WHEN TOO MUCH VITAMIN D is counterproductive.

UPDATE: Reader John Walker writes:

It’s worth noting that the research cited in the New York Times piece studied only a single aspect of the effects of vitamin D: the level of C-reactive protein (CRP) in the blood. But vitamin D has a long list of other well-documented effects, in particular upon the metabolism of calcium: insufficient levels of vitamin D can lead to low bone mineral density, risk of fractures in older people, and in extreme cases, rickets. There is evidence that calcium supplementation in the absence of adequate 25OHD levels in the blood may deposit calcium in soft tissues, including calcification of the arteries. Vitamin D is also linked to immune system performance.

It may be the case, as this research reports, that a serum 25OHD level of 21 ng/mL is optimal for CRP levels, but that says nothing about whether that level is optimal or even sufficient for its other functions. There are certainly indications that higher levels are beneficial in calcium metabolism. Hence choosing an optimum level for its overall effect on health and reducing mortality requires striking a balance among all of the effects of vitamin D, not just one.

As always, the authoritative reference for such matters is the indispensable “PDR for Nutritional Supplements.”

Excellent points.