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Fatness predicts decreased physical activity and increased sedentary time, but not vice versa: support from a longitudinal study in 8- to 11-year-old children


Objective: To examine independent and combined cross-sectional associations between movement behaviors (physical activity (PA), sedentary time, sleep duration, screen time and sleep disturbance) and fat mass index (FMI), as well as to examine longitudinal associations between movement behaviors and FMI.

Methods: Cross-sectional and longitudinal analyses were done using data from the OPUS school meal study on 785 children (52% boys, 13.4%overweight, ages 8–11 years). Total PA, moderate-to-vigorous PA (MVPA), sedentary time and sleep duration (7 days and 8 nights) were assessed by an accelerometer and FMI was determined by dual-energy X-ray absorptiometry (DXA) on three occasions over 200 days. Demographic characteristics, screen time and sleep disturbance (Children’s Sleep Habits Questionnaire) were also obtained.

Results: Total PA, MVPA and sleep duration were negatively associated with FMI, while sedentary time and sleep disturbances were positively associated with FMI (P 0.01). However, only total PA, MVPA and sleep duration were independently associated with FMI after adjustment for multiple covariates (P<0.001). Nevertheless, combined associations revealed synergistic effects among the different movement behaviors. Changes over time in MVPA were negatively associated with changes in FMI (P<0.001). However, none of the movement behaviors at baseline predicted changes in FMI (P>0.05), but higher FMI at baseline predicted a decrease in total PA and MVPA, and an increase in sedentary time (P 0.001), even in normal-weight children (P 0.03).

Conclusion: Total PA, MVPA and sleep duration were independently associated with FMI, and combined associations of movement behaviors showed a synergistic effect with FMI. In the longitudinal study design, a high FMI at baseline was associated with lower PA and higher sedentary time after 200 days but not vice versa, even in normal-weight children. Our results suggest that adiposity is a better predictor of PA and sedentary behavior changes than the other way around.

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Alex’s Notes: Well, apparently the BMI of 10-year-old children is 75% lean mass and 25% fat mass, and the authors of the study at hand hypothesize that changes in fat mass could be partly overlooked when only BMI is examined. As such, this study aimed to examine the associations between physical activity (or lack of), sleep, and fat mass as determined by DXA scan.

The study sample included 834 8-11 year-old Danish students. They wore an accelerometer device to track movement for seven consecutive days and eight nights, and pretty much wore it 24 hours per day. Both the children and parents kept bedtime and wake-time logs, and dietary habits were tracked daily for the week using an online dietary assessment program. Finally, the children had their weight and height measured, and every one of them had body-fat levels assessed via DXA scan.

In a nutshell, total physical activity, moderate-to-vigorous physical activity (MVPA), and sleep duration were associated with less fat mass, while sedentary time and sleep disturbances were associated with greater fat mass. However, only the first three associations remained significant after adjusting for covariates such as caloric intake, age, sex, and parental education.

Perhaps the most interesting finding, however, was that fat mass predicted future levels of physical activity, MVPA, and sedentary time, but not vice-versa. In other words, the fatness of children may be a better predictor of physical activity levels and sedentary behavior changes than the other way around. The authors speculate that,

“Increasing adiposity could simply refrain children from engaging in MVPA relative to total PA. Finally, as children with higher FMI are expected to produce greater energy expenditure for a given amount of movement, a possible physiological explanation for the reverse causality could be fatigue and thereby lowering of especially MVPA.”

Additionally, fat mass was not associated with sleep duration or disturbances in the long-run, but fatter kids tended to have more disturbances and shorter sleep durations.


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