NEW YORK (Reuters Health) - People over the age of 50 often take vitamin D supplements thinking they're making their bones stronger and preventing osteoporosis.
But a new review of past studies finds the supplements don't usually increase bone density. And researchers said they aren't necessary for most healthy adults.
Among people with osteoporosis, bones become weak and fragile due to the loss of bone density that often comes with aging. Fragile bones are more likely to break. A common prevention strategy is to take calcium and vitamin D supplements.
Vitamin D is needed for the body to absorb calcium. But it's not commonly found in foods, unless they're fortified, like most milk. The body makes vitamin D after skin is exposed to sunlight.
Although calcium is necessary for strong bones, there has been some concern about the safety of taking calcium supplements.
"Recent evidence has indicated that calcium, with or without vitamin D, probably increases the risk of heart attacks," Dr. Ian Reid told Reuters Health in an email.
"Therefore, there is a renewed interest in the value of using vitamin D alone for optimizing bone health," Reid added. He is a professor of medicine at the University of Auckland in New Zealand and the lead author of the new study
Reid and his colleagues collected 23 past studies on vitamin D and bone density and re-analyzed their findings.
The studies included a total of 4,082 participants. The participants were in their late 50s, on average, and 92 percent of them were women.
The researchers found vitamin D supplements at any dose didn't make much of a difference for bone density.
"We were surprised at the large number of carefully conducted studies which did not disclose any benefit to bone density from vitamin D supplements, even when the baseline levels of vitamin D in the study subjects were quite low," Reid said.
The results were published in The Lancet.
Each of the studies measured bone density at between one and five sites on the body, like the spine, hip and forearm.
Across the studies, there were 70 separate measurements of bone density at a particular site before and after participants took vitamin D supplements. Of those, six showed some benefit tied to supplement use, two showed bone loss and the rest found no effect of vitamin D.
There was no difference comparing vitamin D2 and D3, the two forms normally sold as supplements.
Reid said he finds the current evidence that calcium supplements increase the risk of heart attacks to be persuasive. So he doesn't recommend people take calcium, unless it's advised by a doctor.
"Individuals interested in lifestyle modifications to optimize bone health should aim to achieve an adequate calcium intake from their diet, without recourse to supplements, and to have regular exercise and sunshine exposure without risking sunburn," Reid advised.
"In practice this means sunlight exposure at the beginning or end of the day in summer."
Some people do need to consider vitamin D supplements, he said.
"Individuals who are permanently indoors, usually as a result of advanced age and frailty, require supplements. Supplements are also required by individuals with very dark skin living at a distance from the equator, and those who are habitually veiled," explained Reid.
Paul Coates, director of the National Institutes of Health Office of Dietary Supplements, confirmed many people in the U.S. take vitamin D supplements, although they trail behind multivitamins and calcium in popularity.
It's possible vitamin D supplements might have other benefits, but Coates told Reuters Health there isn't enough evidence to make recommendations for anything other than bone health. Studies are currently being done to examine vitamin D's relationship to cancer and type 2 diabetes prevention.
Opinions and recommendations on dietary supplements seem to change frequently, making it difficult for consumers to keep up with the science.
Coates suggested the Office of Dietary Supplements website (ods.od.nih.gov) and the National Library of Medicine's MedlinePlus (medlineplus.gov) as two resources.
SOURCE: http://bit.ly/1c6s3oz The Lancet, online October 11, 2013.