Vitamins are organic molecules needed in small quantities for normal metabolism throughout the body. Most vitamin molecules attach to enzymes or coenzymes and help them work properly. Coenzymes are organic, nonprotein catalysts that often act as “molecule carriers.” Many enzymes or coenzymes are not functional without the appropriate vitamins attached to them. This attachment gives coenzymes the proper functional shape. For example, coenzyme A (CoA)—an important carrier molecule associated with the citric acid cycle—has pantothenic acid (vitamin B5) as one of its major components.

Not all vitamins are involved directly with enzymes and coenzymes. Vitamins A, D, and E play a variety of different, but no less important, roles in the chemistry of the body. The form of vitamin A called retinal, for example, plays an important role in detecting light in sensory cells of the retina. Vitamin D can be converted to the hormone calcitriol, which plays a role in the regulation of calcium homeostasis in the body. One role of vitamin E (and vitamin C) is to serve as an antioxidant that prevents free radicals (highly reactive oxygen atoms) from damaging electron-dense molecules in the cell membranes and DNA molecules.

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All but one vitamin, vitamin D, cannot be made by the body itself. Recent research suggests that vitamin D supplements may reduce risks for a range of diseases, including cancers of the breast, colon, ovaries, and prostrate. Bacteria living in the colon make two more: vitamin K and biotin. We must eat vitamins, or molecules we can convert into vitamins, in our food to get the rest. The body can store fat-soluble vitamins—A, D, E, and K—in the liver for later use. Because the body cannot store significant amounts of water-soluble vitamins such as B vitamins and vitamin C, they must be continually supplied in the diet. Table 22-2 lists some common vitamins, their sources and functions, and symptoms of deficiency.


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