A growing body of literature exists regarding the effects of beta-carotene and other carotenoids on chronic diseases in humans. This article reviews and critically evaluates this literature and identifies areas for further research. This review is restricted to studies in humans, with a major emphasis on the most recent literature in the area of carotenoids and selected cancers. Effects of carotenoids on cardiovascular diseases, photosensitivity diseases, cataracts, and age-related macular degeneration are also discussed briefly. Numerous observational studies have found that people who ingest more carotenoids in their diets have a reduced risk of several chronic diseases. However, intervention trials of supplemental beta-carotene indicate that supplements are of little or no value in preventing cardiovascular disease and the major cancers occurring in well-nourished populations, and may actually increase, rather than reduce, lung cancer incidence in smokers. As a consequence of these findings, some of the ongoing trials of beta-carotene and disease prevention have been terminated or have dropped beta-carotene from their interventions. Researchers should now seek explanations for the apparently discordant findings of observational studies vs. intervention trials. The most pressing research issues include studies of interactions of carotenoids with themselves and with other phytochemicals and mechanistic studies of the actions of beta-carotene in lung carcinogenesis and cardiovascular disease. Paradoxically, the finding that lung carcinogenesis and cardiovascular disease can be enhanced by supplemental beta-carotene may ultimately lead to a clearer understanding of the role of diet in the etiology and prevention of these diseases. The conclusion that major public health benefits could be achieved by increasing consumption of carotenoid-rich fruits and vegetables still appears to stand; however, the pharmacological use of supplemental beta-carotene for the prevention of cardiovascular disease and lung cancer, particularly in smokers, can no longer be recommended.
