Breastfeeding duration and long-term body mass index trajectories: a nationwide birth cohort study in Japan

Infant feeding practices are associated with childhood growth, but whether their associations with long-term body mass index (BMI) trajectories differ between boys and girls remains unclear.

We aimed to examine the associations between infant feeding practices and BMI trajectories from early childhood to early adolescence separately in boys and girls.

This secondary analysis used a nationwide Japanese birth cohort (n = 29,264). Latent class mixed models identified BMI z-score trajectories from ages 1.5 to 13 years. Weighted multinomial logistic regression, using inverse probability weighting based on propensity scores, estimated associations between breastfeeding duration/categories and trajectory membership.

Three trajectories were identified: "Decreasing," "Stable," and "Increasing." In boys, exclusive breastfeeding was associated with lower odds of the Increasing trajectory compared with exclusive formula feeding (OR: 0.90; 95% CI: 0.83-0.98). Conversely, in girls, moderate- and long-term breastfeeding were associated with higher odds of the Increasing trajectory compared with never (<1 month) breastfeeding (moderate-term OR: 1.21; 95% CI, 1.11-1.32; long-term OR: 1.18; 95% CI, 1.08-1.28). Mixed feeding in girls was associated with higher odds of both extreme trajectories.

Sex-stratified analyses showed different patterns of association between infant feeding practices and BMI trajectories in boys and girls. Exclusive breastfeeding was associated with lower odds of the Increasing trajectory in boys, whereas moderate- and long-term breastfeeding were associated with higher odds of this trajectory in girls.