Micronutrients, antioxidants and risk of cancer

Currently there is little evidence that vitamins of any type are able to greatly modify the progression of established malignancy with the exception of promyelocytic leukaemia. In contrast, there is considerable laboratory evidence from chemical, cell culture and animal studies that antioxidant vitamins and related micronutrients are able to slow, or possibly prevent the carcinogenic process. There is a good theoretical basis for these findings. Current theories for the mechanism of tumourigenesis suggest that reactive species and prooxidants promote and encourage the process whilst antioxidants are inhibitory and protective. Retinoids and folate with limited or no antioxidant activity may protect DNA in other ways. In man there is support for this role from the extraordinarily high concentrations of ascorbate and possibly alpha-tocopherol at sites where oxidant stress is likely to be most intense, with loss of such antioxidant protection in some conditions which predispose to malignancy. There is also impressive epidemiological agreement, particularly from observational studies, where the lowest fruit and vegetable intake has been consistently associated with increased risk of cancer, especially of the lung and gastrointestinal tract, but much less evidence that such low intakes can encourage the development of cancers which are under hormonal control. Where individual micronutrients have been considered, beta-carotene appears to have the strongest protective effect followed by vitamin C and vitamin E. Whilst the experimental studies have suggested a role for retinoids, this has not been confirmed by the observational studies. Unfortunately, with the exception of oral leukoplakia, studies investigating reversal of premalignant conditions have been disappointing, and two intervention studies aimed at prevention in large populations have produced conflicting results. All this begs the question as to what dietary advice or intervention, if any, should be provided prior to the publication of the many randomized intervention studies that are presently investigating the role of micronutrients in cancer prevention. Gey [73] has produced recommendations for minimum blood concentrations and intakes of antioxidant micronutrients. (ABSTRACT TRUNCATED AT 250 WORDS)