Calcium is needed for- strong bones and proper muscle function. Dairy foods are the best source of calcium. However, studies show that many female athletes who are trying to lose weight cut back on dairy products. Female athletes who don't get enough calcium may be at risk for stress fractures and, when they're older, osteoporosis. Young women between the ages of 11 and 24 need about 1,200 milligrams of calcium a day. After age 25, the recommended intake is 800 milligrams. Low-fat dairy products are a rich source of calcium and also are low in fat and calories.