Known also as vitamin B complex, these are fragile, water-soluble substances,
several of which are particularly important to carbohydrate metabolism.
Thiamine, or vitamin B1, a colorless, crystalline substance, acts
as a catalyst in carbohydrate metabolism, enabling pyruvic acid to be absorbed
and carbohydrates to release their energy. Thiamine also plays a role in the
synthesis of nerve-regulating substances. Deficiency in thiamine causes beriberi,
which is characterized by muscular weakness, swelling of the heart, and leg
cramps and may, in severe cases, lead to heart failure and death. Many foods
contain thiamine, but few supply it in concentrated amounts. Foods richest in
thiamine are pork, organ meats (liver, heart, and kidney), brewer’s yeast,
lean meats, eggs, leafy green vegetables, whole or enriched cereals, wheat germ,
berries, nuts, and legumes. Milling of cereal removes those portions of the
grain richest in thiamine; consequently, white flour and polished white rice may
be lacking in the vitamin. Widespread enrichment of flour and cereal products
has largely eliminated the risk of thiamine deficiency, although it still occurs
today in nutritionally deficient alcoholics.
Riboflavin, or vitamin B2, like thiamine, serves as a
coenzyme—one that must combine with a portion of another enzyme to be
effective—in the metabolism of carbohydrates, fats, and, especially,
respiratory proteins. It also serves in the maintenance of mucous membranes.
Riboflavin deficiency may be complicated by a deficiency of other B vitamins;
its symptoms, which are not as definite as those of a lack of thiamine, are skin
lesions, especially around the nose and lips, and sensitivity to light. The best
sources of riboflavin are liver, milk, meat, dark green vegetables, whole grain
and enriched cereals, pasta, bread, and mushrooms.
Niacin, or vitamin B3, also works as a coenzyme in the release of
energy from nutrients. A deficiency of niacin causes pellagra,
symptoms of which are weakness; sore, red, cracked skin; a red and swollen
tongue; diarrhea; mental confusion; irritability; and, when the central nervous
system is affected, depression and mental disturbances. The best sources of
niacin are liver, poultry, meat, canned tuna and salmon, whole grain and
enriched cereals, dried beans and peas, and nuts. The body
also makes niacin from the amino acid tryptophan. Megadoses of niacin have been
used experimentally in the treatment of schizophrenia, although no experimental
proof has been produced to show its efficacy. In large amounts it reduces levels
of cholesterol in the blood, and it has been used
extensively in preventing and treating arteriosclerosis. Large doses over long
periods cause liver damage.
Pyridoxine, or vitamin B6, is necessary for the absorption and
metabolism of amino acids. It also plays roles in the use of fats in the body
and in the formation of red blood cells, and in maintaining the functions of the
nervous and immune systems as well. Pyridoxine deficiency is characterized by
skin disorders, cracks at the mouth corners, smooth tongue, convulsions,
dizziness, nausea, anemia, and kidney stones. The best sources of pyridoxine are
whole (but not enriched) grains, cereals, bread, liver, avocados, spinach, green
beans, and bananas. Pyridoxine is needed in proportion to the amount of protein
consumed. Excessive intake may damage the sensory nervous system.
Cobalamin, or vitamin B12, one of the most recently isolated
vitamins, is necessary in minute amounts for the formation of nucleoproteins,
proteins, and red blood cells, and for the functioning of the nervous system.
Cobalamin deficiency is often due to the inability of the stomach to produce
glycoprotein, which aids in the absorption of this vitamin. Pernicious anemia
results, with its characteristic symptoms of ineffective production of red blood
cells, faulty myelin (nerve sheath) synthesis, and loss of epithelium (membrane
lining) of the intestinal tract. Cobalamin is obtained only from animal
sources—liver, kidneys, meat, fish, eggs, and milk. Vegetarians are advised to
take vitamin B12 supplements. The malabsorption of B12
among the elderly should also be taken into account.
Other B vitamins
Folic acid, or folacin, is a coenzyme needed for forming body protein and hemoglobin; its deficiency in humans is rare. Low folate intake, however, has been linked with vascular disease and the risk of congenital neural tube defects. Folic acid is effective in the treatment of certain anemias and sprue. Dietary sources are organ meats, leafy green vegetables, legumes, nuts, whole grains, dried beans, dried peas, and brewer’s yeast. Folic acid is lost in foods stored at room temperature and during cooking. Unlike other water-soluble vitamins, folic acid is stored in the liver and need not be consumed daily.
Pantothenic acid, another B vitamin, plays a still-undefined role in the metabolism of proteins, carbohydrates, and fats. It is abundant in many foods and is manufactured by intestinal bacteria as well.
Biotin, a B vitamin that is also synthesized by intestinal bacteria and
widespread in foods, plays a role in the formation of fatty acids and the
release of energy from carbohydrates. Its deficiency in humans is unknown.
|Vitamin A||The B Vitamins||Vitamin C||Ascorbic Acid|
|Vitamin D||Vitamin E||Vitamin K|