Meredith W. Reiches

Links between physical fitness and cardiovascular reactivity and recovery to psychological stressors : A meta-analysis

A meta-analysis of published studies with adult human participants was conducted to evaluate whether physical fitness attenuates cardiovascular reactivity and improves recovery from acute psychological stressors. Thirty-three studies met selection criteria; 18 were included in recovery analyses. Effect sizes and moderator influences were calculated by using meta-analysis software. A fixed effects model was fit initially; however, between-studies heterogeneity could not be explained even after inclusion of moderators. Therefore, to account for residual heterogeneity, a random effects model was estimated. Under this model, fit individuals showed significantly attenuated heart rate and systolic blood pressure reactivity and a trend toward attenuated diastolic blood pressure reactivity. Fit individuals also showed faster heart rate recovery, but there were no significant differences in systolic blood pressure or diastolic blood pressure recovery. No significant moderators emerged. Results have important implications for elucidating mechanisms underlying effects of fitness on cardiovascular disease and suggest that fitness may be an important confound in studies of stress reactivity.

Pooled energy budget and human life history

Human life history contains a series of paradoxes not easily explained by classical life history theory. Although overall reproductive output is higher than in related primates, juvenile growth is slower and age‐specific reproductive rates decline faster with age. A simple energetic model would predict that growth and reproductive rates should be positively correlated and that reproductive effort should not decelerate with age. The pattern of negative correlations in humans suggest the presence of trade‐offs among peak reproductive rate, childhood growth, and reproductive rate at older ages. To address this puzzle, we propose a synthesis of reproductive ecology and behavioral ecology focused on intra‐ and inter‐somatic energy transfers. This integration includes three concepts: the mother as final common pathway through which energy must pass to result in offspring; a distinction between direct and indirect reproductive effort, proposing the latter as a novel net energy allocation category relative to growth and direct reproductive effort; and a pooled energy budget representing the energetic contributions and withdrawals of all members of a breeding community. Individuals at all reproductive life stages are considered in light of their contributions to the pooled energy budget. Am. J. Hum. Biol., 2009.

Is poverty in our genes? A critique of Ashraf and Galor, "The 'out of Africa' hypothesis, human genetic diversity, and comparative economic development," American Economic Review (Forthcoming)

We present a critique of a paper written by two economists, Quamrul Ashraf and Oded Galor, which is forthcoming in the American Economic Review and which was uncritically highlighted in Science magazine. Their paper claims there is a causal effect of genetic diversity on economic success, positing that too much or too little genetic diversity constrains development. In particular, they argue that “the high degree of diversity among African populations and the low degree of diversity among Native American populations have been a detrimental force in the development of these regions.” We demonstrate that their argument is seriously flawed on both factual and methodological grounds. As economists and other social scientists begin exploring newly available genetic data, it is crucial to remember that nonexperts broadcasting bold claims on the basis of weak data and methods can have profoundly detrimental social and political effects.

Puberty as a life history transition.

Background: James Tanners landmark publication, Growth at Adolescence, was not only the first and most comprehensive treatise on the subject of human pubertal development of its time, its core insights have held up remarkably well over time. Review: This review connects Tanners contributions to contemporary understanding of puberty as a process fundamentally driven by neuroendocrine maturation. It introduces the concepts of the ‘hour-glass of puberty’ and ‘somatic strategy’ as heuristic constructs. The ‘hour-glass of puberty’ describes the converging pathways of information flow influencing the timing of the neuroendocrine events of puberty and its ramifying consequences throughout the body. Somatic strategy refers to the pattern of sex-specific, adult body morphology that develops at puberty as the individual undergoes a life history transition from juvenile to adult.

The adolescent transition under energetic stress: Body composition tradeoffs among adolescent women in The Gambia.

Young female adolescents under energetic stress gain lean mass and lose fat mass while older adolescents lose lean mass and maintain fat. These findings support the prediction of life history theory that adolescence represents a period of transition from investment in growth to investment in reproduction.

