secretion of K⁺ is aldosterone, the release of which is triggered by a high concentration of K⁺ in plasma (or a low concentration of Na⁺ or by angiotensin II). When potassium intake is high, secretion of K⁺ by the colon as well as the kidney is increased to eliminate the excess potassium.

During potassium depletion, the kidney reabsorbs most of the filtered K⁺, and essentially no K⁺ is secreted. The small amount of K⁺ excreted in the urine under these circumstances comes from the filtered K⁺ that escaped reabsorption.

The high concentration gradient of K⁺ between the intracellular fluid and the extracellular fluid is important for generation and maintenance of the normal resting membrane potentials across cell membranes and for excitability of nerves and muscles. Higher intakes of potassium may have beneficial effects in preventing hypertension.

Dietary deficiency of potassium does not occur under normal circumstances. Large losses can occur, by either gastrointestinal or renal routes, in cases of prolonged vomiting, chronic diarrhea, use of diuretic agents, some forms of chronic renal disease, and in some metabolic disturbances such as metabolic acidosis. Hypokalemia causes membrane hyperpolarization, and this can interfere with the normal functioning of nerves and muscles, resulting in muscle weakness and decreased smooth muscle contractility. Deficiency symptoms include weakness, anorexia, nausea, drowsiness, and irrational behavior.

Acute hyperkalemia can result from sudden enteral or parenteral increases in potassium intake to amounts of about 18 g per day for an adult. Hyperkalemia causes membrane depolarization, causing muscular weakness, flaccid paralysis, and cardiac arrhythmias. Severe hyperkalemia can cause cardiac arrest and death.

Chloride

Chloride is the principal inorganic anion in the extracellular fluids of the body. Dietary chloride comes almost entirely from sodium chloride, and a small amount comes from potassium chloride. Thus, table salt and foods or beverages that contain NaCl added during food processing or preparation are the major sources of chloride in the diet. The amount of chloride contributed by water is low compared to that contributed by salt.

The estimated minimum requirement for chloride is 750 mg/day for adults, which corresponds to about 1.3 g of sodium chloride per ¼ teaspoon of table salt). Typical salt intake in the United States is higher than this. It is recommended that daily salt intake should not exceed 6 g because of the association of high intake with hypertension.

Loss of fluids through the skin, feces, and urine cause loss of both sodium and chloride. Chloride movement tends to parallel that of sodium, and loss of sodium usually is accompanied by a similar molar loss of chloride. Thus, conditions that cause loss of sodium (e.g., heavy losses through sweating, chronic diarrhea or vomiting, trauma, or renal disease) also cause loss of chloride and can result in hypochloremic metabolic alkalosis.

Chloride is essential for maintenance of fluid and electrolyte balance. Hydrochloric acid is an essential component of the gastric juice secreted by the stomach.

Deficiency of chloride does not occur under normal circumstances. Toxicity from excess intake of chloride is not known to occur, but water-deficiency dehydration can cause hyperchloremia.

See also Choline, Inositol, and Related Nutrients; Gene Expression, Nutrient Regulation of; Immune System Regulation and Nutrients; Malnutrition; Malnutrition: Protein-Energy Malnutrition; Nutrition; Vitamin C; Vitamins; Appendix.

BIBLIOGRAPHY

Chow, Ching K. "Vitamin E." In Biochemical and Physiological Aspects of Human Nutrition, edited by Martha H. Stipanuk. Philadelphia: Saunders, 2000.

Church, Charles F., and Helen N. Church. Food Values of Portions Commonly Used—Bowes and Church, 11th ed. Philadelphia: Lippincott, 1970.

Institute of Medicine. Dietary Reference Intakes for Thiamin, Riboflavin, Niacin, Vitamin B ₆, Folate, Vitamin B ₁₂, Pantothenic Acid, Biotin, and Choline. Washington, D.C.: National Academy Press, 1998.

