Dietary fruit and vegetables protect against somatic mutation in vivo, but low or high intake of carotenoids does not.

Epidemiological studies have demonstrated protective effects of vegetables and fruit on risk of cancer, but underlying mechanisms remain unclear. Intervention studies have in some cases contradicted previous epidemiological evidence, e.g. for beta-carotene supplementation and lung cancer, emphasizing the need for mechanistic data. We assessed in vivo mutagenic effects of several dietary items using the HPRT (hypoxanthine-guanine phosphoribosyl transferase) gene assay with T-lymphocytes from 312 individuals (158 lung cancer cases, 154 population controls), who provided information on diet and smoking habits. HPRT mutant frequency (MF) was significantly decreased in relation to intake of vegetables, citrus fruits and berries, respectively, as well as calculated vitamin C intake from diet. There was a significant U-shaped association with dietary carotenoid intake, with lowest MF near population average carotenoid intakes and higher mutation frequencies both at low and high intakes, and a similar borderline significant association was observed for beta-carotene. Our study is consistent with known diet-cancer associations and provides novel human in vivo mechanistic support for a cancer-protective effect of vegetables and fruit by modulation of somatic mutagenesis. Our results also provide support for the increase in lung cancer risk observed particularly in smokers in studies of beta-carotene supplementation.