ask your pharmacist

How do I choose a calcium supplement?

Calcium carbonate products are usually a good first choice. They're the least expensive and provide the most calcium per tablet. The downside: Calcium carbonate can cause bothersome stomach gas for some people. If this happens, calcium lactate or calcium citrate may be a better choice. But since these products contain less calcium per tablet, you'll need more or larger tablets to get the same amount of calcium as a tablet of calcium carbonate.

To properly digest products containing calcium carbonate or tricalcium phosphate, you need some acid in your stomach. That's why you take these forms of calcium with food, since food stimulates acid production. If you don't have enough stomach acid (a condition called achlorhydria), better calcium options may be calcium citrate or calcium gluconate, which don't require stomach acid for absorption.

Until recently, the various calcium compounds were thought to be equally effective at providing the body with calcium. But a recent study suggests that the calcium from calcium citrate is better absorbed into the bloodstream than calcium from calcium carbonate. The study involved volunteers who took either 500 mg of calcium citrate (Citracal) or 500 mg of calcium carbonate (Os-Cal) with a meal. When researchers analyzed blood samples from the volunteers over the next six hours, they found a greater increase with calcium citrate than with calcium carbonate. But it's not yet known whether this difference in absorption translates into greater protection from the bone loss that occurs in osteoporosis.

Some calcium supplements also contain vitamin D, which helps the body absorb calcium. Examples of calcium supplements with vitamin D include drugstore.com Calcium + D, Healthy Woman, >Posture-D, and >Viactiv-Soft Calcium Chews plus Vitamin D Supplement. Many people don't get enough vitamin D through sun exposure (which stimulates the body to make its own vitamin D) or dietary sources (such as seafood and dairy products), so supplementing with this nutrient can be important. But if you opt for a vitamin D supplement, you won't need to take more 400 International Units (IU) per day.

Your body can absorb only so much calcium at one time. Divide your total daily intake so that each dose is no more than 600 mg of calcium.

Sources:
Levenson, D. "A Review of Calcium Preparations," Nutrition Reviews 52 (1994).
Recker, R. "Calcium Absorption and Achlorhydria," New England Journal of Medicine 313 (1985).
Hansen, C. "Intestinal Calcium Absorption from Different Calcium Preparations: Influence of Anion and Solubility," Osteoporosis International 6 (1996).
Harvey, J. "Superior Calcium Absorption from Calcium Citrate than Calcium Carbonate Using External Forearm Counting," Journal of the American College of Nutrition 9 (1990).
Silverman, H.M., J.A. Romano, G. Elmer. The Vitamin Book. Bantam Books, 1999.
Heller, H. "Pharmacokinetics of Calcium Absorption from Two Commercial Calcium Supplements," Journal of Clinical Pharmacology (1999).
Drug Facts and Comparisons. Facts & Comparisons, 2007.
Medline Plus. Medline, 2007.

This answer prepared 11/2/2000.
This information updated 1/3/2007.