Institute of Medicine. Dietary Reference Intakes for Vitamin C, Vitamin E, Selenium, and Carotenoids. Washington, D.C.: National Academy Press, 2000.

Levine, Mark, et al. "Vitamin C." In Biochemical and Physiological Aspects of Human Nutrition, edited by Martha H. Stipanuk. Philadelphia: Saunders, 2000.

Mahan, L. Kathleen, and Sylvia Escott-Stump. Krause's Food, Nutrition, and Diet Therapy, 9th ed. Philadelphia: Saunders, 1996.

McCormick, Donald B. "Niacin, Riboflavin, and Thiamin." In Biochemical and Physiological Aspects of Human Nutrition, edited by Martha H. Stipanuk. Philadelphia: Saunders, 2000.

National Research Council. Recommended Dietary Allowances, 10th ed. Washington, D.C.: National Academy Press, 1989.

Shane, Barry. "Folic Acid, Vitamin B₁₂, and Vitamin B₆." In Biochemical and Physiological Aspects of Human Nutrition, edited by Martha H. Stipanuk. Philadelphia: Saunders, 2000.

Sheng, Hwai-Ping. "Sodium, Chloride, and Potassium." In Biochemical and Physiological Aspects of Human Nutrition, edited by Martha H. Stipanuk. Philadelphia: Saunders, 2000.

Suttie, John W. "Vitamin K." In Biochemical and Physiological Aspects of Human Nutrition, edited by Martha H. Stipanuk. Philadelphia: Saunders, 2000.

Sweetman, Lawrence. "Pantothenic Acid and Biotin." In Biochemical and Physiological Aspects of Human Nutrition, edited by Martha H. Stipanuk. Philadelphia: Saunders, 2000.

Martha H. Stipanuk

Nutrients

An important aspect of nutrition is the daily intake of nutrients . Nutrients consist of various chemical substances in the food that makes up each person's diet . Many nutrients are essential for life, and an adequate amount of nutrients in the diet is necessary for providing energy , building and maintaining

body organs, and for various metabolic processes. People depend on nutrients in their diet because the human body is not able to produce many of these nutrients—or it cannot produce them in adequate amounts.

Nutrients are essential to the human diet if they meet two characteristics. First, omitting the nutrient from the diet leads to a nutritional deficiency and a decline in some aspect of health. Second, if the omitted nutrient is put back into the diet, the symptoms of nutritional deficiency will decline and the individual will return to normal, barring any permanent damage caused by its absence.

There are six major classes of nutrients found in food: carbohydrates , proteins , lipids (fats and oils), vitamins (both fat-soluble and water-soluble ), minerals , and water. These six nutrients can be further categorized into three basic functional groups.

Carbohydrates

Carbohydrates are the major source of energy for the body. They are composed mostly of the elements carbon (C), hydrogen (H), and oxygen (O). Through the bonding of these elements, carbohydrates provide energy for the body in the form of kilocalories (kcal), with an average of 4 kcal per gram (kcal/g) of carbohydrates (a kcal is equivalent to a calorie on a nutritional label of a packaged food).

Carbohydrates come in a variety of sizes. The smallest carbohydrates are the simple sugars, also known as monosaccharides and disaccharides, meaning that they are made up of one or two sugar molecules . The best known simple sugar is table sugar, which is also known as sucrose , a disaccharide. Other simple sugars include the monosaccharides glucose and fructose, which are found in fruits, and the disaccharides, which include sucrose, lactose (found in milk), and maltose (in beer and malt liquors). The larger carbohydrates are made up of these smaller simple sugars and are known as polysaccharides (many sugar molecules) or complex carbohydrates. These are usually made up of many linked glucose molecules, though, unlike simple sugars, they do not have a sweet taste. Examples of foods high in complex carbohydrates include potatoes, beans, and vegetables. Another type of complex carbohydrate is dietary fiber . However, although fiber is a complex carbohydrate made up of linked sugar molecules, the body cannot break apart the sugar linkages and, unlike other complex carbohydrates, it passes through the body with minimal changes.

