vitamin is a topic covered in the Taber's Medical Dictionary.

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(vīt′ă-mĭn )

[L. vita, life + amine]
An accessory nutrient that serves as a coenzyme or cofactor in an essential metabolic process. Small quantities of the substance assist biological reactions such as oxidation and reduction, or the synthesis of nucleic acids, hemoglobin, clotting factors, or collagen. Vitamin deficiencies produce well-recognized syndromes (such as scurvy [vitamin C deficiency], or beriberi [thiamine deficiency]). Unlike proteins, carbohydrates, fats, and organic salts, vitamins are not energy sources or components of body structures. Instead, they are agents that hasten or facilitate biochemical processes involving these other organic molecules.
SEE: dietary reference intakes; SEE: mineral
Only vitamins A, D, and K are made within the body. The rest must be consumed in the diet. Vitamin A is formed from its precursor, carotene; vitamin D is formed by the action of ultraviolet light on the skin; and vitamin K is formed by the symbiotic action of bacteria within the intestines.

A common classification system distinguishes fat-soluble vitamins (A, D, E, and K) from water-soluble vitamins (B and C). Fat-soluble vitamins are poorly assimilated in diseases that interfere with the digestion of fat, such as steatorrhea, but accumulate in organs like the liver when taken in excess. Water-soluble vitamins are readily lost from the body in urine and sweat and are more likely to be lacking from the body than overabundant.

One's need for vitamins increases in conditions that deplete their stores from the body, such as pregnancy and lactation, alcoholism, and febrile illnesses. Some drugs block the action of specific vitamins, or create illnesses that can be prevented with vitamin supplementation. In patients taking isoniazid for tuberculosis, for example, vitamin supplementation with pyridoxine is needed to prevent peripheral neuropathy.

Refer to the Vitamins Appendix for signs and symptoms of vitamin deficiency.

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