Calcium Marie Dunford, PhD, MD, Nutrition Consultant, Kingsburg, CA Calcium is an essential mineral and a major nutrient needed for proper bone health. Absorption of calcium in adults is low. Vitamin D aids in absorption. During childhood, adolescence, and early adulthood calcium helps to increase bone density. Adults need adequate calcium to slow the loss of calcium from bone. Insufficient dietary calcium intake is common among girls, female adolescents, and women. Female athletes often consume below recommended levels because of low-energy (calorie) diets and the avoidance of dairy products. Athletes suffering from the female athlete triad disordered eating, amenorrhea, and osteoporosis are at great risk for loss of bone calcium and need supplemental calcium. Calcium supplements are commonly prescribed for the prevention and treatment of osteoporosis. Consult with a physician before taking calcium supplements. Calcium intakes above 2,500 mg per day are associated with toxicity. Name Calcium Description Calcium is an essential mineral and a primary component of bone. The best dietary sources are milk and milk products (e.g., cheese and yogurt) and tofu that has been preserved with calcium sulfate. Green leafy vegetables such as broccoli or kale provide less calcium than milk but are important sources for individuals who do not consume dairy products. Orange juice and soy milk may be fortified with calcium. Supplements are in the form of calcium carbonate, calcium citrate, calcium gluconate, or calcium lactate with calcium carbonate containing the highest percentage of calcium (40%). Bone meal or oyster shells may be the source of calcium in some supplements.
Usage All but 1percent of the calcium in the body is found in bones where it is an integral part of the bone structure. The calcium stored in the bones also serves as a bank account providing calcium to the blood. Blood calcium levels are strictly regulated by hormones and must remain stable to supply the calcium that is necessary for muscle contraction, nerve impulse transmission, blood pressure maintenance, and blood clotting. Low calcium intake is associated with osteoporosis, hypertension, and colon cancer. Bone health is influenced by genetic, mechanical (e.g., weight-bearing exercise), hormonal, and nutritional factors. Calcium is the major nutritional factor in the prevention and treatment of osteoporosis, a disease characterized by low bone density. From birth until approximately age 25, bone density increases as calcium is actively deposited in bone. An adequate calcium intake is needed at this time to ensure maximum bone density. After age 40 bones slowly decrease in density as calcium is resorbed from the bone. An adequate calcium intake at this time may help to slow the loss and delay the onset of osteoporosis. After menopause, loss of bone density increases significantly as a powerful hormonal influence, estrogen, is no longer present to help keep calcium in the bone. Post-menopausal calcium intake may help to slow, but cannot prevent, the onset of osteoporosis. Prevalence Calcium supplements are popular especially with middle-aged and older women. The prevalence of calcium supplementation in the athletic population is unknown. A 1999 study of athletes in a Division I university found approximately 20 percent of the female athletes took supplemental calcium whereas only a small percentage of male athletes did. Chemical Mechanism Calcium is found in both food and supplements as a relatively insoluble calcium salt. The acid ph and the presence of food in the stomach help to increase solubility. Absorption of calcium takes place in the small intestine and is considered poor with an estimated average absorption in adults of 30 percent. Men absorb more than non-pregnant women do. Pregnant women have a greater absorptive capacity but only to approximately 50 percent of calcium intake. While the body can increase its absorption slightly, when dietary intake is low, it cannot fully compensate for low dietary calcium simply by increasing absorption. Calcium absorption requires the presence of vitamin D because vitamin D regulates the transport of calcium both through and between the cells of the small intestine. When blood calcium levels are low, vitamin D exerts its influence by increasing calcium absorption from the gut and increasing calcium retention by the kidney. Unfortunately, vitamin D activity decreases with aging and the absorption and retention of calcium in older adults is a problem. The presence of lactose (milk sugar), lactase (the enzyme that breaks down lactose), and the acidic amino acids, lysine and arginine, are all factors that improve calcium
absorption. On the other hand, decreased absorption is associated with diets high in fiber. Foods such as whole grains and spinach are high in phytates and oxalates, compounds known to bind with calcium and reduce absorption. High-protein diets and alcohol intake increase urinary calcium excretion. The metabolism of protein results in excess acid production. Calcium carbonate found in bones is withdrawn and the carbonate is used to neutralize the acid. The calcium is then excreted in the urine. The diuretic action of alcohol also results in the loss of calcium via the urine. Some antacids contain aluminum at levels that increase urinary calcium excretion. Clinical Evidence The use of calcium supplements in the prevention and treatment of osteoporosis is common but not without controversy. Some studies have shown that supplementation with calcium and vitamin D can reduce bone loss and decrease the risk for fractures in adults, including the institutionalized elderly. Other studies have not shown a protective effect. Slowing the loss of calcium from bone with supplements may be related to the site (radius vs. spine vs. hip) or to the length of time since menopause (the body may respond differently during the first five years after menopause than six to ten years postmenopause). Until calcium supplementation is proven to be ineffective, it is prudent to include it as part of the prevention and treatment for osteoporosis. Daily calcium need varies according to age. Adolescents should consume 1,300 mg of calcium daily. It is recommended that adults age 19 to 50 