Browsing by Author "Pontzer, Herman"

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    Constraint and trade-offs regulate energy expenditure during childhood.
    (Science advances, 2019-12-18) Urlacher, Samuel S; Snodgrass, J Josh; Dugas, Lara R; Sugiyama, Lawrence S; Liebert, Melissa A; Joyce, Cara J; Pontzer, Herman
    Children's metabolic energy expenditure is central to evolutionary and epidemiological frameworks for understanding variation in human phenotype and health. Nonetheless, the impact of a physically active lifestyle and heavy burden of infectious disease on child metabolism remains unclear. Using energetic, activity, and biomarker measures, we show that Shuar forager-horticulturalist children of Amazonian Ecuador are ~25% more physically active and, in association with immune activity, have ~20% greater resting energy expenditure than children from industrial populations. Despite these differences, Shuar children's total daily energy expenditure, measured using doubly labeled water, is indistinguishable from industrialized counterparts. Trade-offs in energy allocation between competing physiological tasks, within a constrained energy budget, appear to shape childhood phenotypic variation (e.g., patterns of growth). These trade-offs may contribute to the lifetime obesity and metabolic health disparities that emerge during rapid economic development.
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    Extreme events reveal an alimentary limit on sustained maximal human energy expenditure.
    (Science advances, 2019-06-05) Thurber, Caitlin; Dugas, Lara R; Ocobock, Cara; Carlson, Bryce; Speakman, John R; Pontzer, Herman
    The limits on maximum sustained energy expenditure are unclear but are of interest because they constrain reproduction, thermoregulation, and physical activity. Here, we show that sustained expenditure in humans, measured as maximum sustained metabolic scope (SusMS), is a function of event duration. We compiled measurements of total energy expenditure (TEE) and basal metabolic rate (BMR) from human endurance events and added new data from adults running ~250 km/week for 20 weeks in a transcontinental race. For events lasting 0.5 to 250+ days, SusMS decreases curvilinearly with event duration, plateauing below 3× BMR. This relationship differs from that of shorter events (e.g., marathons). Incorporating data from overfeeding studies, we find evidence for an alimentary energy supply limit in humans of ~2.5× BMR; greater expenditure requires drawing down the body's energy stores. Transcontinental race data suggest that humans can partially reduce TEE during long events to extend endurance.
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    Hip extensor mechanics and the evolution of walking and climbing capabilities in humans, apes, and fossil hominins.
    (Proceedings of the National Academy of Sciences of the United States of America, 2018-04-02) Kozma, Elaine E; Webb, Nicole M; Harcourt-Smith, William EH; Raichlen, David A; D'Août, Kristiaan; Brown, Mary H; Finestone, Emma M; Ross, Stephen R; Aerts, Peter; Pontzer, Herman
    The evolutionary emergence of humans' remarkably economical walking gait remains a focus of research and debate, but experimentally validated approaches linking locomotor capability to postcranial anatomy are limited. In this study, we integrated 3D morphometrics of hominoid pelvic shape with experimental measurements of hip kinematics and kinetics during walking and climbing, hamstring activity, and passive range of hip extension in humans, apes, and other primates to assess arboreal-terrestrial trade-offs in ischium morphology among living taxa. We show that hamstring-powered hip extension during habitual walking and climbing in living apes and humans is strongly predicted, and likely constrained, by the relative length and orientation of the ischium. Ape pelves permit greater extensor moments at the hip, enhancing climbing capability, but limit their range of hip extension, resulting in a crouched gait. Human pelves reduce hip extensor moments but permit a greater degree of hip extension, which greatly improves walking economy (i.e., distance traveled/energy consumed). Applying these results to fossil pelves suggests that early hominins differed from both humans and extant apes in having an economical walking gait without sacrificing climbing capability. Ardipithecus was capable of nearly human-like hip extension during bipedal walking, but retained the capacity for powerful, ape-like hip extension during vertical climbing. Hip extension capability was essentially human-like in Australopithecus afarensis and Australopithecus africanus, suggesting an economical walking gait but reduced mechanical advantage for powered hip extension during climbing.
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    Sitting, squatting, and the evolutionary biology of human inactivity.
    (Proceedings of the National Academy of Sciences of the United States of America, 2020-03-09) Raichlen, David A; Pontzer, Herman; Zderic, Theodore W; Harris, Jacob A; Mabulla, Audax ZP; Hamilton, Marc T; Wood, Brian M
    Recent work suggests human physiology is not well adapted to prolonged periods of inactivity, with time spent sitting increasing cardiovascular disease and mortality risk. Health risks from sitting are generally linked with reduced levels of muscle contractions in chair-sitting postures and associated reductions in muscle metabolism. These inactivity-associated health risks are somewhat paradoxical, since evolutionary pressures tend to favor energy-minimizing strategies, including rest. Here, we examined inactivity in a hunter-gatherer population (the Hadza of Tanzania) to understand how sedentary behaviors occur in a nonindustrial economic context more typical of humans' evolutionary history. We tested the hypothesis that nonambulatory rest in hunter-gatherers involves increased muscle activity that is different from chair-sitting sedentary postures used in industrialized populations. Using a combination of objectively measured inactivity from thigh-worn accelerometers, observational data, and electromygraphic data, we show that hunter-gatherers have high levels of total nonambulatory time (mean ± SD = 9.90 ± 2.36 h/d), similar to those found in industrialized populations. However, nonambulatory time in Hadza adults often occurs in postures like squatting, and we show that these "active rest" postures require higher levels of lower limb muscle activity than chair sitting. Based on our results, we introduce the Inactivity Mismatch Hypothesis and propose that human physiology is likely adapted to more consistently active muscles derived from both physical activity and from nonambulatory postures with higher levels of muscle contraction. Interventions built on this model may help reduce the negative health impacts of inactivity in industrialized populations.
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    The Effect of Lifestyle Change on Health and Early Childhood Growth in Daasanach Pastoralists Living in Northern Kenya
    (2021) Swanson, Zane Shea

