Vitamin D: Too Much of a Good Thing Part 2

"Nothing succeeds like excess."  -Oscar Wilde
"One can never have enough socks."  -Dumbledore

While much has been made of late of a rampant wave of vitamin D deficiency, there has been less focus on how to treat said deficiency.  And while vitamin D is relatively non-toxic, there are issues to be addressed at supraphysiologic levels.   A little over a year ago, the Institute of Medicine increased the daily allowance for vitamin D to a very conservative 600IU daily for most adults while many studies have demonstrated long term safety taking as much as 5,000-10,000IU daily.  The good news is that we can generally take smaller doses for longer periods of time than we can tolerate larger less frequent doses prior to reaching a potential side effect threshold. To give some idea of safety, vitamin D can be purchased over-the-counter in doses as large as 2,000-5,000IU for daily consumption (although we shouldn't ignore the possibility of manufacturing errors).

But what of much larger doses, such as the commonly available by prescription only 50,000IU of vitamin D?  Several of my residents have recently come to me with plans to replete their deficient patients w/weekly 50,000IU doses, which in my simple mind is the equivalent of 7,000+IU per day.  Are you willing to go even higher?  In a small study to be published next month in the Journal of Clinical Endocrinology & Metabolism, the authors gave a single bolus of 600,000IU to 12 adults avg age 76yo.  Serial blood draws over 3 months were made to assess bone turnover compared to another 24 adults serving as controls.  While no changes were noted in bone turnover markers in the controls, the single extraordinarily large dose of vitamin D increased bone metabolism and turnover markers.

This might explain the negative findings from a double-blind, placebo-controlled trial published in JAMA in May 2010 of 2,256 community dwelling women avg age 76yo who were randomized to 500,000IU of vitamin D or placebo once each winter  for almost 3 years.  Quite unexpectedly, the authors noted an increase risk of falls & fractures in those randomized to this very large dose of vitamin D.  Upon further analysis, the fall & fracture risk was greatest in the first three months after administration, coincident with the finding of an increase in bone turnover markers in the more recent JCEM study above.

While infrequent dosing may improve adherence & compliance, the outcomes may not be as expected & desired.  At what point does a single bolus dose of vitamin D as we increase exponentially from 600IU daily to 2,000IU daily to 50,000IU weekly to 500,000-600,000IU intermittently.  We may want to reconsider our recommendations for repleting our deficient patients by using smaller more frequent doses.

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