Vitamins (vi′tah-minz) are organic compounds (other than carbohydrates, lipids, and proteins) required in small amounts for normal metabolism, that cells can not synthesize in adequate amounts. Therefore, they are essential nutrients that must come from foods.
Vitamins are classified on the basis of solubility. Some are soluble in fats (or fat solvents) and others are soluble in water. Fat-soluble vitamins are vitamins A, D, E, and K; water-soluble vitamins are the B vitamins and vitamin C
Fat-soluble vitamins dissolve in fats, and therefore they associate with lipids and respond to the same factors that affect lipid absorption. For example, bile salts in the intestine promote absorption of these vitamins. Fat soluble
vitamins accumulate in various tissues, which is why excess intake can lead to overdose conditions. For example, too much beta carotene, a vitamin A precursor, can tinge the skin orange. Fat-soluble vitamins resist the effects of heat; therefore, cooking and food processing usually do not destroy them. Table lists the fat-soluble vitamins and their characteristics, functions, sources, and recommended daily allowances (RDAs) for adults.
The water-soluble vitamins include the B vitamins and vitamin C. The B vitamins are several compounds that are essential for normal cellular metabolism. They help oxidize carbohydrates, lipids, and proteins. The B vitamins are in many of the same foods, so they are referred to as the vitamin B complex. Members of this group differ chemically and functionally. Cooking and food processing destroy some of them. Vitamin C (ascorbic acid) is one of the least stable vitamins and is fairly widespread in plant foods. It is necessary for collagen production, the conversion of
folacin to folinic acid, and the metabolism of certain amino acids. Vitamin C also promotes iron absorption and synthesis of certain hormones from cholesterol. Table lists the water-soluble vitamins and their characteristics, functions, sources, and RDAs for adults.