Prevention of cancer
Population-based (epidemiological) studies have clearly demonstrated that a diet high in beta-carotene is protective against a variety of cancers. The evidence is much stronger for beta-carotene than for vitamin A, presumably because beta-carotene exerts greater antioxidant and immune enhancing effects than vitamin A.
While there is little doubt that a diet high in carotenes is protective against cancer, the same cannot be said about beta-carotene. Based upon current evidence, it appears that on its own, synthetic beta-carotene supplementation does not prevent cancer. Large cancer prevention trials with synthetic beta-carotene in high-risk groups have produced negative results. In fact, the results of these studies indicate that synthetic beta-carotene supplementation may actually increase the risk for cancer and cardiovascular disease if people continue to smoke. The data strongly suggest that the protection offered by beta-carotene is apparent only when other important antioxidant nutrients (e.g., vitamins C and E and selenium) are provided. The results seem to indicate the need for a diet high in carotenes; if carotene supplementation is desired, people should not smoke, and they should protect against the formation of toxic beta-carotene derivatives by taking extra vitamin C and E and selenium.