Vitamins are small but complex molecules. In addition to helping us to use and store energy from macronutrients, they assist the molecules responsible for vision to perform their function, they serve as regulatory hormones for bone formation, and they act as antioxidants to preserve cellular function.

Each of the vitamins was discovered and its requirement determined by its ability to cure and prevent a particular disease or group of symptoms. For example, the discovery that a substance in limes could cure and prevent the disease called scurvy led to the discovery that our bodies require vitamin C and that scurvy is the result of vitamin C deficiency.

Dietary Reference Intakes (DRIs) are being established for each vitamin. They describe the amount of the vitamin that should prevent symptoms of deficiency in most people, with a little extra added. Today, the diseases that result from vitamin deficiencies rarely occur except in severely malnourished individuals or in those with certain medical conditions.

Rather, in a virtual nutrition revolution, research has progressed beyond the identification and treatment of simple nutrient deficiencies to the realization that some of the vitamins (or foods that contain them) may help maintain health by preventing the development of chronic diseases such as cancer and heart disease in otherwise well-nourished people.

The Food Guide Pyramid is based on the DRIs. It tells us the number of servings, in each group of foods, that will supply us with the recommended allowance of most of the vitamins.

Nutrition research also has begun to support the idea that a few of the vitamins and minerals, notably those referred to as antioxidants, may provide even more benefit if taken in quantities somewhat greater than the recommended amounts. This idea raises some questions. Is there such a thing as too much of a vitamin?

Should these extra vitamins come from food, or is it okay to take a supplement if you just can’t eat that much? And, should the recommended amounts for these vitamins be increased? Although there really is no answer to the last question yet, the answers to the first two questions depend on the type of vitamin.

As we begin our discussion of vitamins, we want to emphasize that although it is virtually impossible to overdose on vitamins from food alone, some vitamin supplements definitely offer too much of a good thing. The 14 essential vitamins can be classified into two groups: water-soluble and fat-soluble.

They are classified on the basis of their molecular structure, which determines the way the vitamins are carried in food and in the bloodstream and the manner in which they are stored in your body. The text that follows describes which vitamins fall into which category and what that means for your health.

Water-Soluble Vitamins

There are 10 water-soluble vitamins. The B complex vitamins have various roles, some of which involve their action, in concert, to regulate the body’s use of energy from food. Folic acid is an important factor in the regulation of growth.

During the early stages of pregnancy, folic acid is important for preventing a type of birth defect known as a neural tube defect. Vitamin C, also known as ascorbic acid, functions in various ways, many of which seem to be related to its antioxidant properties.

As their name implies, water-soluble vitamins dissolve in water. The body strives to maintain the optimal level of each of the water-soluble vitamins for its immediate needs. Surplus water-soluble vitamins are excreted in the urine and through perspiration, because they are not stored in the body to any appreciable extent.

Water-soluble vitamins must be replenished almost daily, preferably by eating foods that are rich in these vitamins. Fruits, vegetables, grains, and beans are excellent sources of the water-soluble vitamins (with the exception of vitamin B12, which is found only in foods of animal origin).

However, if you choose to use multivitamins or single-nutrient supplements to get your water-soluble vitamins, moderation is advised, because high doses of several of the B vitamins can have harmful effects, and high doses of vitamin C may contribute to the formation of kidney stones.

Fat-Soluble Vitamins

The fat-soluble vitamins, A, D, E, and K, are found in the food you eat, absorbed into your bloodstream, and carried throughout your body attached to fat molecules. Because fat-soluble vitamins can be stored in the body, they do not need to be replenished on a daily basis.

Vitamins A and D are stored in the liver, and reserve supplies may be sufficient for as long as 6 months. Reserves of vitamin K, however, may be sufficient for only a few weeks, and the supply of vitamin E can last somewhere between several days and several months.

If taken in excess, usually in the form of a supplement, fat-soluble vitamins can accumulate in the body. Large stores of vitamins A and D can actually become harmful. Fortunately, it is difficult to get an excess of fat-soluble vitamins from food.

For example, beta-carotene, the molecule found in some foods of plant origin that gives carrots and squash their yellow-orange color, is converted to vitamin A in the body.

But because the chemical reaction that converts beta-carotene to vitamin A is carefully regulated, it is nearly impossible to get vitamin A toxicity from eating fruits and vegetables. Foods that provide vitamins D, E, and K would need to be consumed in such quantity that achieving toxic levels of these nutrients is highly unlikely.

However, a high intake of vitamin Kcontaining foods may contribute to abnormal bleeding in people who are receiving blood-thinning (anticoagulant) medication. Therefore, it is important to keep foods that are high in vitamin K relatively constant. Vitamin E supplements also are not recommended for these individuals.

The recent development of diet drugs that work by inhibiting your body’s absorption of fat has raised concerns about the possibility that the use of such drugs could lead to deficiencies in the fat-soluble vitamins.

Preliminary results suggest that when taken as directed, these drugs may interfere with the absorption of beta-carotene and vitamin D from foods. If these findings are confirmed, persons who take these prescribed drugs to manage their weight also will be required to take a daily multiple vitamin that contains the fat-soluble vitamin.

As mentioned in the discussion of fat substitutes, early research on the fat substitute olestra showed that eating olestra-containing snack foods interferes with the body’s ability to absorb some of the fat-soluble vitamins from the other foods we eat.

To compensate for this effect, the manufacturers of snack foods that contain olestra were required to fortify these products with fat-soluble vitamins. Thus, eating olestra-containing snack foods does not seem to create a significant risk for deficiency of fat-soluble vitamins (unless you consistently choose to eat them in place of more nutritious foods).