Last October, the latest salvo in the debate about vitamin D was fired when a combined review and meta-analysis in the Lancet concluded that the supplements do not help keep bones strong in the general population and thus are unnecessary. The paper was done by the same New Zealand researchers whose earlier controversial papers suggested that calcium supplements can cause heart attacks.
But don’t toss your vitamin D capsules just yet. There’s less to this new analysis than many media reports suggested.
Dueling studies on vitamin D
Vitamin D has been one of the most popular supplements in recent years—and not just because of its well-known role in working with calcium to keep bones strong throughout life and help prevent osteoporosis-related fractures. Countless studies (mostly observational) have linked it to a reduced risk of heart disease, multiple sclerosis, some cancers, diabetes, hypertension and many other disorders. Still, the evidence for these other benefits has been inconsistent or at least “does not demonstrate causality,” according to the Institute of Medicine (IOM) in its major 2010 report on vitamin D. Thus, the IOM looked solely at the vitamin’s effect on bone health to come up with its recommended intakes for vitamin D (600 IU a day up to age 70, and 800 IU over age 70, primarily from foods like fortified milk and fatty fish).
So why are researchers still debating the effect of vitamin D on bone health and fracture risk? There have been hundreds of studies on this, and the results have not always been consistent. That’s not surprising, since the studies have varied widely in many ways—notably in the amount of vitamin D tested, the dosing regimen (daily, weekly or monthly), the age and health of the subjects, whether they consumed adequate calcium (from food or supplements), their initial blood levels of the vitamin (if this was even measured), and whether the amount taken was enough to raise blood levels to the optimal range (however that is defined—a bone of contention in itself ).
The new Lancet analysis selected 23 of these studies (92 percent of the subjects were women, average age 59), which tested the effect of vitamin D supplements on bone mineral density for an average of two years, without co-administration of calcium supplements. It found that the supplements, given in a wide range of doses, led to a “small but significant” improvement in bone density only in the femoral neck (the narrow part of the upper thigh bone, which is a common site for hip fractures). The researchers nevertheless questioned the finding, in part because there was no improvement in bone density at other sites. Thus, they concluded that “the widespread use of vitamin D supplements for skeletal protection in adults without specific risk factors for vitamin D deficiency is not justified.”
Two experts weigh in
This is just the latest of more than 70 reviews or analyses on vitamin D for bone health in the past three years alone, according to Robert Heaney, M.D., an expert on vitamin D and calcium at Creighton University in Omaha. Most of these have found some benefit from vitamin D, usually when taken along with calcium supplements, though some have not. The Lancet analysis “reshuffles the same set of studies and adds nothing new,” he said, and like many of its predecessors it is flawed in how the studies included were selected and combined. “Its conclusions, therefore, have to be judged unpersuasive and potentially misleading,” concluded Dr. Heaney, who still recommends vitamin D supplements for older adults.
Bess Dawson-Hughes, M.D., director of the Bone Metabolism Laboratory at the Jean Mayer USDA Human Nutrition Research Center at Tufts University in Boston, and the lead author of two early studies included in the new analysis, had similar criticisms. “There are no new data here, and it’s not a balanced interpretation of what they found,” she said. Moreover, the finding of improved bone density in the femoral neck of people taking vitamin D is notable. “Femoral neck density is the strongest predictor of hip fractures of any site,” she said.
Bone mineral density is only one factor involved in osteoporotic fractures, Dr. Dawson-Hughes also reminds us. Notably, age-related muscle weakness and impaired balance increase the risk of falls and fractures. There’s good evidence that vitamin D can improve muscle function and balance and thus help prevent falls in older people deficient in the vitamin. That’s why the U.S. Preventive Services Task Force, in its 2012 guidelines on fall prevention, recommended that older people take vitamin D supplements (no dose was specified). Dr. Dawson-Hughes’s advice: “Don’t be deterred from taking vitamin D supplements. This study doesn’t refute the more comprehensive assessments that have been made.”