Comparison of Heart Rate Monitor and Accelerometer for Measuring Energy Expenditure Under Field Conditions

Cortisol, as a biomarker of the generalized adaptive stress response, can provide critical information on the physiological effects of behavior. However, group-living animals face multiple interacting stressors from their social and ecological environments. While recent research has revealed the impact of particular social stressors in isolation, few studies have examined how diverse factors contribute to long-term stress hormone variation. We applied multivariate analyses to a 10-year dataset to investigate urinary cortisol variation in wild female chimpanzees (Pan troglodytes schweinfurthii) in Kibale National Park, Uganda. In interindividual contrasts, older females had higher cortisol levels, as did females that were low-ranking for their age. Over time, cortisol was significantly predicted by rates of aggression, particularly female-targeted aggression. The effect of male-female aggression was most pronounced for cycling, swollen females, who are the most frequent targets of sexual coercion. On the other hand, lactating females were strongly affected by group size, such that cortisol levels were elevated when parties contained more females and fewer males. Fruit consumption was an important covariate for lactating females, who experienced higher cortisol when the diet was poor. Our data indicate that both social and energetic factors contribute to stress variation in female chimpanzees, but that energetic stress increases in significance for females facing high reproductive costs. Our study also contributes to growing evidence that direct and indirect competition, while subtle in their expression, can have substantial impacts on female chimpanzees.

Energy Pooling and Implications for the Unique Traits of the Human Life History Strategy

Cortisol, as a biomarker of the generalized adaptive stress response, can provide critical information on the physiological effects of behavior. However, group-living animals face multiple interacting stressors from their social and ecological environments. While recent research has revealed the impact of particular social stressors in isolation, few studies have examined how diverse factors contribute to long-term stress hormone variation. We applied multivariate analyses to a 10-year dataset to investigate urinary cortisol variation in wild female chimpanzees (Pan troglodytes schweinfurthii) in Kibale National Park, Uganda. In interindividual contrasts, older females had higher cortisol levels, as did females that were low-ranking for their age. Over time, cortisol was significantly predicted by rates of aggression, particularly female-targeted aggression. The effect of male-female aggression was most pronounced for cycling, swollen females, who are the most frequent targets of sexual coercion. On the other hand, lactating females were strongly affected by group size, such that cortisol levels were elevated when parties contained more females and fewer males. Fruit consumption was an important covariate for lactating females, who experienced higher cortisol when the diet was poor. Our data indicate that both social and energetic factors contribute to stress variation in female chimpanzees, but that energetic stress increases in significance for females facing high reproductive costs. Our study also contributes to growing evidence that direct and indirect competition, while subtle in their expression, can have substantial impacts on female chimpanzees.

Ovarian Function Across Two Life History Transitions: Puberty and the Postpartum Resumption of Fecundity

Understanding how growing bones adapt to mechanical loading is a fundamental problem in human biology. Exercise-induced changes in bone strength are greater in women who start exercising premenarchally vs. postmenarchally, suggesting that estrogen (E2) may mediate these bone-strain interactions. Here we evaluate the contributions of peripubertal physical activity and estrogen levels to young adult bone strength in subjects from the Penn State Young Women’s Health Study (N=84). We hypothesize that women who 1) had higher E2 levels or 2) were more physically active during puberty will have greater adult bone strength. To test this hypothesis, we divided subjects into tertiles of physical activity and of E2 level. We then compared cross-sectional moment of inertia (CSMI) and section modulus (Z) in the femoral narrow neck, intertrochanteric region, and proximal shaft at age 17 (measured using DXA and the HAS algorithm) among these E2 and activity tertiles. Results indicate that women with the highest E2 levels in the first year after menarche had 11% greater CSMI in the narrow neck and 6-12% greater Z in the narrow neck and intertrochanteric region, vs. women with lower postmenarchal E2 levels. The most physically active women had 16-18% greater femoral CSMI in the narrow neck and intertrochanteric region, and 9-11% greater Z in the narrow neck, vs. less active women. These results support the hypothesis that peripubertal estrogen and physical activity are important determinants of adult bone strength. Physiological factors such as hormone levels may be crucial mediators of human osteogenic responses to exercise.