Although carbohydrates are not considered to be an essential nutrient, the body depends on them as its primary energy source. The body utilizes most carbohydrates to generate glucose, which serves as the basic functional molecule of energy within the cells of the human body (glucose is broken down to ultimately produce adenosine triphosphate, or ATP, the fundamental unit of energy). When the supply of carbohydrates is too low to adequately supply all the energy needs of the body, amino acids from proteins are converted to glucose. However, the typical American individual consumes more than adequate amounts of carbohydrates to prevent this utilization of protein.

Proteins

Proteins are composed of the elements carbon (C), oxygen (O), hydrogen (H), and nitrogen (n). They have a variety of uses in the body, including serving as a source of energy, as substrates (starter materials) for tissue growth and maintenance, and for certain biological functions, such as making structural proteins, transfer proteins, enzyme molecules, and hormone receptors. Proteins are also the major component in bone, muscle, and other tissues and fluids. When used for energy, protein supplies an average of 4 kcal/g.

Proteins are formed by the linking of different combinations of the twenty common amino acids found in food. Of these, ten are essential for the human in the synthesis of body proteins (eight are essential throughout a human's life, whereas two become essential during periods of rapid growth, such as during infancy).

Protein may be found in a variety of food sources. Proteins from animal sources (meat, poultry, milk, fish) are considered to be of high biological value because they contain all of the essential amino acids. Proteins from plant sources (wheat, corn, rice, and beans) are considered to be of low biological value because an individual plant source does not contain all of the essential amino acids. Therefore, combinations of plant sources must be used to provide these nutrients.

Protein deficiency is not common in the American diet because most Americans consume 1.5 to 2 times more protein than is required for the body to maintain adequate health. This excess intake of protein is not considered to be harmful for the average healthy individual. However, when protein intake is inadequate, but total caloric intake is sufficient, a condition known as kwashiorkor may occur. Symptoms of kwashiorkor include an enlarged stomach, loss of hair and hair color, and an enlarged liver. Conversely, if protein and caloric intake are both inadequate, a condition known as marasmus occurs. Marasmus presents with a stoppage of growth, extreme muscle loss, and weakness.

Lipids

Lipids, which consist of fats and oils, are high-energy yielding molecules composed mostly of carbon (C), hydrogen (H), and oxygen (O) (though lipids have a smaller number of oxygen molecules than carbohydrates have). This small number of oxygen molecules makes lipids insoluble in water, but soluble in certain organic solvents. The basic structure of lipids is a glycerol molecule consisting of three carbons, each attached to a fatty-acid chain. Collectively, this structure is known as a triglyceride , or sometimes it is called a triacylglycerol. Triglycerides are the major form of energy storage in the body (whereas carbohydrates are the body's major energy source), and are also the major form of fat in foods. The energy contained in a gram of lipids is more than twice the amount in carbohydrates and protein, with an average of 9 kcal/g.

Lipids can be broken down into two types, saturated and unsaturated, based on the chemical structure of their longest, and therefore dominant, fatty acid. Whether a lipid is solid or liquid at room temperature largely depends on its property of being saturated or unsaturated. Lipids from plant sources are largely unsaturated, and therefore liquid at room temperature. Lipids that are derived from animals contain a higher amount of saturated fats, and they are therefore solid at room temperature. An exception to this rule is fish, which, for the most part, contain unsaturated fat. The important difference between saturated and unsaturated fatty acids is that saturated fatty acids are the most important factor that can increase a person's cholesterol level. An increased cholesterol level may eventually result in the clogging of blood arteries and, ultimately, heart disease .

