Q. What is the best way to take a calcium supplement?

A. Most Americans do not get enough calcium from food sources, so it's smart to make up the difference with a supplement. The average diet supplies 500 to 1,000 mg daily, but the recommended daily intake for adult men and women is at least 1,000 mg. Pregnant or lactating women need 1,200 to 1,500 mg per day. Postmenopausal women not on estrogen therapy, and all adults over 65, need 1,500 mg per day. Be careful not to overdo it, however: Too much calcium (over 2,000 mg per day) may increase your risk for kidney stones and impede absorption of other important nutrients such as zinc, iron, and magnesium.

To get the most out of your calcium supplement, it's best to take it in divided doses, with meals to improve absorption. If you take iron supplements, do not take your calcium and iron pills at the same time because they compete for absorption.

When shopping for a calcium supplement, avoid those derived from unrefined oyster shell, bone meal, or dolomite, because they may contain elevated levels of lead. And check that the brand you buy has the USP (United States Pharmacopeia) symbol, which ensures the product's purity.

Calcium carbonate is a popular, inexpensive form of supplemental calcium, but you may want to consider calcium citrate, which is more easily absorbed if you have lower-than-normal levels of gastric acid, a common condition for older individuals and those taking acid-reducing medications. In a recent study, calcium citrate prevented calcium loss from bone more effectively than calcium carbonate (Osteoporosis International, 2004, vol. 15, no. 4). Calcium malate/citrate, microcrystalline hydroxyapatite, or amino acid chelates are also good options. For some individuals, chewable and liquid calcium supplements may be more easily absorbed than tablets. Because calcium supplements can cause gas or constipation, you may need to try a few different forms to find one that works well for you.

This Q&A was written by Victoria Dolby Toews, MPH, author of User's Guide to Sexual Satisfaction (Basic Health, 2003).