Body fat is essential for health; fat tissue not only stores energy but is an endocrine organ that synthesizes and secretes hormones (including estrogen and testosterone) important for healthy growth and maturation. Excess fat mass during childhood and adolescence tends to track into adulthood and is associated with adverse health outcomes. Obesity prevention has become a key priority for health agencies across Canada and around the world, as nearly one in seven Canadian children and adolescents have obesity as measured by body mass index (BMI). However, BMI does not discriminate between fat and fat-free mass and often reflects healthy increases in fat-free mass during growth and maturation. As such, objective measures of fat mass are vital to improving our understanding of predictors of body fat throughout childhood and adolescence.
In our mixed-longitudinal study, published in Medicine & Science in Sports & Exercise®, we measured total body fat mass in over 300 boys and girls across adolescence using dual-energy X-ray absorptiometry. We assessed physical activity and sedentary behaviors using accelerometry and total energy intake via dietary recalls. Girls and boys mature at different ages (e.g., average age at peak height velocity is 11.5 years for girls and 13.6 years for boys) and growth of body tissues coincides with timing of maturation. Therefore, we aligned girls and boys on biological age (age at peak height velocity) instead of chronological age.
We found that boys and girls gain fat mass differently. The rate of fat mass accrual in girls was four times greater than boys at age at peak height velocity and increased across adolescence. Boys’ fat mass plateaued after age at peak height velocity. Physical activity may protect against excess fat accrual, yet only 14% of girls and 41% of boys in our study met the 60 minutes/day of moderate to vigorous physical activity (MVPA) recommended for health benefits. We found that MVPA negatively predicted fat mass independent of time spent in sedentary activities. For boys, every six minutes/day annual increase in MVPA was associated with 2% lower fat mass. For girls, every four minutes/day greater MVPA was associated with 3% lower fat mass.
Given divergent trajectories of fat mass accrual and physical activity participation in boys and girls, our study emphasizes the need for gender-specific approaches to health-promotion interventions. On average, adolescent girls increased fat mass by 1.4 kg/year during late childhood and adolescence, with lower rates of accrual thereafter. Body fat is particularly important for menstrual function in females. Parents, educators, coaches and sporting agencies should recognize that fat mass accrual is part of healthy growth, and that growth trajectories differ between girls and boys during adolescence.
We hope our work will serve as a building block for future studies to add robust measures of diet, sleep habits (quality and duration), parental characteristics (weight status, health outcomes, employment and education characteristics), the built environment, and socioeconomic status, as these are all related to fat mass accrual. Additionally, a longer follow-up and earlier baseline entry would help determine critical periods when health behaviors such as physical activity may play their most influential role. Exploring these research questions could significantly improve our understanding of the determinants of fat mass accrual and have public health implications across the lifespan.
Jennifer McConnell-Nzunga, PhD, is an analyst with Statistics Canada and an adjunct professor in the Department of Psychology at the University of Regina in Saskatchewan, Canada. Her research focuses on obesity, health behaviors, social determinants of health, and health care utilization using large, longitudinal, linked datasets. Dr. McConnell-Nzunga earned her Ph.D. in the social dimensions of health from the University of Victoria in British Columbia, Canada, where her research focused on childhood obesity measurement, classification and longitudinal determinants.
Leigh Gabel, PhD, is an assistant professor in the Faculty of Kinesiology at the University of Calgary in Alberta, Canada. Dr. Gabel’s research program focuses on the role of physical activity and exercise for musculoskeletal health. Her research uses advanced medical imaging, including DXA and peripheral quantitative computed tomography (pQCT), to assess changes in muscle, fat and bone tissues across the lifespan.
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