ANTIOXIDANT VITAMINS AND CANCER PREVENTION

Environmental factors have been implicated in both the causes and prevention of cancers and considerable epidemiological evidence has accumulated to suggest that good nutritional status of vitamins A, E, C, the carotenes and riboflavin exerts a protective function. Oxygen radicals are implicated in the genotoxic events which can initiate and promote cancer development and this paper outlines the anti-oxidant roles which these vitamins might play. Retinal and riboflavin are preventive anti-oxidants, that is they maintain structural integrity of tissues and therefore good nutritional status may reduce endogenous oxidant production in tissues. In contrast, vitamins E, C and the carotenes are radical quenching anti-oxidants and may be more effective in countering the effects of exogenous stresses. The principal micronutrients of current interest in cancer prevention are the carotenoids and vitamin C. Factors influencing carotenoids in the circulation are discussed, particularly smoking, which not only reduces intake but may also affect absorption and 'metabolism' of carotenes. Also the implications of the recently-reported enterogastric circulation of vitamin C is discussed in terms of its role in countering the endogenous production of nitrosamines. Finally attention is drawn to the possibility that plasma retinol may function as an acute phase reactant and that low concentrations reported in epidemiological studies may be important indicators of occult disease and have no bearing on vitamin A status per se.