Thursday, September 29, 2011

Vitamin D and muscle injuries

I’m on the record as a bit of a vitamin D skeptic. Not a total skeptic, mind you — it’s actually the only supplement of any kind that I take on a regular basis these days. But the claims that vitamin D enhances athletic performance have seemed pretty weak to me so far. However, I’ll dutifully pass along this press release from the American Orthopaedic Society for Sports Medicine, which describes some new research linking vitamin D levels with muscle injuries in NFL football players.

The study: 89 players from one NFL team were tested for vitamin D levels in spring 2010, during pre-season. Not surprisingly, the levels were generally low compared to what’s considered desirable (which seems to be true for pretty much every population group in the developed world):

Twenty-seven players had deficient levels (< 20 ng/ML) and an additional 45 had levels consistent with insufficiency (20-31.9 ng/mL). Seventeen players had values within normal limits (>32 ng/mL).

The team then provided data on time missed due to injuries during the season. Sure enough, players who suffered muscle injuries has “significantly lower levels” of vitamin D. How much lower? It’s not clear: this is conference data, so not yet published in a journal, and unfortunately the press release release doesn’t do a very good job of presenting the data. The average level for players with a muscle injury was 19.9, but it doesn’t tell us what the average for uninjured players was.

First thing to wonder: is it this cause or correlation? Do the players with crappy diets also neglect their strength, flexbility and warm-up routine? Second thing: if it is causal, what’s the mechanism? Why does this work?

Leaving that aside, I’ll just reiterate my hair-splitting distinction between a “performance-enhancing” substance and one that hurts performance if you’re deficient in it. Water helps your performance if you’re dehydrated, but we don’t consider it an ergogenic aid. As far as I can tell, vitamin D falls into the same category: something that you shouldn’t be deficient in, whether you’re an athlete or not. But I’m still not convinced that more is better if you’re in a healthy range.

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