Antioxidant vitamins and mineral supplementation, life span expansion and cancer incidence: a critical commentary
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Experimental evidence indicates a strong connection between oxidative damage, cancer, and aging. Epidemiological observations suggest that a diet rich in fruits and vegetables is associated with lower incidence of some cancers and longer life expectancy; since fruits and vegetables contain natural antioxidants, a considerable effort has been dedicated to understanding their effects in experimental studies and in human trials.
A: Effects of antioxidant-containing food and supplements on oxidation damage in humans. Intervention trials employing a variety of biomarkers have shown either a slight decrease in oxidation damage or no effect. B: Effects of selected antioxidants on mortality and cancer incidence. β-carotene and α-tocopherol, alone or in combination, increase cardiovascular and all-cause mortality or have no effect. In some studies, β-carotene and retinyl palmitate significantly increase the progression of lung cancer and aggressive prostate cancer. Protection against cardiovascular mortality or no effect of vitamin E has been reported, with an increase of all-cause mortality at dosages greater than 150 IU/day. Selenium showed beneficial effects on gastrointestinal cancer and reduced the risk of lung cancer in populations with lower selenium status. For multivitamin and mineral supplementation, no significant reduction of mortality or cancer incidence was observed, but some reports indicate a possible preventive effect in cervical cancer.
The majority of supplementation studies indicate no variation of general mortality and of cancer incidence or a detrimental effect on both. Antioxidant supplements so far tested seem to offer no improvement over a well-balanced diet, possibly because of the choice of the substances tested or of an excessive dosage. However, new natural or synthetic compounds effective in vitro and in experimental studies might still be worth investigating in human trials.