Repeatability of adaptive traits among ethnic Tibetan highlanders.

OBJECTIVES: Connecting traits to biological pathways and genes relies on stable observations. Researchers typically determine traits once, expecting careful study protocols to yield measurements free of noise. This report examines that expectation with test-retest repeatability analyses for traits used regularly in research on adaptation to high-altitude hypoxia, often in settings without climate control. METHODS: Two hundred ninety-one ethnic Tibetan women residing from 3500 to 4200 m in Upper Mustang District, Nepal, provided three observations of hemoglobin concentration, percent of oxygen saturation of hemoglobin, and pulse by noninvasive pulse oximetry under conditions designed to minimize environmental noise. RESULTS: High-intraclass correlation coefficients and low within-subject coefficients of variation reflected consistent measurements. Percent of oxygen saturation had the highest intraclass correlation coefficient and the smallest within-subject coefficient of variability; measurement noise occurred mainly in the lower values. Hemoglobin concentration and pulse presented slightly higher within-subject coefficients of variation; measurement noise occurred across the range of values. The women had performed the same measurements 7 years earlier using the same devices and protocol. The sample means and SD observed across 7 years differed little. Hemoglobin concentration increased substantially after menopause. CONCLUSIONS: Analyzing repeatability features of traits may improve our interpretation of statistical analyses and detection of variation from measurement or biology. The high levels of measurement repeatability and biological stability support the continued use of these robust traits for investigating human adaptation in this altitude range.

Detecting past and ongoing natural selection among ethnically Tibetan women at high altitude in Nepal.

Adaptive evolution in humans has rarely been characterized for its whole set of components, i.e. selective pressure, adaptive phenotype, beneficial alleles and realized fitness differential. We combined approaches for detecting polygenic adaptations and for mapping the genetic bases of physiological and fertility phenotypes in approximately 1000 indigenous ethnically Tibetan women from Nepal, adapted to high altitude. The results of genome-wide association analyses and tests for polygenic adaptations showed evidence of positive selection for alleles associated with more pregnancies and live births and evidence of negative selection for those associated with higher offspring mortality. Lower hemoglobin level did not show clear evidence for polygenic adaptation, despite its strong association with an EPAS1 haplotype carrying selective sweep signals.

Trekking and toddlers: Towards a biocultural analysis of growth among infants and young children in a rural region of the Himalayas.

OBJECTIVES: Rapid socioeconomic change, associated with development and a growing tourism industry is occurring across the Himalayas. The health impact of this rapid economic development is poorly understood, especially for infants and young children. This study investigated the associations between village level economic differences as indexed by economic development and tourism engagement on infant and young child growth and health in a population of ethnic Tibetans living in the western Himalayas of Nepal. METHODS: One hundred and fifty nine infants and young children (ages 1-24 months) were enrolled. Anthropometric data (height, weight, triceps skinfold thickness) were collected at a single time point. Village level measurements of tourism and market engagement were incorporated into a scale measuring tourism, healthcare, trail access, agriculture, and involvement in medicinal trade. Village level disease patterns were calculated from morbidity and mortality recalls collected since 2003. RESULTS: There were no significant associations between infant weight for age z-score (WAZ), length for age z-score (LAZ), or weight-for-length for age z-score (WLZ) and village altitude, village economic development score, or engagement in tourism. Males had significantly higher LAZ, WAZ, and WLZ compared to females; only females showed a decline in LAZ with age. Triceps skinfold thickness z-score (ZTSF) was inversely associated with village level economic development score in male but not female infants; females ZTSF was positively associated with IYC age. CONCLUSIONS: While overall size for age indices (WAZ, LAZ, WLZ) were not associated with altitude or village economic development in this population, ZTSF was inversely associated with village economic development in males but not females.

When women work: Endocrine reactivity in women during everyday physical activity at high altitude.

OBJECTIVES: This study explores the acute endocrine reactivity of testosterone and cortisol in women engaging in everyday physical activity in a high altitude environment. METHODS: Data were collected from 35 women living in the Himalayas, with women recruited from both high (>10 000 ft.) and low altitude villages (<10 000 ft.). Saliva samples were collected at 3 time points (pre-activity, 30 and 60 minutes) and women wore the wGT3X-BT Actigraph during an hour of everyday work to assess the relationship between high altitude, endocrine reactivity, and physical activity. Saliva samples were then analyzed for testosterone and cortisol. RESULTS: Women living at high altitude had lower cortisol and testosterone levels, after controlling for moderate-to-vigorous physical activity, age, and sum of skinfolds. CONCLUSIONS: Testosterone and cortisol increase allocation of energy to costly somatic tissues and the utilization of stored energy. Lower production of these hormones may be beneficial for heightened energetic demands at high altitude.

