What's more, there is not enough evidence to say whether supplements of vitamin D, with or without calcium, prevent osteoporotic fractures in men or younger women, or if they can help keep cancer at bay, according to the Task Force.
The panel's one definitive recommendation is that after menopause, women should not take 400 international units (IU) or less of vitamin D and 1,000 milligrams of calcium to prevent bone fractures. There's not enough evidence to show if larger doses of vitamin D might help.
"There isn't evidence to suggest that 400 IU of vitamin D plus 1,000 milligrams calcium can prevent fractures among postmenopausal women who do not live in assisted living or nursing home facilities," says Task Force member Kirsten Bibbins-Domingo, MD. She is an associate professor of medicine at the University of California at San Francisco.
"We know that vitamin D is important, and that a healthy lifestyle should include sources of vitamin D," she says. "It's just not good for preventing fractures at the doses studied," she says.
That is pretty clear cut, but questions remain about what role, if any, vitamin D may have in cancer prevention. "We need more studies to really clarify what it is good at preventing and at how high of a dose," Bibbins-Domingo tells WebMD. "Our bar for prevention is set pretty high."
The same task force recently reported that vitamin D can prevent falls in community-dwelling older adults. The panel based its recommendations on a review of the medical literature.
These reports led researchers to look into what vitamin D supplements can and can't do, and how much we really need.
Our bodies produce vitamin D when exposed to sunlight. Vitamin D is also added to milk and other foods. But it can be hard to get as much as we need from our diets. As a result, supplements are often needed.
The Institute of Medicine recently raised the recommended daily intake to 600 IU for people aged 1-70 and to 800 IU for adults older than 70. Other groups set the bar even higher.