If you are healthy, living at home, and take calcium and vitamin D supplements to prevent broken bones or cancer, you may want to reconsider, according to the U.S. Preventive Services Task Force (USPSTF). The group's new draft recommendations, published online on June 12, add more fervor to the ongoing debate about the benefits and risks associated with these popular supplements.

The USPSTF based its recommendations on a meta-analysis of 136 clinical trials, observational studies and a systematic review evaluating the effects of supplementation on fracture and cancer risk. It concluded that postmenopausal women should not take supplements containing less than 400 international units (IU) of vitamin D and 1,000 milligrams of calcium, noting that there is not enough evidence to evaluate the effects of taking more than those amounts. There was also insufficient data for the task force to determine whether or not healthy younger women and men of any age should take the supplements.

There's no question that "vitamin D and calcium are essential nutrients for a healthy diet," says task force member Kirsten Bibbins-Domingo, an epidemiologist and biostatistician at the University of California, San Francisco. The heart, muscles and nerves cannot work properly without the minerals, and deficiency can also cause bone loss, osteoporosis and tooth damage. Vitamin D is crucial in part because it helps the body to absorb calcium from food. In May the USPSTF recommended that people over 65 take vitamin D supplements if they have an increased risk of falls, for instance because of mobility problems or a history of falling.

But for healthy people the benefits are less clear-cut, Bibbins-Domingo says. Part of the problem is that trials involving vitamin D and calcium are inconsistent—they test different doses for varying lengths of time in diverse populations, which "makes it extra tricky when you try to put them all together and come up with a final analysis," says Erin LeBlanc, an endocrinologist at the Kaiser Permanente Center for Health Research in Portland, Ore. Earlier this week LeBlanc and her colleagues published a study online in the Journal of Women’s Health suggesting that women over 65 with higher blood vitamin D levels gain less weight over time than women with lower amounts.

Despite the difficulty of making firm conclusions about the data, some experts find the USPSTF recommendations, which are open for public comment until July 10, misleading and potentially dangerous. "I think they went too far," says Susan Ott, a bone specialist at the University of Washington School of Medicine. For one thing, "they ignored a lot of people who are getting really no calcium in their ordinary diet."

Indeed, the recommendations only apply to healthy people who are not deficient in either nutrient. (The task force did not evaluate the benefits of supplementation in people with osteoporosis, but the National Osteoporosis Foundation recommends that people with the bone disease consume at least 1,200 milligrams of calcium and 800 to 1,000 IUs of vitamin D daily through food or supplements.) Although the Institute of Medicine maintains that most Americans get enough vitamin D from food and sunlight, the organization notes that nearly 75 percent of American women between the ages of 31 and 50 do not get the recommended 1,000 milligrams of calcium from their food every day, and that a similar percentage of women older than 50 fail to consume their recommended 1,200 milligrams. (To eat 1,000 milligrams without supplementing, a person would have to eat 16 cups of cooked broccoli or drink three and a third glasses of skim milk daily.) "There are a lot of people out there who are only getting half of what we recommend," Ott says, and the USPSTF recommendations do not apply to them.

Experts have also raised questions about one of the trials published in the meta-analysis that the task force used to make its recommendations. Nearly 70 percent of the postmenopausal women in their overall analysis—a total of 36,282 individuals 50 to 79 years of age—were part of a large 2006 clinical trial supported by the National Institutes of Health and the Department of Health and Human Services called the Women's Health Initiative, which assessed the effects of supplementing with 1,000 milligrams of calcium and 400 IUs of vitamin D per day for an average of seven years. The trial reported that the pills did not prevent broken bones any more than placebos did and, moreover, that they increased the risk of kidney stones by 17 percent. These findings formed much of the basis for the task force's conclusion that postmenopausal women should not take low-dose calcium and vitamin D pills to prevent fractures.

But the USPSTF "simply didn't look very analytically at the data," says Robert Heaney, a bone nutrient specialist at the Creighton University Osteoporosis Research Center in Omaha. Before enrolling in the trial 39 percent of the women were already getting at least 1,200 milligrams of calcium from diet or supplements and 41 percent were already getting the recommended 400 IUs of vitamin D—and they were not advised to change their intake during the trial. As a result, in many cases the women in the treatment group had a daily calcium intake that far exceeded the Institute of Medicine's recommendations, which might help explain the heightened kidney stone risk, Heaney says. And the fact that the treatment was not replacing a deficiency in these women might explain why the intervention did not prevent broken bones. A subgroup analysis of the women who did not take their own calcium supplements revealed that supplementing as part of the trial did reduce hip fracture risk by 30 percent.

Ultimately, Ott agrees with the task force recommendations: women who get enough calcium and vitamin D from their diet shouldn't take pills. "Many people are getting too much," she says. But she fears for the millions of women who feel perfectly healthy yet are in fact deficient in calcium or vitamin D, and who erroneously think as a result of the task force recommendations that they are in the clear. "I'm worried that all those people are going to stop taking their supplements, and then they will be sliding backwards," she says.