Vitamins

September 29, 2011

Vitamins are a group of substances found in foods that are essential for growth, health, and preservation of life itself. The relationship between important food components and health and well-being had been known for hundreds of years, but the specific role of these components was discovered in the 20th century. Vitamins are grouped together as either fat soluble or water soluble.

FAT-SOLUBLE VITAMINS

Fat-soluble vitamins will dissolve in fat and oil, and any extra amount more than the body needs is stored in body fat. Because fat-soluble vitamins can build up in the body, excessive intake of these vitamins can cause toxicity.

Vitamin A

Vitamin A occurs in nature in two forms: retinol (the active form of vitamin A) and carotenoids (precursors used by the body to make vitamin A). Vitamin A helps to maintain skin and the lining of certain internal organs. It is also important for normal vision in dim light. Night blindness, in which the eyes adapt poorly to the dark, is an early sign of vitamin A deficiency. If the deficiency is not corrected, it can lead to eye damage and blindness. This vitamin is essential for normal bone and tooth development, is used in the synthesis of hydrocortisone (a steroid hormone) from cholesterol, and helps stabilize cell membranes. Some carotenoids may have the ability to protect cells from cancer-causing changes.

Liver, fish, and egg yolk are rich sources of vitamin A. The main source of vitamin A in the diet is from carotenes found in dark yellow, green, and orange fruits and vegetables. Fortified milk has added vitamins A and D. Too much vitamin A is toxic and causes loss of hair, anorexia, pain in the joints, and enlargement of the liver and spleen.

Vitamin D

Vitamin D occurs in two forms known as vitamin D2 (ergocalciferol), the vitamin D precursor found in plants, and vitamin D3, the main form present in animal cells. Vitamin D3 is formed in the skin when exposed to ultraviolet light. Vitamin D is a steroid and works like a steroid hormone.

Vitamin D helps in mineralization of bones by regulating calcium and phosphorus absorption from the intestine, and preventing calcium loss into the urine. Deficiency of vitamin D can cause poor mineralization of bone, resulting in osteomalacia in adults and rickets in children.

The amount of vitamin D formed by the action of sunlight on the skin depends on the intensity of sunlight, length of sun exposure, and skin pigmentation. Weak sunlight due to air pollution or during winter may not provide the amount of light needed to form enough vitamin D in the skin. Food sources of vitamin D include animal foods like eggs, liver, butter, and fatty fish. Milk contains added vitamin D.

Symptoms of vitamin D toxicity include increased urination, frequent urination at night, weight loss, diarrhea, and nausea. Severe toxicity leads to calcification and hardening of the soft tissues in the blood vessels, heart, stomach, and kidneys.

Vitamin E

Vitamin E is an important antioxidant. Some studies show that vitamin E might protect against chronic conditions like heart disease, cancer, and cataracts, but other studies do not bear this out. Researchers are working to find out whether vitamin E is beneficial in the prevention of coronary artery disease or whether it has no effect. More research is needed before the answer becomes clear.

Vitamin E is needed for normal growth and helps in the intestinal absorption of unsaturated fatty acids. Most of the vitamin E in the blood is carried on lipopro-teins, and any condition that affects levels of blood fat and cholesterol (lipids) also alters the amount of vitamin E in the blood.

Vitamin E is found in vegetable oils, nuts, seeds, and wheatgerms. The amount of vitamin E in the diet depends on the amount and kind of fat consumed. Vitamin E requirements increase with the intake of polyunsaturated fatty acids. Toxic effects of this vitamin are unknown.

Vitamin K

Vitamin K exists in two forms. Vitamin K1 is produced by green plants and vitamin K2 is produced by animals and also by the intestinal bacteria. Vitamin K is needed to make blood factors needed for normal clotting. Deficiency of vitamin K causes a delay in blood clotting. This vitamin also helps in growth.