Ecological pressures and milk metabolic hormones of ethnic Tibetans living at different altitudes.

BACKGROUND: Very little is known about how milk hormones, shown to influence growth during infancy, may contribute to patterns of altered growth in high altitude living infants. AIM: This study investigated the association between maternal BMI, the metabolic hormones adiponectin and leptin in human milk and infant weight for age z-scores (WAZ) in Tibetans. SUBJECTS AND METHODS: A sample of 116 mothers and infants (aged 0-36 months) were recruited from two locations: the Nubri Valley, Nepal (rural; altitude = 2400-3900 m) and Kathmandu, Nepal (urban, 1400 m). Milk samples, anthropometrics, biological data and environmental information were collected on mothers and infants. Milk was analysed for leptin and adiponectin. RESULTS: Maternal BMI was significantly associated with milk leptin content, but not adiponectin in either group. In the rural high altitude sample, child WAZ declined with age, but no such decline was seen in the urban sample. CONCLUSIONS: Milk leptin and adiponectin were not associated with infant growth in the rural Nubri sample, but were both inversely associated with infant WAZ in the Kathmandu sample. It appears that, in ecologically stressful environments, associations between milk hormones and growth during infancy may not be detectable in cross-sectional studies.

Milk at altitude: Human milk macronutrient composition in a high-altitude adapted population of Tibetans.

OBJECTIVE: The physiological challenges of high altitude have led to population-specific patterns of adaptation. These include alterations to child growth and reproduction, including lactation. However, while breastfeeding has been investigated, nothing is known about milk composition in high altitude adapted populations. MATERIALS AND METHODS: Here, we investigate milk macronutrient composition, volume, and energy in a sample of 82 Tibetans living at high and low altitude in rural villages (Nubri Valley, Nepal) and at low altitude in Kathmandu, Nepal. Milk samples were collected in the morning using hand expression, frozen, and assayed for fat, protein, and total sugars. Reproductive histories and health recalls were also collected. RESULTS: Milk fat averaged 5.2 ±2.0 g/100 mL, milk sugar 7.37 ± 0.49 g/100 mL, and milk protein 1.26 ± 0.35 g/100 mL for a mean energy density of 81.4 ± 17.4 kcal/100 mL. There were no associations between altitude of residence and milk composition; however, overall milk fat was high compared to reference populations. Within the three groups, milk fat was positively associated with infant age (B = 0.103; p < 0.001) and maternal triceps skinfold thickness (B = 0.095; p < 0.01) while milk sugar was significantly and inversely associated with maternal parity and triceps skinfold thickness. DISCUSSION: Milk fat, and consequently milk energy, may be increased in high-altitude adapted Tibetans when compared to populations living at low altitude. The association between milk fat and maternal adiposity suggests that milk composition may be sensitive to maternal adiposity in this sample, likely reflecting increased metabolic costs of producing a high-fat milk.

Closing the Womb Door: Contraception Use and Fertility Transition Among Culturally Tibetan Women in Highland Nepal.

Objectives Whether in metropoles or remote mountain communities, the availability and adoption of contraceptive technologies prompt serious and wide-ranging biological, social, and political-economic questions. The potential shifts in women's capacities to create spaces between pregnancies or to prevent future pregnancies have profound and often positive biological, demographic, and socioeconomic implications. Less acknowledged, however, are the ambivalences that women experience around contraception use-vacillations between moral frameworks, generational difference, and gendered forms of labor that have implications well beyond the boundaries of an individual's reproductive biology. This paper hones in on contraceptive use of culturally Tibetan women in two regions of highland Nepal whose reproductive lives occurred from 1943 to 2012. Methods We describe the experiences of the 296 women (out of a study of more than 1000 women's reproductive histories) who used contraception, and under what circumstances, examining socioeconomic, geographic, and age differences as well as points of access and patterns of use. We also provide a longitudinal perspective on fertility. Results Our results relate contraception usage to fertility decline, as well as to differences in access between the two communities of women. Conclusions We argue that despite seemingly similar social ecologies of these two study sites-including stated reasons for the adoption of contraception and expressed ambivalence around its use, some of which are linked to moral and cosmological understandings that emerge from Buddhism-the dynamics of contraception uptake in these two regions are distinct, as are, therefore, patterns of fertility transition.

Childhood growth and education migration among ethnic Tibetan children from Nepal.

