Biological effects of β-carotene

Several carotenoids show enhancement of the immune response, inhibition of mutagenesis, reduction of induced nuclear damage, and protection from various neoplastic events in cells, tissues, and whole animals. Carotenoids also protect against photo-induced tissue damage. Some carotenoids, including β-carotene, quench highly reactive singlet oxygen under certain conditions and can block free radical-mediated reactions. There is a growing body of literature on the effects of β-carotene in human chronic diseases, including cancer. Evidence from observational epidemiological studies has shown that a high consumption of fruits and vegetables rich in carotenoids is associated with a low risk for cancer. However, some human intervention trials failed to demonstrate prevention of cancer by β-carotene supplements. Several studies have indicated that among subjects who neither smoked cigarettes nor drank alcohol, β-carotene was associated with a marked decrease in the risk of one or more recurrent adenomas but β-carotene supplementation conferred a modest increase in the risk of recurrence among those who smoked. An increase in the risk of lung cancer among smokers and asbestos workers who took β-carotene supplements is also reported. In fact this trial raises the possibility that these supplements may actually have harmful as well as beneficial effects. Alcohol intake and cigarette smoking appear to modify the effect of β-carotene supplementation on the risk of colorectal adenoma recurrence. Similarly, serum β-carotene levels have been associated with a decreased chance of developing cancer. This results show a remarkable consistency for the association of increased lung cancer risk with low amounts of dietary β-carotene or low plasma β-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 β-carotene and reduced cancer risk. For colorectal cancer, the effect will be moderate, if existent. Whatever the results of these trials, carotenoids clearly show biological actions in animals distinct from their function as precursors of vitamin A. This review is an up-to-date and comprehensive analysis of pharmacological, toxicological reports and clinical applications of the β-carotene.