Fruits, such as blueberries, are good antioxidants because they contain phenolics that preferentially react with free radicals. Oxidative damage from free radicals plays an important role in several diseases such as cancer, Alzheimer's disease, and heart disease. It’s also been shown that strenuous exercise in people and dogs produces free radicals and that maintaining antioxidant levels by supplementing the diet with blueberries may prevent exercise-induced oxidative damage.