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
Scragg R, Thadhani R, Camargo CA. Association Between Altitude and Mortality in Incident Dialysis Patients. JAMA. 2009;301(23):2442-2443. doi:10.1001/jama.2009.842