Not all fatty acids are considered harmful. In fact, certain unsaturated fatty acids are considered essential nutrients. Like the essential amino acids, these fatty acids are essential to a person's diet because the body cannot produce them. The essential fatty acids serve many important functions in the body, including regulating blood pressure and helping to synthesize and repair vital cell parts. It is estimated that the American diet contains about three times the amount of essential fatty acids needed daily. Lipids are also required for the absorption of fat-soluble vitamins, and they are generally thought to increase the taste and flavor of foods and to give an individual a feeling of fullness.

Vitamins

Vitamins are chemical compounds that are required for normal growth and metabolism . Some vitamins are essential for a number of metabolic reactions that result in the release of energy from carbohydrates, fats, and proteins. There are thirteen vitamins, which may be divided into two groups: the four fat-soluble vitamins (vitamins A, D, E, and K) and the nine water-soluble vitamins (the B vitamins and vitamin C). These two groups are dissimilar in many ways. First of all, cooking or heating destroys the water-soluble vitamins much more readily than the fat-soluble vitamins. On the other hand, fat-soluble vitamins are much less readily excreted from the body, compared to water-soluble vitamins, and can therefore accumulate to excessive, and possibly toxic, levels. This means, of course, that levels of water-soluble vitamins in the body can become depleted more quickly, leading to a vitamin deficiency if those nutrients are not replaced regularly. Deficiencies of vitamins may result from inadequate intake, as well as from factors unrelated to supply. For instance, vitamin K and biotin are both produced by bacteria that live within the intestines , and a person can become deficient if these bacteria are removed by antibiotics . Other factors that may result in a vitamin deficiency include disease, pregnancy, drug interactions, and newborn development (newborns lack the intestinal bacteria that create certain vitamins, such as biotin and vitamin K).

Minerals

Minerals are different from the other nutrients discussed thus far, in that they are inorganic compounds (carbohydrates, proteins, lipids, and vitamins are all organic compounds). The fundamental structure of minerals is usually nothing more than a molecule, or molecules, of an element. The functions of minerals do not include participation in the yielding of energy. But they do play vital roles in several physiological functions, including critical involvement in nervous system functioning, in cellular reactions, in water balance in the body, and in structural systems, such as the skeletal system.

Because minerals have a very simple structure of usually one or more molecules of an element, they are not readily destroyed in the heating or cooking process of food preparation. However, they can leak out of the food substance that contains them and seep into the water or liquid the food is being cooked in. This may result in a decreased level of minerals being consumed if the liquid is discarded.

There are many minerals found within the human body, but of the sixteen (or possibly more) essential minerals, the amount required on a daily basis varies enormously. This is why minerals are subdivided into two classes: macrominerals and microminerals. Macrominerals include those that are needed in high quantities, ranging from milligrams to grams. Calcium , phosphorous, and magnesium are macrominerals. Microminerals are those necessary in smaller quantities, generally between a microgram and a milligram. Examples of microminerals include copper, chromium, and selenium. Dietary requirements for some minerals have yet to be established.

Water

Water makes up the last class of nutrients, though the fact that it is considered a nutrient is surprising to many people. Water, however, has many necessary functions in the human body. Some of its actions include its use as a solvent (a substance that other substances dissolve in), as a lubricant, as a conduction system for transportation of vital nutrients and unnecessary waste, and as a mode of temperature regulation.

There are many available sources of water other than tap water and bottled water. Some foods have a high water content, including many fruits and vegetables. In addition, the body can make small amounts of water from various metabolic prcesses that result in molecules of water as a by-product. This, however, is by no means sufficient for the body's needs of water. It is generally recommended that people drink eight cups (or nearly 2 liters) of water a day to maintain an adequate supply.

see also Carbohydrates; Fats; Kwashiorkor; Marasmus; Minerals; Nutritional Deficiency; Protein; Vitamins, Fat-Soluble; Vitamins, Water-Soluble; Water.

Susan S. KimJeffrey Radecki

Bibliography

Harper, A. (1999). "Defining the Essentiality of Nutrients." In Modern Nutrition in Health and Disease, 9th edition, ed. M. E. Shills, et al. Baltimore, MD: Williams and Wilkins.

