Excessive free radical production can damage cell membranes and DNA, suppress the immune system and disrupt the function of the brain. Free radicals, however, are also important for generating the stress response that triggers increases in muscle mass and strength that builds physical fitness. The body has an antioxidant system that helps repair damaged tissues and minimizes the effects of free radicals. Antioxidants are produced naturally or consumed in the diet as whole foods or supplements. The brain is particularly susceptible to oxidative damage because of its rapid metabolic rate and high fat content. Anabolic steroids increased free radical production in the brain, which increases the risk of Parkinson’s disease, Alzheimer’s disease and (MS) multiple sclerosis. A study on rats by scientists from the University of Granada in Spain found that high-intensity exercise modified the negative effects of the anabolic steroid stanozolol on the brain. It is not known whether these results apply to humans.