    Understanding the relationships between lifestyle, ecology and physiology is essential for understanding variation in health, life history, and subsistence practice among populations. Previous work has investigated human behavioral ecology and life history across a wide range of human populations, but study with populations experiencing changes to their lifeways remains particularly important. Work with populations that traditionally practice nomadic pastoralism as a subsistence strategy and are experiencing encroaching market pressures offers the opportunity to investigate the effects of stark subsistence and market transitions across a variety of lifestyle factors (e.g., nutrition, physical activity, healthcare, socioeconomic status).Using data collected with the Daasanach Health and Life History Project, this dissertation applies a broad approach to test whether changes in lifestyle (e.g., market integration and sedentarization) affect health and patterns of early childhood growth within a human population through the framework of life history theory. Health, physical activity, growth, nutrition, reproduction, and community composition data have been synthesized to test the effects of life history tradeoffs that arise through socioecological variation. As semi-nomadic pastoralists who currently face the encroaching pressure of sedentarization, the Daasanach living in and around the town of Illeret are well suited to test this hypothesis. In addition, this project will expand the existing body of work concerning life history and health variation in non-industrial populations, specifically adding a population with a subsistence pattern that is currently underrepresented. This addition allows for a new level comparison between the variation in ecology, life history, health, and behavior that characterize our species, advancing our understanding of difference between industrialized and non-industrialized populations, and the breadth of variation in the variables across human populations.

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    Water turnover among human populations: Effects of environment and lifestyle.
    (American journal of human biology : the official journal of the Human Biology Council, 2020-01) Swanson, Zane S; Pontzer, Herman
    OBJECTIVES:To discuss the environmental and lifestyle determinants of water balance in humans and identify the gaps in current research regarding water use across populations. METHODS:We investigated intraspecific variation in water turnover by comparing data derived from a large number of human populations measured using either dietary survey or isotope tracking. We also used published data from a broad sample of mammalian species to identify the interspecific relationship between body mass and water turnover. RESULTS:Water facilitates nearly all physiological tasks and water turnover is strongly related to body size among mammals (r2=0.90). Within humans, however, the effect of body size is small. Instead, water intake and turnover vary with lifestyle and environmental conditions. Notably, despite living physically active lives in conditions that should increase water demands, the available measures of water intake and turnover among small-scale farming and pastoralist communities are broadly similar to those in less active, industrialized populations. CONCLUSIONS:More work is required to better understand the environmental, behavioral, and cultural determinants of water turnover in humans living across a variety of ecosystems and lifestyles. The results of such work are made more vital by the climate crisis, which threatens the water security of millions around the globe.