Epidemiologic evidence for beta-carotene and cancer prevention.

In 1981 it was hypothesized that a high dietary intake of beta-carotene might reduce human cancer rates. Since then, several observational epidemiologic studies have addressed this topic. The results of both case-control and cohort studies show a remarkable consistency for the association of increased lung cancer risk with low amounts of dietary beta-carotene or low plasma beta-carotene concentrations. For stomach cancer, the evidence is also consistent, although the number of studies is more modest. For breast and prostate cancer, the studies indicate no consistent association of plasma or dietary beta-carotene and reduced cancer risk. For colorectal cancer, the effect will be moderate, if existent. For several other cancer sites, the numbers of cases in prospective studies are often small, implying that only strong associations can be detected. For some of these sites, results from retrospective studies are promising. The epidemiologic studies should be carefully interpreted because dietary habits may be misclassified and smoking may reduce plasma beta-carotene concentrations. Observational epidemiology cannot definitively resolve whether associations are indeed due to beta-carotene, or to other components of fruit and vegetables that are rich in beta-carotene. However, overall results are promising and several plausible cancer preventive mechanisms have been reported for beta-carotene. The ongoing human intervention studies will provide more answers regarding cancer prevention by beta-carotene but may need long follow-ups to be conclusive.