BACKGROUND: Ethnographic work among high altitude populations has shown that children are highly mobile-the most recent expression of this is the educational migration of children born at high altitude to boarding schools at lower altitudes. The impact of these patterns of migration on size for age are unknown. AIM: We investigated the association between growth in weight and height and educational migration in ethnic Tibetan children living in and out of their natal communities. SUBJECTS AND METHODS: Five hundred and fifty eight children ages three to sixteen from the Nubri Valley, Nepal participated in this study. Three hundred children were living in natal villages and 258 were attending boarding schools in Kathmandu. Height, weight, and skinfold thicknesses were collected and matched to demographic data from the community. RESULTS: There was no association between altitude of family residence and size for age z-scores. Males had lower z-scores than females; z-scores for both groups declined with age. Differences in size for age among children in boarding schools were associated with two factors: sex and type of boarding school (individual sponsor or group funded). Individuals attending individually sponsored schools had greater size for age compared to children in group funded schools or in their natal villages; younger children in collectively funded schools were smaller than village peers. CONCLUSIONS: Despite popular perceptions, educational outmigration in Himalayan communities may not be associated with improved child growth outcomes and investment in community level schools may be a practical solution for improving child growth and physical and mental health.

Externally-resident daughters, social capital, and support for the elderly in rural Tibet.

This paper focuses on assistance that externally-resident daughters provide for their aging parents in rural Tibet, China, to challenge the notion that rapid modernization invariably threatens family-based care systems for the elderly. The authors discuss social and economic changes associated with modernization that have created new opportunities for parents to send daughters out of their natal households in ways that can benefit them in old age. By investing in a daughter's education so she can secure salaried employment, or by helping a daughter establish a small business so she can earn an independent livelihood, the authors demonstrate how some externally-resident daughters represent a novel form of social capital that parents can draw on for social support. Daughters with income and freedom from extended family obligations are now providing elderly parents with (1) leverage against co-resident children who do not treat them well, (2) temporary places of refuge from ill-treatment at home, (3) caretaking services and financial support when they require hospitalization, and (4) financial resources independent of their household which they can use to pursue age-appropriate activities like pilgrimage. The authors conclude that this new form of social capital vested in externally-resident daughters is having a positive impact on the lives of the elderly in rural Tibet.

Ethnically Tibetan women in Nepal with low hemoglobin concentration have better reproductive outcomes.

Background and objectives: Tibetans have distinctively low hemoglobin concentrations at high altitudes compared with visitors and Andean highlanders. This study hypothesized that natural selection favors an unelevated hemoglobin concentration among Tibetans. It considered nonheritable sociocultural factors affecting reproductive success and tested the hypotheses that a higher percent of oxygen saturation of hemoglobin (indicating less stress) or lower hemoglobin concentration (indicating dampened response) associated with higher lifetime reproductive success. Methodology: We sampled 1006 post-reproductive ethnically Tibetan women residing at 3000-4100 m in Nepal. We collected reproductive histories by interviews in native dialects and noninvasive physiological measurements. Regression analyses selected influential covariates of measures of reproductive success: the numbers of pregnancies, live births and children surviving to age 15. Results: Taking factors such as marriage status, age of first birth and access to health care into account, we found a higher percent of oxygen saturation associated weakly and an unelevated hemoglobin concentration associated strongly with better reproductive success. Women who lost all their pregnancies or all their live births had hemoglobin concentrations significantly higher than the sample mean. Elevated hemoglobin concentration associated with a lower probability a pregnancy progressed to a live birth. Conclusions and implications: These findings are consistent with the hypothesis that unelevated hemoglobin concentration is an adaptation shaped by natural selection resulting in the relatively low hemoglobin concentration of Tibetans compared with visitors and Andean highlanders.

A longitudinal cline characterizes the genetic structure of human populations in the Tibetan plateau.

Indigenous populations of the Tibetan plateau have attracted much attention for their good performance at extreme high altitude. Most genetic studies of Tibetan adaptations have used genetic variation data at the genome scale, while genetic inferences about their demography and population structure are largely based on uniparental markers. To provide genome-wide information on population structure, we analyzed new and published data of 338 individuals from indigenous populations across the plateau in conjunction with worldwide genetic variation data. We found a clear signal of genetic stratification across the east-west axis within Tibetan samples. Samples from more eastern locations tend to have higher genetic affinity with lowland East Asians, which can be explained by more gene flow from lowland East Asia onto the plateau. Our findings corroborate a previous report of admixture signals in Tibetans, which were based on a subset of the samples analyzed here, but add evidence for isolation by distance in a broader geospatial context.

Correction: A longitudinal cline characterizes the genetic structure of human populations in the Tibetan plateau.

[This corrects the article DOI: 10.1371/journal.pone.0175885.].