Vitamin D seems to be all the rage in medicine these days. A family physician colleague commented to me recently that the laboratory test for vitamin D deficiency is becoming the most frequently ordered test in his practice.
This clinical bandwagon is likely a response to data from multiple recent studies that found low vitamin D levels in the majority of children and adults of all ages. While vitamin D has always been thought to play an important role in keeping bones strong, researchers are suggesting that low levels may increase one’s risk for a variety of diseases, including cancer and cardiovascular disease.
Before you go out to your local drugstore to buy mega-doses of vitamin D supplements, though, there are at least two good reasons to proceed with caution.
First, association does not always translate into causation. In other words, just because people with a low (or high) level of a nutrient are more likely to suffer from a particular illness doesn’t mean that the abnormal level caused the illness, nor does it mean that restoring a normal level will cure it. For example, studies have showed that high homocysteine levels are associated with an increased heart attack risk. When I was in residency training, cardiologists routinely prescribed folate supplements to patients who had had heart attacks in order to lower their homocysteine levels and reduce their risk of having another heart attack. However, subsequent studies determined that lowering homocysteine levels does absolutely nothing for these patients.
The second reason to be wary of the vitamin D hype is that we’ve been down this road before, with vitamins A, B, C, and E. An editorial published recently in the American Journal of Epidemiology provides a sobering summary of initial high hopes placed in each “anticancer vitamin du jour” that were subsequently dashed by randomized controlled trials. In the case of vitamin E, high doses actually appear to be harmful.
There’s a lot we don’t know about vitamin D, as an exhaustive review of the evidence for the U.S. Institute of Medicine concluded last year. Although most studies suggest that vitamin D supplements reduce the risk of fractures and falls in older adults, it’s not at all clear what the best dose should be; in a recent study, older women taking a single large dose of vitamin D each year suffered more fractures and falls than women who didn’t.
In the meantime, there are many other things that you can do to keep your bones healthy, including regular weight-bearing exercise, not smoking, and eating a balanced diet that includes two to three servings of dairy products each day.
Kenneth Lin is a family physician who blogs at Common Sense Family Doctor.
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