At this point the general consensus is that dietary supplementation of antioxidant compounds is of either no benefit or mildly harmful to long term health. The only methods of extending life via the introduction of antioxidant compounds involve careful targeting to the mitochondria, such that damaging oxidant molecules generated there are swept up, but the oxidant molecules used in signaling processes elsewhere in cells and tissues are not. The benefits of exercise, for example, rest upon slightly raised levels of reactive oxygen species which can be blocked by high levels of antioxidants in the diet:
In older men, a natural antioxidant compound found in red grapes and other plants - called resveratrol - blocks many of the cardiovascular benefits of exercise. What is emerging is a new view that antioxidants are not a fix for everything, and that some degree of oxidant stress may be necessary for the body to work correctly. This pivotal study suggests that reactive oxygen species, generally thought of as causing aging and disease, may be a necessary signal that causes healthy adaptations in response to stresses like exercise. So too much of a good thing (like antioxidants in the diet) may actually be detrimental to our health.
We studied 27 healthy, physically inactive men around 65 years old for 8 weeks. During the 8 weeks all of the men performed high-intensity exercise training and half of the group received 250 mg of resveratrol daily, whereas the other group received a placebo pill (a pill containing no active ingredient). The study design was double-blinded, thus neither the subjects nor the investigators knew which participant that received either resveratrol or placebo.
"We found that exercise training was highly effective in improving cardiovascular health parameters, but resveratrol supplementation attenuated the positive effects of training on several parameters including blood pressure, plasma lipid concentrations and maximal oxygen uptake. We were surprised to find that resveratrol supplementation in aged men blunts the positive effects of exercise training on cardiovascular health parameters, in part because our results contradict findings in animal studies. It should be noted that the quantities of resveratrol given in our research study are much higher than what could be obtained by intake of natural foods."