Antioxidants are vitamins and other substances that help your body expel “free radicals,” harmful byproducts of cellular oxygen use. Antioxidants are believed to have cancer-fighting properties, aid in heart regulation, and strengthen immune systems. Dark, leafy greens are high in antioxidants such as beta-carotene (which your body turns into vitamin A), selenium, and vitamins C and E. Scientists are still unsure exactly which antioxidants work on which parts of the body, so a balanced diet is still the best diet.