Morrison, Gail, and Hark, Lisa (1999). Medical Nutrition and Disease, 2nd edition. Cambridge, MA: Blackwell Science.

Subar, A. F., et al. (1998). "Dietary Sources of Nutrients in the U.S. Diet, 1989 to 1991." Journal of the American Dietetic Association 98:537.

Wardlaw, Gordon M., and Kessel, Margaret (2002). Perspectives in Nutrition, 5th edition. Boston: McGraw-Hill.

Nutrients

Of the ninety-two naturally occurring elements, only about twenty are indispensable or essential for the growth of plants. Plants, however, absorb many more mineral elements than that from the soil in which they grow. Which of these elements are the essential ones? The best way to answer that is to withhold the element in question from the plants. If then the plants grow poorly or die while plants supplied with the element thrive, the element has been shown to be essential.

Such an experiment cannot be done with soil-grown plants. Soils contain most of the elements in the periodic table of elements. No element can be removed from soil so thoroughly as to deprive plants of that element; the chemical means for doing that would destroy the soil.

Therefore scientists devised a simplified method for growing plants, called solution culture, or hydroponics. In this technique the roots of the plants are not in soil but in water, which contains the dissolved salts of those elements considered to be essential. That way, scientists can control and monitor the chemical composition of the medium in which the plants grow.

Failure of the plants in such an experiment suggests that some essential element is missing, and by trial and error scientists then determine which element cures the deficiency. By this method most of the elements known now to be essential have been identified. Those elements needed in relatively large amounts are called macronutrients; those needed in only small or very small amounts are micronutrients.

By the latter half of the nineteenth century, all the macronutrient mineral elements (see accompanying table) and one micronutrient, iron, had been identified. But throughout the twentieth century additional elements were shown to be micronutrients. It took so long to identify them because early on the water and the nutrient salts used for supplying the macronutrient elements contained substantial impurities, some of which were micronutrients. Investigators therefore supplied, without knowing it, several micronutrients

to their experimental plants. Once this was understood, plant biologists developed ever more refined methods for purifying water and nutrient salts and, little by little, several additional elements were shown to be essential.

When determining the chemical composition of plants, plant nutritionists usually dry the plant first, keeping it at about 70°C (158°F) for forty-eight hours. Fresh plant material is mostly water (H₂ O) so that its dry weight is only around 10 to 20 percent of the initial fresh weight. Carbon and oxygen each make up about 45 percent of the dry matter, and hydrogen 6 percent. These elements can be removed by careful digestion. The inorganic nutrients together make up only about 4 percent of dry plant matter and are left in the digest.

Essential Elements

The table above lists the elements known to be essential to plants, in addition to carbon, oxygen, and hydrogen, and also includes a quantitative indication of their prevalence in plant tissues. For the macronutrient elements, these values are expressed as percent of the dry matter, and for the micronutrients, as micrograms per gram dry matter, or parts per million. The reason for giving a range of values rather than a single one for each element is that these values differ considerably, depending on the kind of plant, the soil in which it grows, and other factors. Three of these elements, sodium, silicon, and cobalt, cannot unequivocally be called nutrients, as explained below.

Living plants use up much water in transpiration . Water is also their main constituent. Carbon, oxygen, and hydrogen are the elements that make up carbohydrates. Plant cells have walls composed mostly of cellulose and related carbohydrate polymers. These three elements make up a high percentage of plant dry matter because quantitatively most of it is cell wall. In addition, it is mainly in the form of sugars (i.e., carbohydrates) that carbon initially assimilated by leaves through photosynthesis is translocated to the rest of the plant body, including the roots.

