Calcium

Calcium is an important mineral for bone and tooth structure, blood clotting, and nerve transmission; it has a DRI of 1,000 milligrams per day for adult men and women. Deficiencies are associated with skeletal malformations (as in rickets), increased skeletal fragility (as in osteoporotic fracture and stress fractures), and blood pressure abnormalities. There are few reports of toxicity from taking high doses of calcium, but it is conceivable that a high and frequent intake of calcium supplements may alter the acidity of the stomach (making it more alkaline), thereby interfering with protein digestion. Since there is competitive absorption between bivalent minerals (calcium, zinc, iron, and magnesium) in the small intestine, it is also possible that a high amount of calcium may interfere with the absorption of other minerals if they are present in the gut at the same time. Therefore, taking high-dose calcium supplements with a meal that contains iron, for example, may result in the malabsorption of iron and eventually lead to iron-deficiency anemia.

Food sources of calcium include dairy products (milk, cheese, yogurt), dark green vegetables (collards, spinach, chard, mustard greens, broccoli, green peppers), and dried beans and peas (lentils, navy beans, soy beans, and split peas). Calcium and several other minerals (especially iron, magnesium, and zinc) are easily bound to the oxalic acid in dark green vegetables, making the minerals unavailable for absorption. Therefore, although dark green vegetables are potentially good sources of calcium and several other minerals, these foods don't make the minerals easily available to us unless they are properly prepared. Oxalate is water soluble, so by dipping the vegetables for a few seconds into boiling water (blanching), a good deal of the oxalate is removed but the minerals remain. The vegetables can then be prepared as needed. This technique dramatically improves the delivery of calcium from vegetables and has been used by cultures that have not traditionally consumed dairy products (especially in Asia) for thousands of years. As a side benefit, vegetables that are blanched may also be more acceptable to eat. Oxalic acid has a bitter taste. Therefore, removal of oxalate has the added benefit of making the vegetables more pleasant tasting.

Numerous studies have assessed the relationships among calcium intake, physical activity, and bone density. Athletes most often take calcium supplements to reduce the risk of fracture (i.e., by improving bone density), not for the purpose of improving physical performance. Physical activity is known to enhance bone density, just as physical inactivity is known to lower bone density. However, the development and mineralization of bone are complex processes involving several factors, including growth phase (childhood and adolescence are associated with faster bone development), hormonal status (especially estrogen for women), energy adequacy, vitamin D availability, and calcium intake.

Since the early 1990s, the increased availability of an accurate bone-density measuring device called DEXA (dual energy X-ray absorptiometry) has dramatically improved the ability to measure bone density and determine fracture risk. Studies that used DEXA indicate that children and adolescents who have a calcium intake at or slightly above the RDA (up to 1,500 milligrams) have higher bone densities. Adequate calcium intake in adults may not increase bone density, but it lays the groundwork for stabilizing the bone. It seems prudent, therefore, to make certain that calcium intake is maintained at the DRI level, that adequate physical activity is maintained (not a problem for most athletes), and that the intake of vitamin D is adequate. A survey of elite gymnasts indicates that sunlight exposure is more related to bone mineral density than is calcium intake. This points to the integrated relationship between calcium and vitamin D and how critical it is to have nutrients working together to ensure optimal health.⁶¹

Another concern of many female athletes is amenorrhea (cessation of menses), which is strongly associated with either poor bone development in young athletes or bone demineralization in older athletes. The causes of amenorrhea are complex and include inadequate energy intake, eating disorders, low body fat levels, poor iron status, psychological stress, high cortisol levels, and overtraining. Put simply, elite female athletes who train hard are at risk. Anything that might lower risk, such as maintaining a good iron status and consuming enough energy, is useful for lowering the risk of developing amenorrhea. Even if an amenorrheic athlete has sufficient calcium intake, that alone would not suffice to maintain or develop healthy bones because the lower level of circulating estrogen associated with amenorrhea would inhibit normal bone development or maintenance.