As the human body evolved to utilise oxygen, it has also evolved elaborate defences in order to limit free radical damage. These come in the form of antioxidant enzymes, which the body produces naturally within the cells, such as superoxide dismutase. Dietary antioxidants can also be acquired from the food that we eat. People with poor diets, depleted of nutrients, may be at greater risk, as the body's antioxidant enzymes require nutrients such as vitamins A, C, E and minerals including zinc, selenium, copper and manganese in order to function properly. Antioxidants are substances that slow oxidation by neutralising free radicals and work by being able to donate or receive electrons. Both the enzymes and the nutrients play a role in breaking the damaging chain reactions. Research suggests that regular exercisers do have a much higher level of their natural antioxidant enzymes to help protect them.