Today we are increasingly aware of the role played by free radicals in aging and disease. This has prompted many health-minded people to begin taking antioxidant supplements. But are these really helpful? 
Low levels of free radicals are constantly being produced within our cells, and all cells contain molecules (i.e. antioxidants) that can neutralize them. These neutralizing molecules are either produced within the cell or derived from our diet. High levels of free radical production combined with low levels of antioxidants in cells produces oxidative stress, and results in damaged cells. Yet not all free radicals are bad guys!
Healthy cells actually need low levels of free radicals. Small increases in free radicals act like chemical messengers telling the cells to adapt to various stimuli, including exercise. Taking away these messengers can be potentially harmful. It is the balance between these that really counts.
Exercise does increase the production of free radicals. The amount produced is influenced by the intensity and duration of activity, and by certain environmental factors (i.e., hot temperatures and high altitude). Low to moderate intensity exercise in people with low fitness will produce more oxidative stress than in people who are very fit. Consequently, regular moderate exercise is actually beneficial in maintaining low levels of free radicals and minimizing oxidative stress.
So, does that mean that people who exercise regularly should take antioxidant supplements? Maybe if you do extreme exercise, like 100-mile races. But recent studies have found that high doses of antioxidant supplements can actually prevent important exercise-induced adaptations in the skeletal muscles, specifically free radical’s signals to increase production of antioxidant enzymes. Antioxidant supplementation can also blunt the training adaptation that occurs with aerobic exercise.
Right now, there is no convincing evidence that active adults or athletes (who include fruits and vegetables in their well-balanced diet) need antioxidant supplementation. In fact, evidence shows that high-levels of supplementation might be detrimental.
The One who designed each detail and function of our beings said, “Look! I have given you every seed-bearing plant throughout the earth and all the fruit trees for your food… Then God looked over all he had made, and he saw that it was very good!” (Genesis 1:29, 31) Who could have done better?