• Nitrogen is a component of all amino acids, and as proteins are amino acid polymers, of all proteins. Nucleic acids and other essential compounds also contain nitrogen.
• Phosphorus is part of several compounds essential for energy transfer, of which adenosine triphosphate (ATP ), the "energy currency" of cells, is the best known. Nucleic acids and several other classes of biochemical entities also contain phosphorus as an integral component.
• Three sulfur -containing amino acids and other compounds needed in metabolism account for the essentiality of sulfur.
• Potassium is not an integral part of any compound that can be chemically isolated from plants. However, it activates some seventy enzymes , and along with other solutes regulates the water relations of plants.
• Calcium is part of the middle lamella, the layer between the cell walls of adjacent cells. Another function is maintenance of the integrity of cell membranes. Calcium is also a cofactor (nonprotein part) of several enzymes. It functions to signal environmental changes in plant cells.
• Magnesium is a constituent of the chlorophyll molecule and activates numerous enzymes.
• Iron is a part of many metabolites, including those primarily involved in energy acquisition (photosynthesis), utilization (respiration), and nitrogen fixation.
• Manganese activates a number of enzymes and is part of the protein complex that causes the evolution of oxygen, O₂, in Photosystem II of photosynthesis.
• Zinc is a constituent of several enzymes.
• Copper is also a constituent of several enzymes.
• Nickel , the element required in the least amount, is a constituent of the enzyme urease. A deficiency of it causes an excessive accumulation of urea.
• Boron has several functions in plant growth; severe boron deficiency causes the growing tips of both roots and shoots to die.
• Chlorine (in the form of chloride ion) is required in Photosystem II of photosynthesis. Severely chlorine-deficient plants wilt, suggesting some unknown function in water relations.
• Molybdenum is a constituent of enzymes active in the acquisition of nitrogen.
• Cobalt is required by the symbiotic nitrogen-fixing bacteria associated with the root nodules of legumes and some other plants.
• Sodium is prominent in many soils of arid and semiarid regions, and native wild plants growing on these saline soils grow best with an ample supply of it. Crops, however, often suffer under saline conditions. Plants with the C₄ photosynthetic pathway require sodium as a micronutrient.
• Silicon is essential for plants of the family Equisetaceae, the horse-tails or scouring rushes. Although apparently not absolutely essential for plants in general it has nevertheless many beneficial effects; it has been called quasi-essential.

Deficiency and Toxicity Symptoms

When some element is deficient or present in such high concentration as to be toxic, plants often have symptoms somewhat characteristic of the particular condition afflicting them. For example, yellowing of leaves, or chlorosis, often indicates a deficiency of nitrogen. Nevertheless, visual identification of deficiencies or toxicities is not a reliable procedure. For example, sulfur deficiency may result in symptoms very similar to those of nitrogen deficiency. Therefore even experts check their visual impression by analyzing the tissue to find out whether its content of the suspected element is in fact below the value deemed adequate for that particular crop or present in excess. Often, such unrelated conditions as diseases caused by fungi or bacteria may result in the development of symptoms that mimic those of nutrient disorders.

see also Biogeochemical Cycles; Fertilizer; Halophytes; Hydroponics; Nitrogen Fixation; Soil, Chemistry of.

Emanuel Epstein

Bibliography

Bennett, W. F., ed. Nutrient Deficiencies and Toxicities in Crop Plants. St. Paul, MN: American Phytopathological Society, 1993.

Epstein, Emanuel. Mineral Nutrition of Plants: Principles and Perspectives. New York: John Wiley & Sons, 1971.

——. "Silicon." Annual Review of Plant Physiology and Plant Molecular Biology 50 (1999): 641-64.

Taiz, Lincoln, and Eduardo Zeiger. Plant Physiology, 2nd ed. Sunderland, MA: Sinauer Associates, 1988.

Nutrients

Nutrients are any chemicals that are required for life. Nutrients can be of two basic types: (1) inorganic substances absorbed by autotrophic organisms such as plants and certain microorganisms for use in their synthetic reactions and metabolism; or (2) biomass ingested as nourishment by animals and heterotrophic microorganisms.

