Measures of Resting Metabolic Rate, Physical Fitness, and Leisure-Time Physical Activity Among Underweight and Normal Weight High School Girls [Exercise Physiology & Fitness]

Background: Resting metabolic rate is a function of body mass and is closely related to the size of the fat-free mass in adults. Among adolescents, physical activity has been shown to be associated with lower fat mass. However, it is unclear what affects leisure-time physical activity has on the resting metabolic rate. The purpose of this investigation was to study relationships among measures of health-related physical fitness, leisure-time physical activity, and resting metabolic rate among underweight and normal weight high school girls. Methods: Sixteen girls (15.0 ± 1.1 yr) whose body mass indexes based on gender-specific BMI-for-age were below the 85th percentile participated in this study. Leisure-time physical activity, operationalized as duration, work, caloric expenditure, and average intensity was quantified by a 21-day leisure-time physical activity diary and compendium of physical activities with MET intensities. Percent fat was predicted from skinfold thicknesses. Resting metabolic rate was assessed via open-circuit gas analysis. Physical fitness and leisure-time physical activity variables were compared to resting metabolic rate. Regression analyses were conducted to ascertain the variance of resting metabolic rate that could be explained by leisure-time physical activity and physical fitness variables. Results: Resting metabolic rate correlated strongly with fat-free mass (r = .69, p < .01), body mass (r = .67, p < .01), and body mass index (r = .65, (p < .01 ), and moderately with percent fat (r = .51, p < .05), leisure-time physical activity time (r = .56, p < .05), leisure-time physical activity work (r = .56, p < .05), and leisure-time physical activity caloric expenditure per kg body mass (r = .56, p < .05). Resting metabolic rate was best explained by fat-free mass, R² = .477, p < .01. Discussion: This study's findings for predictors of and variables associated with resting metabolic rate are in agreement with existing data among adults, with resting metabolic rate best explained by fat-free mass. In addition to variables generally associated with resting metabolic rate, leisure-time physical activity operationalized as caloric expenditure per kg body mass was strongly associated with resting metabolic rate. Conclusions: Resting metabolic rate is a function of fat-free mass and is strongly associated with body mass and caloric expenditure per kg body mass in underweight and normal weight high school girls.
Keyword(s): exercise/fitness, physical activity