Childhood obesity is a global health crisis. Current World Health Organization (WHO) guidelines recommend at least 60 minutes per day of moderate-to-vigorous physical activity (MVPA) for children and adolescents to maintain healthy weight. However, most studies supporting these guidelines are cross-sectional or short-term, and the long-term effects of objectively measured movement behaviors on body composition remain unclear. Furthermore, the mechanisms by which movement behaviors influence body composition, whether directly or indirectly via metabolic pathways, are not fully understood. This study aimed to address these knowledge gaps by investigating the longitudinal associations of objectively measured sedentary time (ST), light physical activity (LPA), and MVPA with changes in body composition, specifically fat mass, in a large birth cohort. The study also explored potential causal relationships and mediating roles of metabolic factors, such as glucose, insulin, and lipids, in these associations.

Existing literature on the impact of physical activity on childhood obesity presents mixed results. While the WHO guidelines advocate for at least 60 minutes of MVPA daily, many school-based interventions show little to no effect on body mass index (BMI). A significant gap exists in long-term longitudinal studies using objective measures of movement behavior and body composition. Animal models and small human studies have suggested several biological pathways linking sedentary behavior and adiposity, including inflammation, altered glucose and lipid metabolism, and muscle atrophy. However, the causal relationships and the roles of mediating factors in human populations, particularly during growth and development, remain unclear. This lack of longitudinal evidence hinders the development of effective obesity prevention strategies.

This study used data from the Avon Longitudinal Study of Parents and Children (ALSPAC), a UK birth cohort. A total of 6059 participants with at least one time-point measure of movement behavior and body composition were included in the primary analyses. Accelerometer data (ActiGraph) were used to objectively measure ST, LPA, and MVPA at ages 11, 15, and 24 years. Dual-energy X-ray absorptiometry (DEXA) scans measured body composition (total fat mass, trunk fat mass, lean mass) at the same ages. Fasting blood samples were collected to assess glucose, insulin, lipids, and high-sensitivity C-reactive protein at ages 15, 17, and 24. Statistical analyses included generalized linear mixed-effect models (GLMM) to examine longitudinal associations, mediation analyses using structural equation modeling to assess the roles of metabolic factors, and temporal causal path analyses to investigate causal relationships. Multiple imputation was used to handle missing data.

The study found significant longitudinal associations between movement behaviors and fat mass. Each minute per day of cumulative ST from ages 11–24 was associated with a 1.3 g increase in total fat mass. In contrast, each minute per day of LPA was associated with a 3.6 g decrease, and each minute per day of MVPA with a 1.3 g decrease. Children consistently achieving ≥60 min/day of MVPA showed a larger decrease in fat mass (2.8 g per minute) compared to those with <40 min/day. Mediation analyses revealed that increased insulin and LDL cholesterol partially suppressed the positive association between ST and fat mass. Temporal causal analyses indicated that higher fat mass at age 11 predicted lower MVPA at age 15, while higher MVPA at age 15 predicted lower fat mass at age 24. Sex-specific analyses showed similar trends, with females generally having higher fat mass and males engaging in more MVPA.

These findings provide strong evidence for the independent and potentially causal role of movement behaviors in influencing fat mass accumulation during childhood and adolescence. The consistent association between increased ST and higher fat mass, irrespective of LPA and MVPA levels, highlights the detrimental effects of prolonged sedentary time. The significant fat-mass-lowering effect of LPA, comparable to or even exceeding that of MVPA, suggests that LPA should be a key target in obesity prevention strategies, particularly for children who struggle with or are unwilling to engage in high-intensity exercise. The mediating roles of insulin and LDL cholesterol underscore the importance of considering metabolic health in obesity prevention programs. The temporal causal relationships suggest that early interventions targeting MVPA in early childhood may be crucial to prevent the development of obesity and subsequent reduction in MVPA during adolescence.

This study demonstrates the long-term impact of sedentary time, light physical activity, and moderate-to-vigorous physical activity on body composition in children. The strong association between sedentary time and increased fat mass, coupled with the significant fat-reducing effects of LPA and MVPA, emphasizes the need for a comprehensive approach to obesity prevention that includes both reducing sedentary behavior and increasing physical activity at all intensity levels, with a particular focus on early childhood interventions. Future research should investigate the optimal balance between LPA and MVPA for long-term weight management and explore the underlying mechanisms through which movement behaviors impact body composition during growth and development in diverse populations.

The study population was predominantly White, limiting the generalizability of the findings to other racial and ethnic groups. The study relied on self-reported data for some variables, which might introduce recall bias. Cohort attrition could have led to some selection bias, however, the characteristics of the excluded participants were similar to those included. The unavailability of fasting blood samples at all three time points might have affected the mediation analyses.