consume 1,000 mg per day. After age 50, 1,200 mg of calcium daily is needed. Post-menopausal women not receiving hormone replacement therapy (estrogen) should consume 1,500 mg per day. As people age, energy (calorie) intake may decline and many older people find it difficult to obtain 1,200 mg of calcium daily through food alone. Obtaining 1,500 mg solely from dietary sources is almost impossible. Insufficient dietary calcium intake is common among girls, female adolescents, and women. Boys and men consume more food and thus more calcium. Scientific Research Studies of athletes dietary intake report that male athletes age 19 to 50 generally receive enough calcium. Female athletes often consume below recommended levels and this is attributed to low-energy diets and avoidance of dairy products. Of particular concern are those female athletes who suffer from disordered eating, amenorrhea, and osteoporosis, a syndrome known as the female athlete triad. In a quest to be thin they may engage in disordered eating and become amenorrheic. The decrease in estrogen production coupled with low calcium and energy intake can significantly decrease bone density at a young age. The risk for fracture is also increased. In one study, bone densities of 14 amenorrheic female athletes were compared with those of 14 female athletes with normal menstruation. The average age of the 14 athletes with amenorrhea was 25 years. However, the density of their bones was equivalent to what would be expected of a 51-year-old. Although the women were
chronologically young, because of their amenorrhea, they were physiologically similar to post-menopausal women. When the amenorrheic women resumed menstruation bone density increased but it remained below the average values for women of the same age who had never been amenorrheic. Those diagnosed with the female athlete triad receive medical, psychological, and nutritional treatment including calcium supplements (1,500 mg daily) with vitamin D (400-800 IU daily). Administration Calcium supplements are ingested orally as pills or tablets. Supplemental calcium may also be obtained from antacids (e.g., Tums). Calcium carbonate contains the most calcium per pill but absorption may be less than other forms. Calcium citrate is often prescribed for older women because it is not dependent on adequate gastric acid for optimal absorption. Calcium supplements are better absorbed when taken with food. Dosage The Tolerable Upper Intake Level (UL) for calcium is 2,500 mg per day. Intakes above the UL are associated with toxicity. The recommended calcium intake for an adult is between 1,000 and 1,500 mg daily depending on age and menopausal status. Physicians often prescribe calcium supplements of 800 to 1,000 mg per day. This assumes a dietary calcium intake between 400 and 700 mg per day, the amount commonly consumed by people in North America. The amount of calcium in a supplement also affects its absorption. Small doses (400-500 mg) taken more frequently are better absorbed than a single large dose. Many calcium supplements also contain vitamin D, a nutrient known to increase calcium absorption. The ability of the tablet to dissolve depends in part on how it is manufactured. A simple test of dissolution is to put the calcium supplement in one half cup of vinegar. Stir occasionally. After thirty minutes the tablet should be dissolved. Contraindications Individuals with hyperparathyroidism, kidney disease, or a history of kidney stones should consult with a physician before taking calcium supplements. Precautions/Warnings Calcium supplements should not be self-prescribed. Calcium supplements can inhibit the absorption of iron, zinc, magnesium, and phosphorous. When taken with tetracycline, the two compounds bind impairing the absorption of both. Calcium supplements increase the risk of kidney stones in those individuals who are susceptible to them and in rare cases contribute to milk alkali syndrome, a condition of high blood calcium. A common side effect of calcium supplementation is constipation. Several organizations concerned with bone health have concluded that calcium obtained from food is preferable to calcium obtained in supplements. Supplemental calcium obtained from bone meal, oyster shells, or dolomite (limestone)
may contain contaminants such as lead. If these sources are chosen it would be prudent to contact the supplement manufacturer and find out the amount of lead contained. Some antacids (e.g., Rolaids) contain aluminum or magnesium hydroxides and increase calcium excretion. If antacids are used as a source of calcium check the ingredient list for the presence of aluminum or magnesium hydroxide. Banned/Permitted Calcium supplements are not banned by any athletic governing organization. Legality Calcium is a legal substance. References 1. Bellantoni, M.F. Osteoporosis prevention and treatment. Am Fam Phys. 1996;54(3):986-992, 995-996. 2. Dawson-Hughes, B. Calcium supplementation and bone loss: a review of controlled clinical trials. Am J Clin Nutr. 1991;54 (1 suppl):274s-280s. 3. Drinkwater, B.L., Nilson, K., Chesnut, C.H., Bremner, W.J., Shainholtz, S. and Southworth, M.B. Bone mineral content of amenorrheic and eumenorrheic athletes. N Engl J Med 1984;311(5):277-281. 4. Hosking, D.J., Ross, P.D., Thompson, D.E., Wasnich, R.D., McClung M., Bjarnason, N.H., Ravn, P., Cizza G., Daley, M., Yates, A.J. Evidence that increased calcium intake does not prevent early postmenopausal bone loss. Clin Ther 1998;20(5):933-944. 5. Krumbach, C.J., Ellis, D.R. and Driskell, J.A. A report of vitamin and mineral supplement use among university athletes in a division I institution. Int J Sport Nutr. 1999;9(4):416-425. 6. New, S.A. Bone health: the role of micronutrients. Br Med Bull. 1999;55(3):619-633. 7. Manore, M. and Thompson, J. Sport Nutrition for Health and Performance. Champaign, IL, Human Kinetics; 2000. 8. Sizer, F.S. and Whitney, E.N. Nutrition: Concepts and Controversies. 8th ed. Stamford, CT, Wadsworth; 2000. 9. Yates, A.A., Schlicker, S.A. and Suitor, C.W. Dietary Reference Intakes: The new basis for recommendations for calcium and related nutrients, B vitamins, and choline. J Amer Dietet Assoc. 1998;98(6):699-706 Degree of Confidence 1.0 Human Kinetics Publishers, Inc.