Association of physical activity and body composition with insulin sensitivity in a community sample of adolescents

Objective: To examine how body composition and physical activity are related to insulin sensitivity and secretion in adolescents. Design: Cross-sectional. Setting: Baltimore, Maryland. Participants: Fifty-six healthy adolescents (34 boys and 22 girls; mean [SD] age, 13.3 [1.3] years; 95% were African American) who had been recruited at infancy from a health care clinic serving a low-income, urban community. Main Exposures: Physical activity was measured for 5 to 7 days by a uniaxial accelerometer placed on the right ankle. Proportion of time spent in play-equivalent physical activity (PEPA) was defined as 1800 or more counts per minute. Body mass index (BMI) (calculated as weight in kilograms divided by height in meters squared) was converted to an age- and sex-specific z score. Main Outcome Measures: Insulin sensitivity, insulin secretion, and disposition index calculated from a fasting oral glucose tolerance test. Results: Thirty-nine percent of the adolescents had a BMI in the 85th percentile or higher; half of those were overweight (BMI≥95th percentile). Play-equivalent physical activity and BMI z score were not correlated. In multivariate analyses, BMI z score and time spent in PEPA together explained 21% of the variance in insulin sensitivity and 18% in insulin secretion. Independent of each other, high BMI z score and low proportion of PEPA were significantly associated with low insulin sensitivity (partial r ²=0.14 and 0.10, respectively) and high insulin secretion (partial r²=0.10 and 0.10, respectively), but not with disposition index. Conclusions: In a cohort of urban, predominantly African American adolescents, both body composition and physical activity were independently associated with insulin sensitivity. At this point, insulin secretion is appropriately regulated.