Christine Rosenbloom, a professor of nutrition at Georgia State University and spokesperson for the American Dietetic Association, provides the following answer:

Vitamins and minerals in supplements are synthetic forms of the nutrients. The word "synthetic" doesn't necessarily mean inferior, however. Even those supplements that claim to have "natural" ingredients contain some synthetic ingredients. Indeed, if a pill contained only natural ingredients, it would be the size of a golf ball.

For the most part, our bodies appear to absorb synthetic forms as well as they do natural forms. The one exception seems to be vitamin E, which in natural form (RRR-alpha tocopherol) is better absorbed than in synthetic form (all-rac-alpha tocopherol). But most supplements now contain more natural vitamin E, so it is well absorbed in pill form.

For absorption to occur, a pill must dissolve and disintegrate. So when shopping for supplements, look for the USP symbol. This symbol indicates that the U.S. Pharmacopeia, an independent testing organization, has tested the supplement to make sure it will dissolve in your stomach. The absorption of nutrients in pill form is not well studied, but if they dissolve in the stomach, they are probably well absorbed.

Look for a supplement that contains about 100 percent of the daily values for nutrients. Don't spend extra money on products that are marked "high potency," "stress formula," or "laboratory approved." The supplement industry is not well regulated and claims can be made without much scientific proof. And don't forget, "food first." Foods contain substances other than vitamins and minerals for good health. Fruits, vegetables and whole grains contain phytochemicals, or plant chemicals, that can help to fight the development and progression of many chronic diseases, including cancer.

Luke Bucci, vice president of research for Schiff natural vitamins and supplements, offers this explanation:

For the 13 acknowledged vitamins (A, B1, B2, B3, B6, B12, folate, biotin, pantothenate, C, D, E and K), sometimes the usual forms found in multiple vitamin pills are identical to those found in foods, and sometimes not. Pharmaceutical manufacturers have singled out the most useful and most stable forms of each vitamin over the past 100 years for use in vitamin pills. With only a few exceptions, the vitamins in pills are utilized and handled by the body just as efficiently, or more so, than the vitamin forms found in foods. Indeed, some of the vitamin forms (called vitamers) found in foods are less active and less easily converted into activated forms than the vitamers used in pills.

One exception is vitamin E. The natural forms, called d-alpha-tocopherols, are absorbed and utilized twice as well as the synthetic forms, denoted by dl-alpha tocopherol. There might also be some differences in utilization between synthetic and natural beta carotene (the vitamin A precursor), vitamin Ds and vitamin Ks.