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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.
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.
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