Dietary levels of plant phenols and other non-nutritive components: could they prevent cancer?

Several non-nutritive components in fruits, vegetables, herbs and spices have been found to inhibit tumour formation in experimental animals exposed to carcinogens. The active non-nutritive components vary with respect to their chemical structures, and may be classed as phenols, terpenes, indoles, isothiocyanates, allyl sulphides or others. They also seem to work by different mechanisms, being inducers or inhibitors of various enzymes, antioxidants, scavengers of reactive metabolites, or inducers of apoptosis. The dietary levels are generally in the order of 1-100 mg/day for most classes of compounds in the Danish population, and similar levels are expected in most northern European countries. These levels are very low compared with the levels used in most animal experiments, where non-nutritive factors have individually been shown to have inhibitory actions on tumorigenesis. Human long-term intervention trials with antioxidants have generally been discouraging. In human short-term intervention studies, where increased dietary levels of specific vegetables or fruits are studied, doses are also comparatively low. Effects on important enzymes have been reported in several such studies, indicating that low levels of non-nutritive factors could influence carcinogenesis by specific mechanisms. Meta-analyses of cohort studies on specific food items rich in specific non-nutritive components, indicate that carotenoid- or glucosinolate-rich foods protect against some cancers, while flavonoid rich food items do not uniformly show protective effects.