In several animal models, beta-carotene and other antioxidant nutrients inhibit oral carcinogenesis. Epidemiologic studies consistently relate low intake of these nutrients with high cancer risk. Smokers have lower beta-carotene levels in plasma and oral mucosal cells than nonsmokers. Eight clinical trials have now shown that beta-carotene and vitamin E produce regression of oral leukoplakia, but chemoprevention studies in oral leukoplakia have limitations, which we review. All available evidence supports a significant role for antioxidant nutrients in preventing oral cancer.
Evidence supports the potential role of beta-carotene in cancer prevention.
Although observational epidemiologic studies are not entirely consistent, many show an inverse association between dietary intake or blood levels of beta-carotene and subsequent cancer risk. However, because of the long latency period for cancer, which may be a decade or more, the six-year duration of treatment in this trial may have been inadequate to detect an anticancer effect. A Chinese trial demonstrated a modest reduction in cancer mortality from a combined regimen of beta-carotene, vitamin E, and selenium. The effect of the individual agents could not be assessed, and because the trial was carried out among a nutritionally deficient population, its results may not have direct relevance to well-nourished individuals. The Physicians' Health Study, which is testing beta-carotene among 22,071 US male physicians, will have an average duration of treatment of 12.5 years at its scheduled termination in late 1995.