Vitamin D and calcium are dietary requirements, but it's unclear how much is best for us. New draft findings by the United States Preventive Services Task Force conclude that for healthy, postmenopausal women, daily supplementation with low levels of vitamin D -- up to 400 international units -- combined with 1,000 milligrams of calcium, does not reduce fracture risk.

However, this amount of supplementation is associated with a small but significantly increased likelihood of developing painful kidney stones, according to the task force.

Because relatively few, high quality studies have been done, the task force was unable to draw conclusions about the fracture-preventing merits of supplementation in healthy men and in premenopausal women.

Nor did studies offer conclusive evidence regarding vitamin D and cancer prevention, according to the task force. Evidence also was inadequate to draw conclusions about supplementation with higher doses.

The task force did not review studies on supplementation that were focused on individuals who already had osteoporosis or vitamin D deficiencies. Nor did the task force investigate other putative benefits of vitamin D.

However the task force did previously recommend vitamin D supplementation to prevent falls among men and women age 65 and older. Falls are a major cause of hip fracture and early death among the elderly, especially among those with osteoporosis.

Vitamin D Benefits, Risks, Not Adequately Studied "We know that vitamin D and calcium are essential for a healthy diet," said task force member Kirsten Bibbins-Domingo, MD, PhD, a UCSF epidemiologist and internist. "We're not making recommendations about the treatment of osteoporosis or of vitamin deficiency. We're making recommendations that apply to generally healthy women who are seeking to supplement their diets to prevent fractures."

The task force reviewed already-published studies to draw conclusions. The public comment period for the new draft recommendations runs through July 10.

To reach its recommendation that healthy, postmenopausal women not take low-dose vitamin D and calcium supplements to ward off fractures, task force members examined 16 studies in which participants took supplements. Formulations and dosages of vitamin D and calcium varied among the studies.

Among them was the Women's Health Initiative (WHI). The WHI included 36,282 health postmenopausal women ages 50 to 79. As part of the study, many women were assigned to take 400 international units of vitamin D3 and 1,000 milligrams of calcium carbonate each day. Results of other studies of low-dose supplementation were consistent with WHI findings, Bibbins-Domingo said.

To prevent fracture, supplementation for healthy, postmenopausal women at low vitamin D doses, with or without calcium, "does not work," Bibbins-Domingo said, and comes with a "small but measurable risk for kidney stones."

Whether benefits outweigh risks for harm at higher supplemental doses remains unclear.

"Many women are taking higher doses of vitamin D already, and that is where there is not sufficient evidence for us to make a recommendation one way or another," Bibbins-Domingo said. The same goes for many other questions related to vitamin D in various patient groups.

"We need more research. There are not enough high-quality studies," she said.