Extra vitamin D and calcium may offer some protection against fractures in elderly people, but have little or no impact on cancer risk, according to a fresh look at the medical evidence.
Some research has suggested that vitamin D, with or without calcium, might help stave off cancer, but recent trials have slashed those hopes.
"It turns out that as a group, all of the micronutrient supplements have been disappointing," said Dr. Michael Pollak, who heads the division of cancer prevention at McGill University in Montreal, Canada, and was not involved in the new work.
"Even one of the best candidates, which is vitamin D, is certainly no slam dunk," he told Reuters Health.
The new report, out Monday in the Annals of Internal Medicine, was commissioned by the government-backed U.S. Preventive Services Task Force to inform its public recommendations.
It pulls together 19 gold standard experiments -- so-called randomized controlled trials -- on vitamin D with or without calcium. The trials lasted anywhere from seven months to seven years and ranged in size from a few thousand participants to tens of thousands.
Only three of them reported on cancer, however. While one small study found some protection against cancer in postmenopausal women taking vitamin D and calcium, the larger studies found no benefits.
"I don't have confidence in any of the findings because they could be chance findings," lead researcher Mei Chung, of Tufts Medical Center in Boston, told Reuters Health.
Last month, another randomized controlled trial was published in the Journal of Clinical Endocrinology and Metabolism. Although it wasn't included in Chung's report, it confirms her results.
In that study, among seniors taking 800 IU of vitamin D daily for a few years, 32 out of every 100 died during the study, while 33 out of every 100 people who did not get the supplement died.
That small difference could easily have been due to chance, the researchers found. There were no differences in deaths from cancer or heart disease either, just as calcium also proved unhelpful.
According to Chung, one large U.S. study, known as the Women's Health Initiative, also showed that women taking the supplements had higher rates of kidney and bladder stones.
Marji McCullough, a nutritional scientist at the American Cancer Society, said her organization does not advise dietary supplements to prevent cancer.
"Various researchers have recommended that, but large consensus panels have not," she told Reuters Health. "There is no compelling evidence currently that taking supplements will lower your cancer risk."
The Institute of Medicine recommends that most adults get 1,000 to 1,200 milligrams (mg) of calcium per day and 600 to 800 IU of vitamin D. It sets a recommended upper limit at 2,000 mg of calcium and 4,000 IU of vitamin D.
However, Chung's team did find a small reduction in fracture risk among elderly people living in an institution such as a nursing home, with extra vitamin D and calcium preventing two out of every 100 expected fractures.
But the risk reduction was smaller for people living on their own, and might have been due to chance, she added.
Chung, who is assistant director of the Evidence-based Practice Center at Tufts, said that in an earlier report from 2009, which looked at several possible health benefits, only the fracture benefit was convincing.
Pollak said it's possible that a few people who have low levels of vitamin D may get some benefit from it, but that doesn't warrant everybody taking extra vitamins.
"You can have too much of a good thing," he told Reuters Health.
For people interested in lowering their cancer risk, he added, there are better ways to go than supplements.
"Don't smoke and stay as close as you can to your ideal body weight," Pollak urged. "Those two things will definitely lower you cancer risk and they will have many other health benefits as well -- and there are no possible downsides."
The U.S. Preventive Services Task Force could not say when its new vitamin D guidelines will be released.