MONDAY, Dec. 19 (HealthDay News) -- A new analysis on the effects of vitamin D on bone health shows that it cuts fracture risk in older adults, but only when taken with calcium supplements.

The review of nearly 50 studies on vitamin D -- present in a small number of foods and produced naturally in the skin with sun exposure -- by the U.S. Preventive Services Task Force (USPSTF) also indicates that it's too soon to tell if vitamin D supplements can help prevent cancer.

Report author Mei Chung, assistant director of the Evidence-based Practice Center at Tufts Medical Center in Boston, said she wasn't able to advise specific recommended doses based on the review, which concluded that a daily vitamin D regimen of between 300 and 1,100 international units (IUs) combined with 500 to 1,200 milligrams (mg) of calcium reduces fracture risk in those over 65.

"I think vitamin D likely has more benefits than we currently know and also [presents] little harm," said Dr. Robert Graham, a vitamin D researcher and internist at Lenox Hill Hospital in New York City, who did not participate in the USPSTF report, published Dec. 20 in the Annals of Internal Medicine.

"An acceptable level is always a moving target," Graham added. "It's a very controversial topic, although I honestly don't know why it's so controversial."

The USPSTF review comes about a year after a more extensive report by the Institute of Medicine (IOM), the health arm of the National Academy of Sciences, which said that most Americans and Canadians up to age 70 need no more than 600 IUs of vitamin D per day to maintain health, while those 71 and older may need as much as 800 IUs.

The IOM report took nearly 1,000 published studies into account, while the latest review incorporated 19 randomized controlled trials and 28 observational studies to determine the benefits and harms of vitamin D with or without calcium supplementation on clinical outcomes of cancer and fractures.

In a related study also published in the Annals of Internal Medicine issue, the USPSTF noted that there's still no evidence to support vitamin D supplementation to prevent heart disease. Limited data suggest that high dosages can reduce the risk for all kinds of cancer, but more research is needed to draw a firm conclusion, the USPSTF said. And concern remains about proper dosing, since too much vitamin D can raise the risk for kidney and urinary tract stones.

Graham said most people should ask their doctors to test their blood levels of vitamin D to ensure they're not deficient.

"More physicians are checking vitamin D in their patients," he said. "I think there's greater awareness. At some point we have to decide what's good, what's bad, what's safe and what's not safe. I think we've learned from the last 10 years of this data, that there's still a lot we don't know about vitamin D."

Chung, also an investigator at Tufts Medical Center's Institute for Clinical Research and Health Policy Studies, said that research focusing on bone mineral density -- a measurement of bone thickness -- could be useful to further pin down correct vitamin D doses for various age groups.

"It could be a shorter trial that could enroll many more people . . . and compare a variety of doses to see which dose we could probably use to the best effect," she said. "If we use fracture as an outcome, it will take much longer and cost a lot of money to enroll a lot of subjects."

More information

The U.S. National Library of Medicine has more on vitamin D.