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SUMMARY: Vitamins are small molecules which cannot be synthesized by humans and must be obtained directly from the diet or supplements. In contrast to certain essential amino acids and fats, vitamins usually function as catalysts for important chemical reactions in the body, and thus are needed in very small quantities compared to other nutrients. Most vitamins are associated with specific chemical reactions, and lack of that vitamin leads to a particular short-term deficiency disease. The Daily Values indicate the amount of each micronutrient to prevent such short-term deficiencies. The role of and requirement for micronutrients to maintain long-term optimal health is the subject of intense study, and is likely to lead to different quantities and even other molecules essential for long-term good health.

Back in seventeenth century, sailors discovered that eating limes prevented scurvy, a disease which we now know results from a lack of vitamin C. It was not until the 1930's that scientists discovered what the critical nutrient was in limes - vitamin C, also known as ascorbic acid. The first vitamin discovered was vitamin B1, or thiamine. Because it was "vital" to life, and contained a group of atoms known as an "amine", it was called "vitamine"

Vitamins are small molecules essential in small quantities which cannot be synthesized in humans from the basic food molecules we eat, which are proteins, carbohydrates, fats, and nucleic acids. To give an idea how small they are, a vitamin weighs about 1/100 as much as a protein. These small molecules must be obtained in the diet or supplement form. They catalyze important reactions in the body which keep us healthy.

There are two types of vitamins, those soluble in water and those soluble in fat. The water-soluble vitamins dissolve in water, and therefore work in the water or aqueous part of your body. As you excrete water regularly in the form of urine, you also lose this type of vitamin quickly, on the order of 12-72 hours. You therefore cannot store water soluble vitamins for very long, and must get them regularly. In contrast, fat soluble vitamins are stored in the fat or lipid parts of your body like cell membranes and nerve linings. These fat soluble vitamins therefore are needed much less often.

Several factors influence how much of a vitamin is useful. First, you need to obtain the vitamin in your diet, as food or supplements. Second, the vitamin must be effectively absorbed from the digestive system. High doses of certain vitamins and minerals can reduce how much of the micronutrient is absorbed, interfere with absorption of other micronutrients, and cause unpleasant side-effects. It is thus important not to take too much of a nutrient in supplement form. Second, micronutrients are excreted from the body, and the rate at which they are excreted influences how often you need to replace them. Vitamin C, for instance, is absorbed from the digestive system and excreted from the kidney over 12 hours, so there may be significant advantage in getting vitamin C at least every 12 hours (ref). Third, some vitamins can accumulate to hazardous levels, such as vitamin A, calcium, iron, and selenium, while some such as vitamin C appear to have no hazard even at extremely high dosages.