P2.191: Changes in physical activity and sedentary time from early to middle childhood and their associations with fat mass, fat free mass and body fat percentage

Background: In the United States, childhood obesity is rising and physical activity and sedentary behaviors are thought to be key determinants. Most of this evidence comes from cross-sectional studies which cannot establish a temporal relationship. Further, many studies use self-reported measures of activity, which are prone to misreporting, and Body Mass Index (BMI), which cannot distinguish between lean and fat mass. The objective was to prospectively examine the relationship between moderate-to-vigorous physical activity (MVPA), sedentary time (ST) and adiposity from early to middle childhood using objective and more granular measures of behaviors and adiposity.

Methods: Participants were children (n=536) in Colorado. Data were collected at 2 timepoints: ages 4-6 years and 8-12 years. A sub-sample (n=212) had measures of MVPA and ST at both time points. Physical activity and ST were measured using ActiGraph accelerometers worn for 7-days (model wGT3X-BT, Pensacola, FL). Weight and height were measured using anthropometry. BMI (kg/m2) was calculated. Fat Mass Index (FMI), Fat Free Mass Index (FFMI), and body fat percentage (BF%) were measured using air displacement plethysmography (Cosmed, Italy). Data were analyzed using mixed effect models with adjustments for sociodemographic characteristics.

Results: In cross-sectional analyses, each additional 30 min/day of MVPA correlated with significantly lower FMI (-0.23 [95%CI, -0.34 to -0.12]), lower BF% (-1.2 [95%CI, -1.7 to -0.72]), and higher FFMI (0.13 [95%CI, 0.057 to 0.21]). Each additional 30 min/day in ST was linked to significantly higher FMI (0.083 [95%CI, 0.0092–0.16]) and BF% (0.43 [95%CI, 0.098–0.77]). Among the sub-sample of children with repeated measures of MVPA and ST, a 30 min/day MVPA increase between early and middle childhood was significantly associated with lower FMI (-0.28 [95%CI, -0.53 to -0.034]) and lower BF% (-1.1 [95%CI, -2.1 to -0.080]) at follow-up. Conversely, a 30 min/day ST increase was linked to higher BF% (0.72 [95%CI, 0.082–1.4]) at follow-up. Neither MVPA nor ST were associated with BMI.

Conclusion: Increases in MVPA from early to middle childhood were associated with greater lean mass and less fat mass. Over the same period, increases in sedentary time were associated with a higher body fat percentage. No associations were found with BMI. These findings demonstrate the potential benefits of promoting active lifestyles in early childhood; and are consistent with the American Academy of Pediatrics guidance on physical activity counseling in pediatric clinical settings. Importantly, a 30 min/day change in either behavior corresponded to ~1% BF% reduction, suggesting that improvements in MVPA and ST should be promoted alongside other obesity prevention strategies. The association of an active lifestyle with FMI, FFMI, and BF%, but not BMI raises questions about whether studies that use BMI as the sole indicator of adiposity may underestimate the effects of movement behaviors on adiposity.