Intrauterine growth retardation has long been recognized at high altitude. Since growth-retarded babies have a decreased chance of survival, intrauterine growth retardation would be expected to have been selected against in populations long resident at high altitude. We have previously reported that Tibetan babies born at 3,658 m weighed more than their North or South American altitude counterparts. This study sought to determine whether Tibetans were protected from altitude-associated intrauterine growth retardation. We compared birth weights in Tibetans living at low altitude in Kathmandu, Nepal (elevation 1,200 m), or at high altitude in Lhasa, Tibet Autonomous Region, China (elevation 3,658 m). Birth weights were similar in 45 low-altitude and 34 high-altitude Tibetan births regardless of whether all infants or only full-term births were considered, or whether birth weight was adjusted for variation in maternal parity, gestational age, and infant sex. In comparison with literature observations, the altitude-associated difference in birth weight was smallest in Tibetans, intermediate in South America, and greatest in North America. These data support the hypothesis that Tibetans are protected from altitude-associated intrauterine growth retardation and suggest that selection for optimization of birth weight at high altitude has occurred in Tibetans.
PIP: Researchers compared data on infants born to 45 Tibetan women who delivered at the Patan Hospital in Kathmandu, Nepal (1200 m), in 1990 with data on 34 infants born to Tibetan women who delivered at the People's Provincial Hospital in Lhasa (3658 m) in the Tibet Autonomous Region of China in 1987-1988 and in 1991. All the women were healthy. The aim of the study was to determine whether birth weight differs in Tibetans born at low altitude compared with those born at high altitude. The mean birth weight of all Tibetan infants born at high altitude essentially matched that of those born at low altitude (3222 g and 3313 g, respectively). Further, the 2 groups exhibited similar weight gain as gestational age increased. The frequency of preterm post-term and low-birth-weight infants and mean gestational age were essentially the same for both high and low altitude infants. The researchers compared this study's results with those from studies in high altitude areas of Bolivia, Peru, and Colorado, USA. Tibetans had the smallest attitude associated difference in birth weight, followed by infants in South America and then by infants in the US (72 g, 282-270 g, and 352 g, respectively). In fact, the altitude associated differences in birth weight in South America and the US were significant (p .01) while they were not significant in Tibetans. These findings strongly suggested that Tibetans have experienced natural selection for optimization of birth weight at high altitude. They supported the hypothesis that genetic adaptation protects Tibetans from altitude associated intrauterine growth retardation.