Longer Breastfeeding Is Associated With Healthier Body Mass Index Trajectories and Lower Risk of Overweight and Obesity in Youth Aged 9–12 Years

ABSTRACT Background The association between breastfeeding and adolescent obesity remains unclear because most previous studies included youth with obesity. We explored these associations from ages 9 to 12 amongst initially healthy weight youth. Methods The analytic sample included 4161 youth (10 586 observations across three assessments; all healthy weight at baseline; 50.7% female; 52% breastfed more than 6 months) from the Adolescent Brain Cognitive Development (ABCD) Study. Linear mixed‐effects models examined breastfeeding × age × sex and breastfeeding × puberty × sex interactions on BMI trajectories (kg/m 2 ). Logistic regression estimated the risk of unhealthy weight (BMI ≥ 85th percentile) by ages 11–12. All analyses were stratified by sex. Results Three‐way interactions were not significant. Significant breastfeeding × age interactions ( β = −0.005, p < 0.001) indicated that breastfed youth exhibited lower BMI trajectories, particularly when breastfed for more than 6 months ( β = −0.006, p < 0.001). Significant breastfeeding × puberty interactions showed lower BMI trajectories across pubertal stages amongst breastfed youth ( β = −0.06, p = 0.017), observed only in boys and especially in those breastfed beyond 6 months ( β = −0.11, p = 0.005). Breastfeeding was associated with a reduced unhealthy weight risk at ages 11–12 (RR = 0.78, p = 0.002), exclusively in boys and amongst those breastfed more than 6 months (RR = 0.52, p < 0.001). No associations were found in girls. Conclusion Amongst initially healthy weight youth, breastfeeding, especially beyond 6 months, was associated with healthier BMI trajectories and lower unhealthy weight risk in boys. Although these sex‐specific patterns warrant further research, our findings suggest that the associations between breastfeeding and body weight development extend into adolescence.