Body Composition, Fitness, and Mental Health in Preadolescent Children.

Importance: There is an increasing prevalence of poor fitness, concurrent with rising obesity, anxiety, and depression among children in the US. However, the association of fitness, adiposity, and lean tissue with anxiety and depression in preadolescent children is understudied. Objective: To estimate differences in the association between specific tissues (adipose and lean tissue) and cardiorespiratory fitness with anxiety and depression symptoms among children. Design, Setting, and Participants: Cross-sectional study between 2019 and 2023. Participants were a volunteer sample of children aged 8 to 11 years from the Greater Boston, Massachusetts, region without prior mental health diagnosis. Exposures: Body composition assessed via dual-energy x-ray absorptiometry (body fat percentage [BF%], visceral adipose tissue [VAT], and lean mass) and cardiorespiratory fitness measured by peak oxygen uptake (VO2peak) relative to body weight. Main Outcomes and Measures: Children self-reported trait anxiety (State-Trait Anxiety Inventory for Children) and depressive symptoms (Children's Depression Inventory). Independent multivariable hierarchical regression analyses were conducted for each outcome with sociodemographic factors in step 1 and BF%, VAT, lean mass, or fitness in step 2. Results: Among 207 children (mean [SD] age, 10.0 [0.7] years; 119 male [57.5%]; 1 [0.5%] American Indian or Alaska Native, 26 [12.6%] Asian, 39 [18.8%] Black or African American, 120 [58%] White, and 21 [10.1%] mixed race or other), anxiety was positively associated with BF% (standardized regression coefficient [β] = 0.15; 95% CI, 0.23 to 6.33; P = .04) and VAT (β = 0.15; 95% CI, 0.04 to 1.82; P = .04), and negatively associated with lean mass (β = -0.16; 95% CI, -14.77 to -1.21; P = .02) and VO2peak (β = -0.19; 95% CI, -0.28 to -0.04; P = .01). Depression showed positive associations with VAT (β = 0.27; 95% CI, 0.34 to 1.11; P < .001), and negative associations with lean mass (β = -0.16; 95% CI, -6.39 to -0.43; P = .03) and VO2peak (β = -0.16; 95% CI, -0.11 to -0.01; P = .03). Conclusions and Relevance: In this cross-sectional study of preadolescent children, greater lean mass and higher fitness were associated with fewer anxiety and depression symptoms, while higher VAT was associated with increased symptoms of both. BF% was only associated with greater anxiety. These findings highlight the roles of body composition in mental health and underscore the value of early identification of physical health markers to support children's well-being and development.