Calcium is the most abundant mineral in the human body. Of the two to three pounds of calcium contained in the average body, 99% is located in the bones and teeth. Calcium is needed to form bones and teeth and is also required for transmission of signals in nerve cells, and muscle contraction. The importance of calcium for preventing osteoporosis is probably its most well-known role.

Although calcium plays at least some minor role in lowering blood pressure, the mechanisms involved appear complex and somewhat unclear. The level of calcium in the blood is tightly regulated by parathyroid hormone (PTH), and low intake of calcium causes elevations in PTH, which in turn have been implicated in the development of hypertension. High calcium intake has also been associated with a reduced risk of cardiovascular disease in postmenopausal women.

By reducing absorption of oxalate, a substance found in many foods, calcium may be able to indirectly reduce the risk of kidney stones. However, people with a history of kidney stones must talk with a doctor before supplementing with calcium because such supplementation might actually increase the risk of forming stones for the small number of people who absorb too much calcium.

Calcium also appears to partially bind some fats and cholesterol in the gastrointestinal tract. Perhaps as a result, some older research suggests that calcium supplementation may help lower cholesterol levels.

Warning: Calcium supplements should be avoided by prostate cancer patients

Two new studies published in the November issue of The American Journal of Therapeutics provide additional scientific evidence that calcium citrate may be the optimal form of calcium supplementation. The studies suggest consumers might benefit more by taking a calcium citrate supplement as opposed to common calcium carbonate supplements.

Researchers at the Center for Mineral Metabolism and Clinical Research, University of Texas Southwestern Medical Center, Dallas, Texas conducted a "Meta-Analysis of Calcium Bioavailability: A Comparison of Calcium Citrate with Calcium Carbonate" (Sakhaee et. al., 1999), which evaluated 15 studies on the bioavailability of two of the most common forms of calcium supplements, calcium citrate and calcium carbonate. All but one study showed significantly greater absorption of calcium from calcium citrate than calcium carbonate by an average of 22%-27%, regardless of whether the candidate had eaten a meal or not.

"In the case of calcium supplements, bioavailability refers to the amount of calcium a person actually absorbs from a supplement, rather than the amount of elemental calcium the supplement contains before it is taken," explains lead author Khashayar Sakhaee, MD, University of Texas Southwestern Medical Center. "Although calcium carbonate contains more elemental calcium than calcium citrate, it is not as readily available to the body overall. The meta-analysis confirms our hypothesis that calcium citrate provides superior bio-availability of calcium.