High Arterial Oxygen Saturation in the Acclimatized Lowlanders Living at High Altitude

Blood oxygen saturation(SpO_(2))is a key indicator of oxygen availability in the body.It is known that a low SpO_(2)at high altitude is associated with morbidity and mortality risks due to physiological hypoxemia.Previously,it was proposed that the lowlander immigrants living at high altitude should have a lower SpO_(2)level compared to the highlander natives,but this proposal has not been rigorously tested due to the lack of data from the lowlander immigrants living at high altitude.In this study,we compared arterial oxygen saturation of 5929 Tibetan natives and 1034 Han Chinese immigrants living at altitudes ranging from 1120 m to 5020 m.Unexpectedly,the Han immigrants had a higher SpO_(2)than the Tibetan natives at the same high altitudes.At the same time,there is a higher prevalence of chronic mountain sickness in Han than in Tibetans at the same altitude.This result suggests that the relatively higher SpO_(2)level of the acclimatized Han is associated with a physiological cost,and the SpO_(2)level of Tibetans tends to be sub-optimal.Consequently,SpO_(2)alone is not a robust indicator of physiological performance at high altitude.

Seasonality and Sex-Biased Fluctuation of Birth Weight in Tibetan Populations

Birth weight(BW)is a key determinant of infant mortality.Previous studies have reported seasonal fluctuation of BW.However,the responsible environmental factors remain disputable.High-altitude environment provides a great opportunity to test the current hypotheses due to its distinctive climate conditions.We collected BW data of~9000 Tibetan singletons born at Lhasa(elevation:3660 m)from 2014 to 2018.Using regression models,we analyzed BW seasonality of highland Tibetans.Multivariate models with meteorological factors as independent variables were employed to examine responsible environmental factors accounting for seasonal variation.We compared BW,low-BW prevalence and sex ratio between high-land and lowland populations,and we observed a significant seasonal pattern of BW in Tibetans,with a peak in winter and a trough in summer.Notably,there is a marked sex-biased pattern of BW seasonality(more striking in males than in females).Sunlight exposure in the 3rd trimester and barometric pressure exposure in the 2nd trimester are significantly correlated with BW,and the latter can be explained by seasonal change of oxygen partial pressure.In particular,due to the male-biased BW seasonality,we found a more serious BW reduction and higher prevalence of low-BW in males,and a skewed sex ratio in highlanders.The infant BW of highland Tibetans has a clear pattern of seasonality.The winter BW is larger than the summer BW,due to the longer sunlight exposure during the late-trimester.Male infants are more sensitive to hypoxia than female infants during the 2nd trimester,leading to more BW reduction and higher mortality.