Plants absorb a wide range of mineral nutrients, which they utilize in their photosynthetic reactions and other metabolic processes to manufacture all of the biochemicals that they require for growth and reproduction. Some nutrients are required by plants in relatively large quantities. These are called macro-nutrients and include compounds of the following elements: carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur. Of these, carbon is required in the largest quantity, typically making up about one-half of the dry weight of plant tissues, while hydrogen, oxygen, nitrogen, and calcium occur in concentrations of one to several percent. Phosphorus, potassium, magnesium, and sulfur typically account for 0.1% to 1% of the dry weight of plants. Micronutrients are required by plants in much smaller quantities and include metals such as copper, iron, and zinc.

Animals must eat plants or other animals to obtain virtually all of their nutrients. After ingestion, animals typically digest their food and thereby break it down into relatively simple biochemicals and inorganic chemicals, which are then absorbed through the gut and used in the animal’s metabolism. The largest nutritional need of animals is for energy to support their respiration and growth, and sources of fixed energy such as plant or animal biomass are ingested for this purpose. There are also a few micronutrients that animals require in small quantities, but they cannot synthesize. These include biochemicals called vitamins, which must be ingested with food. Some animals can also utilize mineral forms of certain nutrients, which they may take directly from the inorganic environment without eating biomass. For example, many large grazing mammals will utilize salt licks when they are available, because these animals crave sodium, which is not usually present in sufficiently large concentrations in the plants that they eat.

Appropriate nutrition for all organisms is a matter of both quantity and balance. For good nutritional health, all of the essential inorganic and organic nutrients must be available, but they must be obtainable in an appropriate balance. A severe shortage of even a micronutrient required in trace quantities can result in severe metabolic dysfunctions, and even death of organisms.

See also Vitamin.

Nutrients

Nutrients are any chemicals that are required for life. Nutrients can be of two basic types: (1) inorganic substances absorbed by autotrophic organisms such as plants and certain microorganisms for use in their synthetic reactions and metabolism ; or (2) biomass ingested as nourishment by animals and heterotrophic microorganisms.

Plants absorb a wide range of mineral nutrients, which they utilize in their photosynthetic reactions and other metabolic processes to manufacture all of the biochemicals that they require for growth and reproduction. Some nutrients are required by plants in relatively large quantities. These are called macronutrients and include compounds of the following elements: carbon , hydrogen , oxygen , nitrogen , phosphorus , potassium, calcium , magnesium , and sulfur . Of these, carbon is required in the largest quantity, typically making up about one-half of the dry weight of plant tissues, while hydrogen, oxygen, nitrogen, and calcium occur in concentrations of one to several percent . Phosphorus, potassium, magnesium, and sulfur typically account for 0.1% to 1% of the dry weight of plants. Micronutrients are required by plants in much smaller quantities and include metals such as copper , iron , and zinc.

Animals must eat plants or other animals to obtain virtually all of their nutrients. After ingestion, animals typically digest their food and thereby break it down into relatively simple biochemicals and inorganic chemicals, which are then absorbed through the gut and used in the animal's metabolism. The largest nutritional need of animals is for energy to support their respiration and growth, and sources of fixed energy such as plant or animal biomass are ingested for this purpose. There are also a few micronutrients that animals require in small quantities, but they cannot synthesize. These include biochemicals called vitamins, which must be ingested with food. Some animals can also utilize mineral forms of certain nutrients, which they may take directly from the inorganic environment without eating biomass. For example, many large grazing mammals will utilize salt licks when they are available, because these animals crave sodium , which is not usually present in sufficiently large concentrations in the plants that they eat.

Appropriate nutrition for all organisms is a matter of both quantity and balance. For good nutritional health, all of the essential inorganic and organic nutrients must be available, but they must be obtainable in an appropriate balance. A severe shortage of even a micronutrient required in trace quantities can result in severe metabolic dysfunctions, and even death of organisms.

See also Vitamin.