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Students of the "if-a-little-is-good-a-lot-must-be-better" school might want to look hard at the latest vitamin D study in this week's JAMA.
Researchers at the University of Melbourne found that giving annual megadoses of vitamin D to elderly women with brittle bones may have actually increased the women's rate of falls and bone fractures, instead of reducing their risk as the researchers had hoped.
Now, these were very big doses of the sunshine vitamin -- 500,000 IU spread across ten days, once a year for four years. That's more than 600 times the daily dose of 700 to 800 that's been shown in earlier studies to decrease fracture risk by up to 25 percent.
Why, you might ask, would doctors even think about giving patients such huge vitamin doses?
As Dr. Bess Dawson-Hughes, a long-time bone researcher at Tufts University who wrote an editorial about the work in the same issue of JAMA, says it's quite common for doctors to kickstart their patients who are low on vitamin D with 50,000 IU twice a week for six to eight weeks.
The University of Melbourne's Kerrie Sanders and colleagues say they were motivated to try even bigger doses because it's hard to get people to stick with with a daily regimen of pills. They also knew from other work that fat-soluble vitamin D is stored in the body for long periods, and released a little at a time.
Giving a megadose in autumn, they figured, would help boost the women's blood levels to a good range throughout the dark days of winter when vitamin D levels typically dip. And there was no reason, they say, to think it would hurt -- the women's blood levels, even at the peak was much lower than toxic levels.
But after several years, women in high-dose vitamin D group had 15 percent more falls, and 26 percent more fractures than women who swallowed dummy pills instead.
Why that would happen is still far from clear, but at least one other similar study has shown the same result. Both Sanders and Dawson-Hughes are still optimistic that vitamin D, given in lower daily doses, can help build bones. But the effect of very high doses, Sanders says, warrants further study.