The less air that UV light needs to penetrate, the less UVB light ozone in the air deflects, and the more UVB gets through to the ground. All other things being equal, more UV gets through at high altitudes, explaining the bad sunburns you can get while skiing. This should mean that vitamin D levels are higher at higher altitude, but few studies have looked at this issue.
Dr Valeria Hirschler and colleagues of the University of Buenos Aires, recently measured vitamin D levels in 290 Koya Indian schoolchildren living at high altitude in the fall (May in Argentina).
Hirschler V, Maccallini G, Molinari C, Aranda C; on behalf of San Antonio de los Cobres Study Group. Low vitamin D concentrations among indigenous Argentinean children living at high altitudes. Pediatr Diabetes. 2013 Jan 17.
San Antonio de los Cobres in Salta, Argentina, is a town with a population of about 4,000 people, 500 of which are children. It is located in a remote mountainous region. With its elevation of more than 11,000 feet, (approximately2.3 miles) above sea level, it is one of the highest inhabited regions in Argentina. Its latitude is tropical at 24 degrees south. Most of the men work in mines while the women care for sheep and llamas. The people are heavily clothed, so despite being in a sunny climate close to the equator at high altitudes, children might be at risk for deficiency.
In fact, in this study, concentrations of 25(OH)D averaged 10 ng/ml and ranged from 5 to 28 ng/ml in the 290 children. Despite the high altitude and the low latitude, 50% of the kids had severe deficiency and 100% were below 30 ng/ml.
They also found some remarkable associations. Analysis showed that 25(OH)D concentrations were significantly and inversely associated with glucose concentrations, BMI, systolic BP, and insulin levels in otherwise healthy children. As this was a cross-sectional study, causation cannot be determined, but other studies indicate low vitamin D levels may elevate blood sugar, insulin levels and blood pressure.
Besides the heavy clothing, it is possible that genetic factors explain these low 25(OH)D levels, although one would think that the law of natural selection would tend to push 25(OH)D levels in the opposite direction. That is, if natural selection were to occur, it would favor higher, not lower vitamin D levels as higher levels favor better health.