An influential panel of experts questioned two big reasons people take vitamin D supplements.
The U.S. Preventive Services Task Force concluded in draft recommendations released Tuesday that taking less than 400 international units of vitamin D and 1,000 milligrams of calcium every day doesn't reduce the risk for bone fractures among postmenopausal women. And so the task force recommended against doing that.
And the panel concluded that there was insufficient evidence that higher doses protect the bones of postmenopausal or premenopausal women, or reduce the risk for cancer.
"This is the first recommendation we have looking specifically at the prevention of fractures and the prevention of cancer," Kirsten Bibbins-Domingo of the University of California, San Francisco, a task force member.
Evidence has been mounting in recent years that vitamin D might have many health benefits. Experts believe that because people are spending so much more time inside and covering up when they're outside that they are not getting as much vitamin D from the sun as they need. As a result, many are advocating that people take large doses daily.
"It has potential benefits in cancer prevention, cancer treatment, cardiovascular prevention, infection prevention, prevention of autoimmune disease," said Bruce Hollis of the Medical University of South Carolina.
That prompted the task force to examine two of the most widely touted supposed benefits.
The task force analyzed data from the best sixteen studies that have looked at vitamin D and calcium to prevent bone fractures.
"It really is not effective for preventing fractures. So for post-menopausal women who are seeking to do that, this type of supplementation really isn't effective for that purpose," Bibbins-Domingo said.
Now, Bibbins-Domingo noted that calcium and vitamin D do seem to help prevent falls among elderly people. It just doesn't seem to work for preventing postmenopausal women from getting bone fractures, and the jury's still out for younger women or the use of higher doses for older women, she said.
When it comes to cancer, the task force could only find three studies it considered really good enough. The bottom line? Again, it's just too soon to tell.
"We just don't have enough data at this time. We don't have enough studies that have specifically looked at cancer. So we don't--we have insufficient evidence. We can't really make a statement on that," she said.
Bibbins-Domingo acknowledges that the task force can't rule out that higher doses may have some benefits.
But in the meantime, she says, there are some real well-known risks of taking supplements. "There is a small but increased risk of kidney stones by taking vitamin D and calcium supplementation," she said.
Kidney stones may seem minor compared to cancer. But Bibbins-Domingo says when you're giving a healthy person a drug for a long time you have to be careful.
"In prevention, we're taking people generally who are otherwise healthy--who don't have symptoms, who are taking in this case a supplement potentially for a long period of time in order to achieve some health goal in the future. And I think for those purposes we have to set the bar high and really understand whether those purposes can be achieved," she said.
But vitamin D proponents like Hollis dismissed the task force's conclusions. For one, thing, he says, the doses studied were far too low to have any benefits.
"The amounts of vitamin D they're recommending are extraordinarily low. They're the-- they're basically the level an infant should get. To me it's appalling. I think the report is a sham," he said.
And Hollis and others say they are convinced higher doses are safe and have lots of benefits.
"We've monitored thousands of patients taking these levels of vitamin D that everybody's worried about that there'll be toxicity. yet the human body can make that much in the sun," he said.
Bibbins-Domingo said researchers can't rule out that other doses might turn out to be effective. "It may be that higher doses are beneficial. But that would be an area that we would certainly want to see the studies to know the benefits are there and outweigh any potential risk of higher doses," she said.
So it's clear the debate over the risks and benefits of vitamin D supplements isn't going to end any time soon.
The task force plans to take public comment until July 10 before issuing final recommendations.