Food sources of vitamin K are liver, cauliflower, spinach, and other green leafy vegetables. People who are taking blood thinners such as warfarin (Coumadin) need to be careful to limit their intake of vitamin K because it interferes with the effect of the blood thinners. A deficiency of vitamin K can be produced by the elimination of dietary sources or by prolonged treatment with antibiotic, which kills the intestinal bacteria.

WATER-SOLUBLE VITAMINS

Water-soluble vitamins will dissolve in water and any extra amount more than the body needs is excreted in the urine. These vitamins are obtained from the daily diet and are not stored in the body.

Vitamin C

Vitamin C (ascorbic acid) is involved in the formation of collagen. Collagen helps to maintain body structure. Collagen is found in cartilage, bone, teeth, and the lining of blood vessels. Vitamin C is important in wound healing and helps the body respond to stress, injury, and infection.

Vitamin C helps convert folic acid to the active form folinic acid, is used in the synthesis of steroid hormones from cholesterol, in the release of iron for transport in the body, and in the conversion of ferric iron to ferrous iron in the gastrointestinal tract. Vitamin C is also a powerful antioxidant.

Vitamin C is present in fresh, frozen, or raw fruits and vegetables. Dairy products are poor sources of vitamin C. Prolonged cooking at high temperature destroys vitamin C. To retain vitamin C in cooked foods, cooking should be done with the minimum amount of water for the shortest possible period of time in a covered pot. Steaming of fruits and vegetables helps to retain vitamin C.

Thiamine

Thiamine functions as a coenzyme in the breakdown of carbohydrates. It regulates muscle tone of the gastrointestinal tract and the heart. It is essential for normal functioning of nerves.

Deficiency of thiamine results in a disease called beriberi. Nerve, heart, and brain damage occur in beriberi. The nerve damage causes numbness and tingling of the legs and wasting and weakness of the muscles of the extremities. Mental depression may occur. Chronic alcoholism can lead to thiamine deficiency, which results in a disease called Wernicke’s syndrome. The symptoms of Wernicke’s syndrome include confusion, abnormalities of muscle movement, and problems moving the eyes.

Some sources of thiamine include lean pork, organ meats, wheatgerm, whole grains, dried beans, nuts, seeds, enriched flour, and brewer’s yeast.

Riboflavin

Riboflavin is a yellow pigment with green fluorescence. This vitamin is used in the conversion of vitamin B6 and folate to their active forms. It is involved in several metabolic reactions. A deficiency of riboflavin leads to cracks in the skin at the corners of the mouth (cheilitis or angular stomatitis). The tongue may develop a magenta hue because of inflammation (glossitis). Changes in the eyes include eye inflammation, invasion of the cornea by blood vessels, and eye pain in bright light. Inadequate dietary intake and chronic alcoholism can cause riboflavin deficiency.

Good sources of riboflavin are milk, meat, fish, poultry, and whole-grain or enriched cereal and cereal products. Riboflavin is relatively insoluble and therefore it is not lost by the usual cooking methods.

Niacin

Niacin occurs as nicotinic acid in plants and nicotin-amide in animal tissues (this is not the same as nicotine which is found in tobacco). Niacin is an important component of coenzymes I and II, and plays an important role in cellular respiration. It is also involved in the metabolism of proteins, fats, and carbohydrates. The precursor of niacin is tryptophan, which is also one of the essential amino acids found in protein. Therefore, a diet deficient in protein can also cause niacin deficiency.

Deficiency of niacin leads to a condition called pellagra. This is also called the disease of the three Ds: dermatitis (skin inflammation), diarrhea, and dementia. Niacin deficiency may occur in chronic alcoholism and gastrointestinal disorders.

Niacin has the ability to reduce serum lipids (blood fats and cholesterol) and decrease the release of fatty acids from fat tissues. Niacin is sometimes prescribed in large amounts to lower cholesterol levels. At these high doses, niacin can cause side effects in some people such as flushing or liver problems. Proteins from animal foods are the richest sources of niacin. Most vegetables contain about 1% tryptophan and are a moderately good source of niacin.

