Tuesday, July 6, 2010

Is Vitamin D vastly overrated?

Vitamin D seems to be all the rage in medicine these days. A family physician colleague commented to me last week 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.


This post was first published on my CommonSense MD blog at Family Health Guide.


  1. Hi Kenny,
    still recall supervising you (er, um I mean staying out of your way!) at LGH.
    We recently had a spirited discussion about this at one of of our FM conferences and came to the same conclusion that association and causation are not necessarilty related. This resulted in a great teachable moment about how lowering blood sugar does not necessarily translate into improved patient oriented outcomes.

    Will Melahn

  2. Hi Will - the folate-for-homocysteine example came straight out of my 1st year rotation with the Heart Group (not that they weren't otherwise terrific docs). And then there's estrogen replacement therapy, antiarrhythmics to suppress atrial fibrillation, avoiding beta blockers in CHF ... the list goes on and on. I wouldn't be surprised if Vitamin D will turn out to be a similar story - so I'll wait until the RCTs are done first.

  3. Milan Moore, MD, MPHJuly 16, 2010 at 8:52 PM

    I agree that study design is critically important. Our understanding of physiology makes some studies not only suspect, but, indeed, laughable.

    Imagine this: “in a recent study, older women taking a single large dose of a beta blocker each year suffered more fractures and falls than women who didn’t.” Indeed, substitute almost anything and the study outcome is practically the same. I challenge your readers to create a list of nutrients or pharmaceuticals which are safely and optimally administered as a single large dose annually (1. Reclast, 2. ???.) Therefore, the sited article is a specious argument about Vit D, and only creates unfounded skepticism.

    Remember, the association between folic acid deficiency and neural tube defects was recognized in the 1950’s, but the FDA recommendations for supplementation did not appear until 1992 and 1996. The risks of Vit D are extremely low (no reports of toxicity at doses below 30,000 IU/day) and the potential benefits (of 2000 IU/day) are tremendous. Check out www.grassrootshealth.net from the Univ of California system.

    Caution is prudent, but sometimes waiting too long has adverse consequences for your patients. ….. BTW which pharmaceutical company is going to fund those RCT’s??? You might be waiting a lifetime.