Adequate intakes of beta-carotene and vitamin C have been linked in some studies to a reduced risk of cancers of the esophagus, stomach, pancreas, lung, colon, rectum, prostate, breast, ovaries and cervix. Several studies found that people with low dietary intakes or blood levels of antioxidants have a higher risk of these cancers.
In the studies that showed protective effects, fruits and vegetables —not supplements—were the key sources of these nutrients.
In fact, studies with supplements prove how difficult it is to distinguish between the effect of a food and the effect of one of its many components. At best, studies of antioxidant supplements have produced disappointing results, and a few have raised the possibility that high doses of a single antioxidant may be harmful. For example, the Women’s Health Study found that vitamin E supplements provide no protection against cancer, and two other studies found that beta-carotene supplements increase the risk of lung cancer in smokers.
Some researchers hypothesize that the large amounts of beta-carotene in the supplements increased lung cancer risk because the beta-carotene blocked the absorption of other carotenoids that are protective against cancer.
These studies should not stop you from eating foods that supply beta-carotene, which is only one of the hundreds of carotenoids found in foods. In fact, it’s virtually impossible to get too much of any nutrient from nonfortified foods alone, although a temporary condition called carotenemia, in which the skin takes on an orange-yellow color, can occur in people who consume excessive amounts of carrots or carrot juice.
Other carotenoids may protect against cancer. For example, some studies suggest that tomatoes and tomato-based products (such as tomato sauce, tomato juice and ketchup) are linked to a reduced risk of prostate cancer. Tomatoes are a leading source of lycopene, a carotenoid that gives tomatoes their red color.
Additional studies are needed to firmly establish whether lycopene, alone or in combination with other dietary components, reduces prostate cancer risk. The greatest protective effect of tomatoes appears to come from cooked tomato products, such as tomato sauce, since cooking concentrates the lycopene content and increases its absorption in the body. Watermelon is another good source of lycopene.
Selenium is a mineral associated with a reduced risk of various cancers. Selenium acts indirectly as an antioxidant; it is an essential component of enzymes that inactivate free radicals. Research indicates that people with a high intake of this mineral and those who live in areas where the selenium content of the soil is high have a lower risk of lung, colon, and other cancers.
Selenium is found in Brazil nuts, sunflower seeds, fish, turkey, wheat germ, and other grains, fruits and vegetables. The amount of selenium in plant foods depends on the amount of the mineral in the soil where the food is grown.
Preliminary studies suggesting that selenium supplements may reduce cancer risk are far from conclusive. In fact, one study found that men with naturally high levels of selenium doubled their risk of developing high-grade prostate cancer by taking selenium supplements. Further research is necessary to clarify the benefits and risks of this supplement.