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Vitamins are essential nutrients and must be supplied through diet as they either are not made in the body or are made in insufficient quantity. Although they are essential to life and health, they neither build tissues nor supply energy to the body. A prolonged shortage of any of the vitamins can lead to a “deficiency disease”. However, all 13 vitamins can be obtained in adequate quantity from a well-chosen diet. The work of vitamins is to help bring about the body's chemical responses. They act as catalysts in the processing of other nutrients (proteins, fats, and carbohydrates), help form red blood cells and hormones, create genetic materials, and regulate the nervous system.
Vitamins are classified into two groups based whether they are soluble in fat or in water: the water-soluble vitamins (B complex and C) and the fat-soluble vitamins (A, D, E, and K). The water-soluble ones are widely found in plant and animal tissue, but some of them are destroyed to a variable degree during the processing of foods and in cooking. Being water-soluble, these vitamins are easily eliminated from the body in sweat or urine; since only small amounts are stored, they must be steadily present in the diet. Fat-soluble vitamins, on the other hand, are less widely distributed in nature. Being fat-soluble and not soluble in water, they are not easily excreted and can even build up to toxic levels over a period of time if substantially more than the Recommended Dietary Allowances are consumed daily. A deficiency in a fat-soluble vitamin may take many more months to develop than a deficiency of a water-soluble vitamin.
Although vitamins are needed for thousands of bodily actions every day, they are required in very small amounts. The total amount of vitamins one person needs each day for maintaining good health measures less than 1/8 teaspoon. Such small amounts are sufficient because vitamins are not usually depleted in doing their work for the body. Instead, the body “recycles" vitamins, releasing and reutilizing them until they are replaced by new ones.
The two units of measure commonly used to express vitamin amounts are milligrams and micrograms. To understand these units, think of an ounce-which equals 28.3 grams. A milligram is 1/1000 of a gram, or slightly less than .03 ounces. The recommended allowance for the average adult is only 60 milligrams of vitamin C per day, for example. The recommended allowance of some vitamins, such as B12, is even smaller. These are measured in micrograms, that is 1/1,000 of a milligram or 1 millionth of a gram. Thus, a single ounce of B12 would be enough to supply one day's requirement for nearly 9 million people. Until recently, international units (IU) were the common measure used to express the biological activity of fat-soluble vitamins. But since these vitamins are found in nature in different forms, with varying degrees of biological effect, this imprecise system has created confusion. To help clear matters, fat-soluble vitamins are now measured in their pure form according to weight (micrograms and milligrams). However, during this period of transition, both systems of measurement will continue to be employed simultaneously. | 