Folic acid

This is also known as folacin or folate. Folic acid is involved in the synthesis of choline, DNA, RNA, and amino acids. Folic acid along with vitamin B12 is important in the production of red blood cells in the bone marrow. Severe folic acid deficiency causes a type of anemia in which large immature red blood cells are released into the bloodstream.

Intestinal bacteria synthesize folic acid. The richest sources of folic acid are liver, asparagus, spinach, wheat, yeast, and dried beans. Dark green leafy vegetables are also good sources of this vitamin.

Mild folic acid deficiency has been linked to birth defects in the brain and spinal cord (anencephaly and spina bifida). To prevent these birth defects, women who may become pregnant should take at least 0.4 mg (400 fig) of folic acid daily. This amount is contained in most adult multivitamins. Women who previously had an affected child or who have spina bifida themselves should take 4,000 mg (4 g) daily, under the supervision of their doctor.

Vitamin B12 (cobalamin)

Vitamin B12 is used in the synthesis of DNA and RNA. It is necessary for the synthesis of the myelin sheath that encases nerve fibers. It is also involved in the metabolism of carbohydrates and fats and in the development of mature red blood cells. Like folic acid deficiency, deficiency of vitamin B12 results in a type of anemia in which large immature red blood cells are released into the bloodstream. It can also cause nerve damage, impaired cell division, and altered protein synthesis.

Vitamin B6

Vitamin B6 occurs naturally in three forms: pyri-doxine, pyridoxal, and pyridoxamine. Vitamin B6 is essential for the conversion of tryptophan to niacin, the breakdown of glycogen to glucose, the metabolism of unsaturated fatty acids, synthesis of antibodies, and formation of neurotransmitters in the brain from amino acids. Vitamin B6 deficiency causes seborrheic dermatitis, glossitis (inflamed magenta-colored tongue), cheili-tis (cracks in the corners of the mouth), nerve damage, and blood disorders. Fish, poultry, meats, walnuts, peanuts, wheatgerm, and brown rice are good sources of vitamin B6.

Biotin

Biotin is a sulfur-containing vitamin. As a component of many enzymes, biotin is involved in carbohydrate and fat metabolism. It is also involved in the breakdown of some amino acids. It is vital to growth and maintenance of skin, hair, nerves, and bone marrow. Biotin is widely distributed in foods. Liver and yeast are the richest sources, but nuts, meats, and seafood are also good sources of biotin.

Pantothenic acid

The term pantothenic acid is derived from the Greek word pantothene meaning “everywhere.” It is widely distributed in plants and animals. Pantothenic acid has a vital role in the metabolism of carbohydrates, proteins, and fats for energy. It is involved in the release of energy and the synthesis of fatty acids, steroids, cholesterol, acetylcholine, and the porphyrin ring of the hemoglobin molecule.

Kidney, liver, eggs, yeast, wheatgerm, and dried peas are the richest sources of this vitamin. Pantothenic acid is widely distributed in many types of foods.

See Also: Diet, Nutrition, Prenatal care

Suggested Reading

  • Chan, A. C. (1998). Vitamin E and atherosclerosis. Journal of Nutrition, 128(10), 1593-1596.
  • Combs, G. F. (1998). The vitamins: Fundamental aspects in nutrition and health (2nd ed.). San Diego, CA: Academic Press.
  • DeLuca, H. F. (1982). New developments in the vitamin D endocrine system. Journal of the American Dietetic Association, 80(3), 231-237.
  • Farrington, K., Miller, P., Varghese, Z., Baillod, R. A., & Moorhead, J. F. (1981). Vitamin A toxicity and hypercalcaemia in chronic renal failure. British Medical Journal Clinical Research Edition, 282(6281), 1999-2002.
  • Institute of Medicine, Food and Nutrition Board. (1998). Dietary reference intakes for thiamin, riboflavin, niacin, vitamin B-6, folate, vitamin B-12, pantothenic acid, biotin, and cholin. Washington DC: National Academy Press.
  • Shils, M. E., et al. (Eds.). (1999). Modern nutrition in health and disease (9th ed.). Baltimore: Williams & Wilkins.

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