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Doctors have long known that vitamin D is essential to good health. Get enough of it and it ensures strong bones and teeth. But a new study this week suggests an even more extraordinary benefit: a lower risk of death.

The new paper, published in the Sept. 10 issue of the Archives of Internal Medicine, is a meta-analysis of 18 previously published studies on the vitamin. None of the original experiments was specifically designed to study how vitamin D affects mortality — the trials involved conditions such as bone fractures, bone mineral density, congestive heart failure and colorectal cancer — but all of them tracked participants' death data. Overall, researchers found, people who took daily vitamin D supplements were 7% less likely to die during the study — from any cause — than people who didn't.

The study's authors still don't know exactly how the vitamin may reduce people's death risk, but their findings are in line with a spate of recent research linking the vitamin to a wide range of health benefits. Not only does it promote calcium absorption and bone maintenance, but vitamin D also appears to stimulate the immune system, inhibit cellular proliferation and spur cell differentiation — in turn, those processes could reduce the aggressiveness of cancer tumors or keep artery-clogging plaques from growing. Indeed, studies have suggested that low levels of vitamin D may be associated with a higher risk of death from certain cancers, heart disease and diabetes.

The current analysis looked at data on 57,311 participants, most of whom were middle-aged or elderly and in generally good health. Those in intervention groups took daily doses of vitamin D — ranging mostly from 400 IU to 833 IU per day, with a study size–adjusted mean intake of 528 IU a day. Compared with people who weren't given supplements, the test groups had up to a five times greater blood level of vitamin D and a significantly reduced risk of death. Though there's no medically recommended optimum level of the vitamin, "throughout human evolution when the vitamin D system was developing, the 'natural' level... was probably around 50 ng/mL or higher," writes Dr. Edward Giovannucci, a professor of nutrition and epidemiology at the Harvard School of Public Health, in an accompanying editorial. "In modern societies, few people attain such high levels, and levels below 10 ng/mL or 15 ng/mL are not uncommon."

If people can't get enough natural vitamin D from food or sun exposure, which synthesizes it in the skin, then daily supplements may be a good alternative — and the current study shows that an intake of up to 800 IU a day is safe. In the 18 studies that researchers analyzed for the current report, none of the participants taking supplements — even at a 2,000 IU daily dose — surpassed the 50 ng/mL mark. According to Giovannucci, people should reasonably shoot for levels of 30 ng/mL to 40 ng/mL, and doctors should consider testing patients who are at risk for deficiencies in vitamin D.

• Sora Song
• A new report shows that people who take a daily dose of the vitamin are less likely to die from any cause