Sex-Specific Effects of Relative Fat Mass on Attention-Deficit/Hyperactivity Disorder: Insights from the 1999–2004 National Health and Nutrition Examination Survey

BACKGROUND: Attention-deficit/hyperactivity disorder (ADHD) is a common neurodevelopmental disorder in childhood. Conventional adiposity indicators have been linked to ADHD. Relative fat mass (RFM), a novel adiposity measure, has demonstrated advantages in predicting metabolic and cardiovascular disease risk, conditions that share overlapping pathophysiological mechanisms with ADHD. This study aimed to investigate the association between RFM and ADHD, with a particular focus on sex-specific differences. METHODS: Data were obtained from the 1999-2004 National Health and Nutrition Examination Survey, including 5089 children aged 6-14 years. RFM was calculated using height and waist circumference, and ADHD was defined based on physician diagnosis reported in the questionnaire. Multivariable logistic regression models were applied to assess the association between RFM and ADHD. Smooth curve fitting was conducted to validate the results, and subgroup analyses were performed separately for boys and girls. RESULTS: A significant sex-specific association between RFM and ADHD was observed. Among boys, higher RFM levels were inversely associated with ADHD (Model 3: odds ratio [OR] = 0.967, 95% confidence interval [CI]: 0.946-0.989), whereas among girls, higher RFM levels were positively associated with ADHD (Model 3: OR = 1.043, 95% CI: 1.007-1.081). Smooth curve fitting confirmed these opposite linear trends in both sexes. Subgroup analyses further demonstrated that this sex-specific pattern was consistent across strata defined by age, health insurance status, maternal smoking during pregnancy, and birth weight. CONCLUSIONS: There is a significant sex-specific association between RFM and ADHD, showing an inverse relationship in boys and a positive association in girls. These findings suggest that the influence of adiposity distribution on ADHD differs by sex, highlighting the importance of considering sex differences when evaluating risk factors for ADHD.