Vital minerals are found in many foods. Those supplying iron help oxygen exchange. Calcium and phosphorus are major components of bone, together with magnesium. Iodine is needed for the hormone controlling metabolic rate. Although sodium chloride (salt) is found in most diets, extra quantities may be needed in hot countries where people lost salt in sweat. Potassium is important in nerve and muscle function and fluorine, found chiefly in drinking water, protects children's teeth.