Too much Vitamin D is bad--maybe

Update 7/8/12: Yet another study shows that high amounts of vitamin D reduce the risk of fractures in elderly women. This makes me even less inclined to accept the results discussed below.

Vitamin D deficiency has been in the news lately as a culprit in a variety of illnesses, including diabetes, immune system diseases, cardiovascular disease and even some cancers. What about having an excess of vitamin D? That may not be good either. Researchers from the University of Copenhagen found that vitamin D, like everything else, is best in moderation. Maybe.


The scientists measured vitamin D levels in almost 250,000 participants between 2004 and 2010. Subjects were then followed to see who lived and who died over the next few years. The lowest mortality rate was observed for patients with intermediate serum vitamin D levels (between 50-60 nmol/liter). People with both lower and higher levels had an increased risk of dying.

I’m not surprised that there’s a sweet spot for vitamin D levels. That said, I’m dubious about this study. The researchers only took one blood sample per subject, and then looked at who had died over the next few years. I’m no clinician, but this seems like a tenuous connection at best. After all, a person’s vitamin D levels are bound to fluctuate depending on diet and sun exposure. To top it off, the researchers admit that their study did not include data on either cause of death, or on attributes well known to affect mortality risk (smoking, body mass index, etc.).

Meanwhile, another large study has shown that vitamin D, in combination with calcium, significantly reduces mortality in elderly people.  Lars Rejnmark of Aarhus University Hospital and an international team of doctors and scientists compiled data from thirteen randomized controlled studies with over 70,000 participants, most of whom were women in their 60’s and 70’s. Their analysis showed a clear (though small) benefit of taking ten times the amount of vitamin D used in the Danish study.

Taken together, this makes me wonder whether the Copenhagen scientists were measuring what they thought they were measuring. If it's true that too much vitamin D is as bad or worse than too little, we'd definitely need more and better studies to prove it.