Wednesday, December 1, 2010

Supplemental data on vitamin D supplementation

Could the promotional enthusiasm over vitamin D supplementation be screeching to a halt?  Ok, that's too much to hope, but at least the Institute of Medicine has introduced a note of sanity into the discussion.

The IOM was charged with assessing the existing data to determine whether in fact half of us (or more) are deficient in vitamin D, whether deficiency actually does cause all the diseases it's been linked with in recent years -- cancer, diabetes, heart disease, the flu and other infectious diseases, autism, disorders of the immune system such as multiple sclerosis; indeed, most of the diseases of modernity -- whether excess vitamin D intake could be a problem, to establish adequate vitamin D levels, and to suggest gaps in our knowledge about vitamin D and how to fill them.  They also looked at whether calcium supplementation was necessary.

Their report, released on Tuesday, is summarized thus:
The IOM finds that the evidence supports a role for vitamin D and calcium in bone health but not in other health conditions. Further, emerging evidence indicates that too much of these nutrients may be harmful, challenging the concept that “more is better.
Dr Michael Hollick of Boston University is the most prominent proponent of the more-is-better view, and he ain't backin' down now.  Few people do back down from public postures, though of course if they're right they shouldn't.

Adequate vitamin D levels have been difficult to establish, in part because the causes of chronic disease are so difficult to identify.  As the NYT says,
It is not clear how or why the claims for high vitamin D levels started, medical experts say. First there were two studies, which turned out to be incorrect, that said people needed 30 nanograms of vitamin D per milliliter of blood, the upper end of what the committee says is a normal range. They were followed by articles and claims and books saying much higher levels — 40 to 50 nanograms or even higher — were needed.
In a population with high prevalence of chronic disease, it's very easy to establish a link with inadequate vit D (defined, essentially, as the level found in most of the unhealthy population), for someone who wants to believe it.  But, it's often impossible to say which came first, the purportedly low vitamin D levels or disease.  Someone who's chronically ill is less likely to be out in the sun, making vitamin D, than someone who's fit and healthy.  This is a classic case of the correlation doesn't equal causation caveat not being heeded.

The Institute of Medicine says, vitamin D is required to maintain strong bones, but even this is not unequivocal, since African American women have significantly lower vitamin D than European Americans, but significantly lower fracture risk as well.

And, in fact, says the NYT:
Evidence also suggests that high levels of vitamin D can increase the risks for fractures and the overall death rate and can raise the risk for other diseases.  While those studies are not conclusive, any risk looms large when there is no demonstrable benefit.  Those hints of risk are "challenging the concept that 'more is better,' " the committee wrote.

So, once again, what seemed like a simple story, a simple correlation and a simple cure for the ills of modern civilization turns out to have been simplistic.

That's a lesson that really needs to be learned: if something is so elusive it's unlikely to be that important in general or on its own.  And this means that simple hypotheses about function, and especially about evolutionary fitness, can easily become simplistic -- knowingly overlooking complexity in order to tell (or sell) a good story.  Instead of such simplistic Just-So causal or evolutionary stories, a major challenge for modern biology is -- or at least should be -- to face complexity on its own terms.  That's hard as we so often stress on MT, because the career, bureaucracy, and reward systems favor simple, quick-fix kinds of assertions.

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