BackgroundEating behavior is associated with weight gain in infancy and childhood. Few studies found a bidirectional association between weight gain and eating behavior development in childhood, but there is little data on the association in early infancy, a period critical for the programming of obesity risk. ObjectiveWe investigated the bidirectional association between appetite traits and weight gain during the first year of life. MethodsParticipants were part of a cohort of 432 infants born in Cyprus. Appetite traits were measured using the Baby Eating Behavior Questionnaire or the Child Eating Behavior Questionnaire at age 2 to 4 wk, 6 mo, and 12 mo. Weight and length were collected at birth, 4 wk, 6 mo, and 12 mo. Multivariable linear regression was used to analyze associations between appetite traits at 2 to 4 wk and 6 mo and weight for age z-score change (WFAZC) between 4 wk and 6 mo and 6 and 12 mo. Associations were also analyzed in the opposite direction, between WFAZC from birth to 4 wk, 4 wk to 6 mo, and 6 mo to 12 mo and appetite traits at 4 wk, 6 mo, and 12 mo. ResultsSatiety responsiveness (SR) at 2 to 4 wk was associated with lower WFAZC from 4 wk to 6 mo (β: −0.17; 95% CI: −0.30, −0.04) and SR at age 6 mo was associated with lower WFAZC from 6 to 12 mo (β: −0.09; 95% CI: −0.17, −0.02). WFAZC from 4 wk to 6 mo was associated with higher enjoyment of food at 12 mo (β: 0.11; 95% CI: 0.01, 0.20), higher food responsiveness at 12 mo (β: 0.17; 95% CI: 0.04, 0.30), and lower SR at both 6 mo (β: −0.11; 95% CI: −0.21, −0.01) and 12 mo (β: −0.14; 95% CI: −0.24, −0.03). ConclusionsWe found a bidirectional association between weight gain and appetite traits in infancy, suggesting that the effect of postnatal weight gain on obesity development is partly mediated by programming of appetite traits.