Replacing sedentary time with MVPA cuts adiposity, improves fitness in kids

Patterns of activity appear to affect adiposity and fitness in young boys and girls, according to a recent study.

“Our findings suggest that reducing sedentary time (ST) by increasing light physical activity (LPA) may not be enough to affect fitness and adiposity. Instead, replacing lighter intensity activities with moderate-to-vigorous physical activity (MVPA) may be more strongly associated with these outcomes in children,” said researchers.

Providing wrist-worn accelerometers, researchers measured ST and activity levels in 443 children (mean age, 10.2 years; 51.2 percent female). The average ST per day was 9 hours, of which 1.9 percent was accumulated in long bouts. Daily MVPA in boys and girls was 84 and 79 minutes, respectively. [J Sports Sci 2020;38:114-120]

Isotemporal substitution analysis revealed that MVPA significantly affects adiposity in girls only. For instance, replacing ST with MVPA led to a significant reduction in adiposity (coefficient, –0.62; p=0.005), as did replacing LPA with MVPA (coefficient, –1.02; p=0.004). On the other hand, replacing ST with LPA significantly increased adiposity (coefficient, 0.40; p=0.034).

Similar effects were found when bouts of ST were considered. Replacing long ST with shorter bouts, for example, reduced adiposity in girls (coefficient, –0.69; p=0.043), as did replacing it with MVPA (coefficient, –1.32; p=0.001). Replacing short ST bouts with MVPA likewise reduced adiposity (coefficient, –0.62; p=0.004).

Notably, replacing long bouts of ST with LPA had no significant effect, while replacing short bouts led to a significant increase in adiposity (coefficient, 0.42; p=0.026). Opting for MVPA rather than LPA led to a decrease in adiposity (coefficient, –1.04; p=0.003).

No such effect of MVPA was found on adiposity in boys. There were, however, improvements in fitness. Replacing total ST and LPA (p<0.001 for both) with MVPA led to significant improvements in fitness. The same was true for substituting MVPA for long (p=0.003) and short (p<0.001) ST bouts.

Girls likewise derived fitness benefits from MVPA: Replacing LPA, total ST, and long and short bouts of ST with MVPA led to significant improvements.

“These findings suggest that activity pattern is associated with adiposity in girls and fitness in boys and girls. Further, assessing ST by long and short bouts may have limited value when using wrist-worn accelerometry in children,” said the researchers.

In the present study, participants were given wearable accelerometers the objective measurement of physical activity. They were instructed to wear the devices on their nondominant hands for 24 hours a day, for 8 consecutive days. Fat mass index was derived from bioelectrical impedance measurements and was used as the primary indicator of adiposity; VO₂max was used for the evaluation of fitness.

“Future studies should systematically explore associations of total and bouts of ST with adiposity and fitness using activity monitoring that can distinguish between seated and standing posture in order to more accurately measure ST,” the researchers said.