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Any woman who has been faithfully swallowing her daily calcium supplements in hopes of staving off osteoporosis and colorectal cancer can be forgiven for being confused by news last month that two major studies found that the pills provide little or no benefit against either condition. But as is so often the case with complicated health studies, it pays to dig beyond the headlines.

First the news. A U.S. study of more than 36,000 healthy postmenopausal women determined that taking a standard calcium-and-vitamin-D supplement for seven years had no significant effect for most of them on preventing fractures in the spine, arms and hips, although it did lead to a 1% improvement in hip-bone density. Yet women who managed to take the vitamin-mineral combo at least four days out of five had a statistically significant 29% fewer hip fractures. And women over 60 suffered 21% fewer broken hips.

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A companion study found no beneficial effect on the rate of colorectal cancer. But those women were not at any particular risk of colorectal cancer. Other studies have concluded that men and women who have already had one precancerous polyp surgically removed from their intestinal tract develop fewer subsequent polyps if they take calcium supplements.

The take-home message? Calcium and vitamin D supplements are no magic bullets, but if you're going to take them, try to take them every day.

These latest studies, which are part of the giant U.S. Women's Health Initiative, are not likely to be contradicted anytime soon. Investigators tested the benefits of calcium and vitamin D in the most scientifically rigorous way possible—with a double-blind, placebo-controlled trial.

But there are still some quirks in the data. More than half the participants were also on hormone therapy, which is known to increase bone density. Moreover, most of the studies' subjects were already getting more calcium and vitamin D from their diet than the average American woman. Maybe supplements work best in people who need them most. Finally, the women were on the chunky side, which also protects bones.

With all those caveats in mind, however, it seems clear that the benefits of calcium and vitamin D supplements are small for most women and probably fairly modest even for those at greatest risk of cancer or bone loss. In addition, the studies found that women who take extra calcium have a 17% greater risk of developing kidney stones.

Fortunately, there are other ways to increase bone density. Weight-bearing exercises help, and there are several prescription drugs that have been proved to work. For more information, talk to your doctor. And be sure to have a colonoscopy by age 50—or 45 if you are of African descent. (You may need one even earlier if you have a family history of colorectal cancer.) It's a lot to keep track of, but nobody ever said prevention was easy.

Sources of Vitamin D and Calcium:
• Vitamin D (400 IU to 600 IU needed daily): sunlight (as much as 20,000 IU in 15 min. at midday), salmon (103 IU per oz., or 28 g), fortified milk (12.5 per fl. oz. [30 ml]), fortified orange juice (12.5)
• Calcium (1,000 mg to 1,500 mg needed daily): Atlantic sardines (108 mg per oz. [28 g]), canned salmon (68), low-fat plain yogurt (52), tofu (30), canned white beans (21) and regular skim milk (35 per fl. oz. [30 ml])

• Christine Gorman
• Separating fact from fiction on the benefits of calcium and vitamin D supplements