Simply put, supplements are no substitute for fruits and vegetables. To obtain the full spectrum of health-promoting phytochemicals (plant-derived compounds), you must consume the foods themselves. Further, nutrients and phytochemicals in supplements do not always assume the same form as those in foods.
Synthetic beta-carotene, for example, contains primarily one form of beta-carotene, namely, all-trans beta-carotene. In addition to all-trans beta-carotene, many plant foods contain significant amounts of the several cis forms of beta-carotene, which may be stronger antioxidants than the all-trans form (the prefixes "cis" and "trans" refer to the three-dimensional configurations of the beta-carotene molecule, as determined by which side of the molecule certain side chains are positioned). In one Israeli study, people who received a supplement containing both cis and trans beta-carotene forms showed evidence of increased antioxidant activity, whereas those who received synthetic all-trans beta-carotene did not show such an increase.1
The potential dangers of supplementation were highlighted by a couple of recent studies showing that smokers who received synthetic beta-carotene supplements had a higher risk of developing lung cancer over time than those who did not receive the supplements. Accordingly, smokers are now warned against taking such supplements. The question we must now ask is, "Why would beta-carotene have this effect in smokers?" First, we must realize that smokers have depleted stores of many antioxidants, including beta-carotene. One reason is that cigarette smoke contains powerful prooxidants that tie up the body's antioxidants. Another reason may be that the diet of many smokers contains fewer vegetables and more of the immediate gratification-producing animal products. It is quite possible, therefore, that synthetic beta-carotene supplementation produces a much higher ratio of all-trans beta-carotene to cis beta-carotene in smokers than it does in nonsmokers, since the latter have much more cis beta-carotene to begin with. Having proportionally less cis beta-carotene may have been what was harming the smokers. Had they received a natural supplement containing cis beta-carotene as well, the results might have been different: that is my hypothesis, at least.
Vitamin E provides another good example. Some studies have shown that vitamin E from foods, as opposed to that from supplements, may help prevent certain diseases. Why should that be? In nature, vitamin E consists primarily of eight forms, four tocopherols, called alpha-, beta-, gamma-, and delta-tocopherol, and four tocotrienols, with the same prefixes. Although alpha-tocopherol is the best known of these forms, gamma-tocopherol predominates in the diet. This gamma-tocopherol may be a better antioxidant than alpha-tocopherol with regard to certain oxidants, and many tissues of the body seem to concentrate it. These findings explain why higher concentrations of gamma-tocopherol in the blood are associated with a lower risk of heart disease and of prostate cancer. The tocotrienol forms of vitamin E may be even better antioxidants.
Unfortunately, vitamin E supplements, even those marketed as "mixed tocopherols," generally consist primarily of alpha-tocopherol. Therefore, supplements are not a good substitute for the full spectrum of vitamin E that comes from the diet. The richest sources of vitamin E in the diet include soyfoods, whole grains (wheat germ), pistachios, pecans, peanuts, walnuts, Brazil nuts, blackberries, and blueberries. Sesame seeds are not only rich in gamma-tocopherol, they also contain a compound called sesamin that inhibits the liver's conversion of gamma-tocopherol to an inactive metabolite, thereby increasing concentrations of gamma-tocopherol in the body to a much greater extent.
Can taking ordinary vitamin E supplements be harmful, then? In large dosages, it is quite possible. A recent major clinical trial showed that, in patients who have blood vessel disease or diabetes, supplementation with 400 I.U. of natural vitamin E (alpha-tocopherol) not only does not prevent cancer or cardiovascular disease but may actually increase the risk of heart failure.2 Again, we seek an explanation for this possible adverse effect. We know that when people take megadoses of alpha-tocopherol, the concentrations of gamma-tocopherol in their blood plummet. It may be, then, that supplementation with alpha-tocopherol results in a "crowding out" effect in which the body suffers from a lack of the other forms of vitamin E, such as gamma-tocopherol.
In summary, supplements are not the key to good nutrition and may even be harmful. Most people should not even need to take a multivitamin/multimineral supplement. Such a supplement might possibly be beneficial to an elderly person consuming the average, not-so-healthful diet, but even there the evidence is conflicting. To reiterate the obvious, good nutrition means consuming a wide variety of whole plant foods, including vegetables, whole grains, fruits, and nuts and seeds in moderation.
1Ben-Amotz A, Levy Y. Bioavailability of a natural isomer mixture compared with synthetic all-trans beta-carotene in human serum. Am J Clin Nutr 1996;63:729-734.
2The HOPE and HOPE-TOO Trail Investigators. Effects of long-term vitamin E supplementation on cardiovascular events and cancer. JAMA 2005;293:1338-1347.