A high dietary calcium intake combined with vitamin D can increase bone density and reduce fractures in older women and, probably, men. The Institute of Medicine recommends that persons 19 to 50 years of age consume 1,000 mg per day of calcium and that persons older than 50 years consume 1,200 mg per day. The average American consumes less than 800 mg of calcium per day. Medical Letter consultants reviewed the efficacy and safety of current calcium supplements (see the accompanying table).

Calcium supplements are derivatives of natural products, such as oyster shell or bone. Some are marketed primarily as antacids. Calcium carbonate and phosphate preparations have the highest concentration of elemental calcium, about 40 percent. Calcium citrate contains 21 percent elemental calcium, and calcium lactate and gluconate contain 13 and 9 percent, respectively. A recent meta-analysis found that absorption of calcium citrate was 27 percent higher than that of calcium carbonate when taken on an empty stomach and 22 percent higher when taken with meals. It did not make a difference which preparation of calcium was taken or in which dosage.

Taking calcium with food in dosages of 500 mg or less increases absorption. Calcium preparations must be dissolved before they can be absorbed; the absorption rate for calcium is about 20 to 30 percent. The absorption of calcium supplements differs by preparation. Foods such as spinach, rhubarb and wheat bran can decrease calcium absorption. Calcium can interfere with absorption of iron, zinc, bisphosphonates and tetracycline. Absorption also requires adequate doses of vitamin D. The recommended daily intake of vitamin D is 200 IU for adults younger than 50 years and 600 IU for those older than 70 years. Vitamin D supplementation is especially important in elderly persons because skin synthesis and absorption of vitamin D may be impaired.

In postmenopausal women with low dietary calcium intake, specially prepared 500-mg tablets of calcium citrate malate were more effective in preventing bone loss than 500-mg tablets of calcium carbonate, although the difference was not significant. In another study, 500 mg of calcium citrate taken with breakfast produced serum calcium levels significantly higher than those demonstrated after 500 mg of calcium carbonate.

The most common adverse effects of calcium supplements are constipation, intestinal bloating and excess gas. Adverse effects occur most frequently with calcium carbonate. Switching preparations or increasing fluid intake may relieve symptoms. Patients who form calcium-containing stones are generally advised not to take calcium supplements. However, a low intake of calcium can aggravate the risk of stone formation by increasing absorption and urinary excretion of oxalate. High calcium intakes can increase stone formation in patients with absorptive hypercalciuria. Those with renal hypercalciuria may experience increased bone loss if calcium intake is too low.

Medical Letter consultants reiterate the need for adequate calcium supplementation in all persons, regardless of age or sex. Calcium carbonate may cause more adverse events. Calcium citrate may be better absorbed than the other calcium salts. The consultants conclude that any calcium supplement taken in dosages of 500 mg or less with meals may improve absorption.