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To the Editor: In their cohort analysis of incident dialysis patients, Dr Winkelmayer and colleagues¹ speculated that the inverse association observed between altitude and all-cause mortality in both dialysis patients and the general US population may be due to hypoxia-induced regulation of enzymes to reduce cardiovascular risk.

Another possible explanation of their findings is that populations living at high altitudes have higher vitamin D levels, which reduces their risk of cardiovascular disease compared with those living at sea level.² UV-B radiation, which is the primary source of vitamin D in humans, increases with altitude by about 15% per 1000 m because of decreasing ozone.² Moreover, the photoconversion of 7-dehydrocholesterol to previtamin D₃ has been shown to increase with altitude, particularly above 2000 m.³

There is evidence from cohort studies of vitamin D status in healthy populations and randomized trials of vitamin D supplementation for bone-related benefits to indicate that vitamin . . . [Full Text of this Article]

Robert Scragg, MBBS, PhD
r.scragg@auckland.ac.nz
School of Population Health
University of Auckland
Auckland, New Zealand

Ravi Thadhani, MD, MPH; Carlos A. Camargo Jr, MD, DrPH
Massachusetts General Hospital
Boston

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