vitamin: Vitamin B Complex
Commonly grouped as the vitamin B complex are eight water-soluble vitamins.
Thiamine (vitamin B1 or antiberiberi factor) is a necessary ingredient for the biosynthesis of the coenzyme thiamine pyrophosphate; in this latter form it plays an important role in carbohydrate metabolism. Good sources are yeast, whole grains, lean pork, nuts, legumes, and thiamine-enriched cereal products. This vitamin is a factor in the maintenance of appetite, normal intestinal function, and in the health of the cardiovascular and nervous systems. A deficiency of the vitamin may lead to beriberi; the disease was first shown to result from a dietary deficiency by Dutch physician Christiaan Eijkman. The recommended dietary allowance for adults is 1.2 to 1.4 mg for men and 1.0 to 1.1 mg for women.
Riboflavin (vitamin B2 or lactoflavin) is used to synthesize two coenzymes that are associated with several of the respiratory enzymes of plants and animals (including humans) and is therefore important in biochemical oxidations and reductions. Deficiency leads to fissures in the corners of the mouth, inflammation of the tongue showing a reddish purple coloration, skin disease, and often severe irritation of the eyes. The recommended dietary allowance for adults is 1.4 to 1.7 mg for men and 1.2 to 1.3 mg for women. Riboflavin is widely distributed in plant and animal tissues; milk, organ meats, and enriched cereal products are good sources.
The B vitamins niacin (nicotinic acid) and niacinamide (nicotinamide) are commonly known as preventives of pellagra, which in 1912 was shown by American medical researcher Joseph Goldberger to result from a dietary deficiency. Niacin was first synthesized in 1867. The amino acid tryptophan is the precursor of niacin. Niacin and niacinamide function in the biochemistry of humans and other organisms as components of the two coenzymes nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP); these operate in many enzyme-catalyzed oxidation and reduction reactions. The deficiency state in humans causes skin disease, diarrhea, dementia, and ultimately death. The deficiency state in dogs analogous to pellagra in humans is called blacktongue disease. Lean meats, peanuts and other legumes, and whole-grain or enriched bread and cereal products are among the best sources of niacin. The recommended daily dietary allowance for adults is 16 to 19 mg niacin equivalents (60 mg of dietary tryptophan to 1 mg of niacin) for men and 13 to 14 mg for women.
Pyridoxine, pyridoxal, and pyridoxamine make up the vitamin B6 group. They all combine with phosphorus in the body to form the coenzyme pyridoxal phosphate, which is necessary in the metabolism of amino acids, glucose, and fatty acids. The best sources of B6 vitamins are liver and other organ meats, corn, whole-grain cereal, and seeds. Deficiency can result in central nervous system disturbances (e.g., convulsions in infants) due to the role of B6 in serotonin and gamma-aminobutyric acid synthesis. More generally the effects of deficiency include inadequate growth or weight loss and anemia due to the role of B6 in the manufacture of hemoglobin. The recommended dietary allowance for adults is 2.0 to 2.2 mg for men and 2 mg for women. Additional doses are required in pregnancy and by those taking oral contraceptives or the tuberculosis drug izoniazid. Severe nerve damage has been reported from megadoses.
Pantothenic acid, another B vitamin, is present in perhaps all animal and plant tissues, as well as in many microorganisms. Good sources of it include liver, kidney, eggs, and dairy products. It is a component of the important substance coenzyme A, which is involved in the metabolism of many biochemical substances including fatty acids, steroids, phospholipids, heme, amino acids, and carbohydrates. The adrenal gland is an important site of pantothenic acid activity. There is no known naturally occurring deficiency state and no known toxicity to pantothenic acid. The estimated safe and adequate daily intake for adults is 4 to 7 mg.
Biotin is a B vitamin that functions as a coenzyme in the metabolism of carbohydrates, fats, and amino acids. Although it is vitally necessary to the body, only exceedingly small quantities are needed, and since biotin is synthesized by intestinal bacteria, naturally occurring biotin deficiency disease is virtually unknown. The disease state can be produced artificially by including large quantities of raw egg white in the diet; the whites contain avidin, a biotin antagonist. Especially good sources of this widely distributed vitamin include egg yolk, kidney, liver, tomatoes, and yeast. There is no known toxicity to biotin. The estimated safe and adequate daily intake for adults is 100 to 200 micrograms.
Folic acid (pteroylglutamic acid, folacin, or vitamin B9) occurs abundantly in green leafy vegetables, fruits (e.g., apples and oranges), dried beans, avocados, sunflower seeds, and wheat germ. Derivatives of this vitamin are directly involved in the synthesis of nucleic acids; for this reason cells in the body that are subject to rapid synthesis and destruction are especially sensitive to folic acid deprivation. For example, the retarded synthesis of blood cells in folic acid deficiency results in several forms of anemia, while failure to replace rapidly destroyed cells in the intestinal wall results in a disease called sprue. Inadequate amounts of folic acid in the diet of pregnant women have been strongly associated with neural tube defects (i.e., spina bifida and anencephaly) in newborns; fortification of flours, cornmeal, rice, and pasta (in a manner similar to the fortification of milk with vitamin D) has been required in the United States since 1998. Adequate folic acid also reduces the risk of premature birth. A U.S. study published in 1998 involving 80,000 women showed significant reduction of heart disease among those whose diets included adequate amounts of folate and vitamin B6. Several chemical antagonists to the action of folic acid have been developed in the hope that they might inhibit the growth of rapidly dividing cancer cells; one such compound, methotrexate, is used to treat leukemia in children. The recommended daily dietary allowance for adults is 400 micrograms. Para-aminobenzoic acid (PABA), which is incorporated into the folic acid molecule, is sometimes listed separately as a B vitamin, although there is no evidence that it is essential to the diet of humans.
The molecular structure of vitamin B12 (cobalamin), the most complex of all known vitamins, was announced in 1955 by several scientists, including British biochemists A. R. Todd and Dorothy Hodgkin. In 1973 the vitamin was reported to have been synthesized by organic chemists. Vitamin B12 and closely related cobalamins are necessary for folic acid to fulfill its role; both are involved in the synthesis of proteins. American physicians G. R. Minot and W. P. Murphy in 1926 fed large amounts of liver to patients with pernicious anemia and cured them; the curative substance in this case was probably vitamin B12. However, pernicious anemia in humans is caused not by a vitamin B12 deficiency in the diet but rather the absence of a substance called the intrinsic factor, ordinarily secreted by the stomach and responsible for facilitating the absorption of B12 from the intestine. When a person's body cannot produce the intrinsic factor, the standard treatment today is to inject vitamin B12 directly into the bloodstream. Minot and Murphy's therapy worked because the liver they fed their patients contained such large quantities of B12 that sufficient amounts of the vitamin were absorbed without the assistance of the intrinsic factor. Inadequate absorption of B12 causes pernicious anemia, nervous system degeneration, and amenorrhea. The only site of cobalamin synthesis in nature appears to be in microorganisms; neither animals nor higher plants are capable of making these vitamin B12 derivatives. Nevertheless, such animal tissues as the liver, kidney, and heart of ruminants contain relatively large quantities of vitamin B12; the vitamin stored in these organs was originally produced by the bacteria in the ruminant gut. Bivalves (clams or oysters), which siphon microorganisms from the sea, are also good sources. Plants, on the other hand, are poor sources of vitamin B12. The recommended daily dietary allowance for adults is 3 micrograms.
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