Intellectual Merit: The proposed research has three objectives, each of which makes timely and significant contributions to human biology. First, it develops activity monitoring with accelerometers as an innovative and non-invasive method for estimating energy expenditure in human ecology field studies. Second, it explores the relationship of energy expenditure profiles in adolescent females in a subsistence agricultural society to body composition and pubertal maturation. Third, it investigates whether the relationship between energy expenditure and adolescent development is modified by early life energy availability.
The project takes advantage of a unique opportunity in The Gambia to directly address the effects of early life factors in a human population where differences in maternal energetic status during pregnancy are known. Between 1989 and 1994, the British Medical Research Council gave standardized nutritional supplements to a sub-set of pregnant women in the West Kiang District. This project follows up both the children of both supplemented and unsupplemented individuals to investigate the interaction of developmental and current energetic conditions on adolescent development.
Broader Impact: Populations in developing nations show evidence of a shift over time toward sedentism and associated health problems, including obesity and diabetes. This study will help to establish a field technique capable of generating high-quality, comparative data on energy expenditure with minimal invasiveness and low cost. It will quantify energy expenditure in activity in a subsistence agricultural population and its relation to body composition and pubertal progression, providing an important comparison for urban industrial settings. It will constitute a rare randomized test of the impact of prenatal conditions on adolescent physiology and development. Furthermore, it will build capacity in an undergraduate research assistant, who will participate in data collection and complete a senior thesis, and in three Gambian field assistants in the West Kiang District, who will be learn accelerometer techniques. This doctoral dissertation research project will also provide training and development opportunities for a female graduate student.
This project set out to examine the impact of seasonal changes in energy availability on the activity patterns and body composition of adolescent women who were subsistence farmers in a seasonal environment in The Gambia, West Africa. While much is known about the activities and energy expenditure of adult men and women, and more is beginning to be known about the activities and energy budgets of children in subsistence societies, much less information is available on activity patterns and changes in body composition in adolescence, when the bodies of young men and women are shifting energy from childhood growth to adult reproductive function. Particularly in energetically stressful environments of the kind in which humans likely evolved, adolescence is a crucial period for optimizing energy use in order to optimize both adult size and the length of the reproductive lifespan. The intellectual merit of this project is to shed light on the way that the bodies of adolescent women allocate energy in activity and body composition under conditions of energetic stress, identifying possible adaptations in human behavior and physiology during this vital final window of linear growth. Its broader impact beyond the community of physical anthropologists and evolutionary biologists was to create job opportunities for local Gambian field workers, train those field workers in the use of activity and energy expenditure telemetry with accelerometers and heart rate monitors, and provide opportunities to undergraduate students to collect thesis data and gain international science experience. Data collected for this study generated several significant findings. First, weight change and energy expenditure data revealed that adolescent female weight loss in the rainy agricultural season is due to food shortage relative to activity, while weight gain in the dry harvest season is due to increased food availability but not decreased activity. Second, older adolescent women spend more energy in activity than younger adolescent women, and married adolescents, who tend to be older, spend more energy in activity than their unmarried peers. Finally and most intriguingly, the way that adolescent female bodies allocate energy within their bodies changes over the course of puberty. In times of energy constraint and relative energy abundance, younger, less developed, and faster-growing adolescent women allocate energy to lean mass--that is, to bone and muscle and organs--even when this means using up some of the small amount of fat mass that their bodies have stored. Older, more developed adolescent women who are no longer investing so much energy in getting taller, on the other hand, maintain fat mass during the rainy agricultural season when food supplies are short and lose lean mass. From the perspective of evolutionary biologists and physical anthropologists who are interested in the ways that bodies budget their energy, this division in allocation across puberty suggests that growing taller--or growing a bigger skeleton, including a pelvis that is large enough to give birth to a full term fetus--is a high priority for human female adolescents, even under conditions of energetic stress. Once growth nears completion, however, bodies prioritize reproductively important adipose tissue, which helps to meet the caloric costs of pregnancy and nursing. The variation in energetic priorities seen within the sample emphasizes the extent to which adolescence is a period of transition rather than a moment of change, and the patterns of age and developmental stage seen within that transition indicate that adolescent humans likely evolved to respond to variability in energy availability by modulating their short term investments in fat and lean mass.
