Summary Context There is an increasing prevalence of overweight during early childhood in the most impoverished areas in Brazil, although there is a lack of evidence regarding its onset. Objectives To investigate the incidence and risk factors associated with overweight among low‐income children during their first year of life. Methods We analysed data from a prospective birth cohort study conducted in Brazil, which followed‐up children at birth, 3rd, 6th and 12th months (n = 196). The overweight incidence (zBMI/A > 2SD) was analysed using the Kaplan–Meier survival estimator and its associated factors by the Coxs regression model. Missing data were addressed with multiple imputations and results on the final adjusted model were calculated by pooling the estimates generated for each imputed dataset. Results The overweight incidence was 17 events/100 children‐year, and most cases occurred before the 6th month. The final model showed that children exclusively breastfed up to 30 days (vs. >30 days; HR 2.68; 95%CI 1.11–6.49) and whose mothers consumed ultra‐processed foods more than 4 times/day (vs. ≤4 times/day; HR 3.02; 95%CI 1.28–7.13) presented a higher risk of developing overweight. Conclusion The overweight incidence was high in this impoverished population. Shorter exclusive breastfeeding duration and an unhealthy household food environment provided a short‐term risk increase.

Background: Although childhood obesity is increasing in low-income regions, theoretical models cannot be adequately applied due to the lack of prospective studies with under 2-year-old children from impoverished populations. Objective: To analyse direct and indirect effects of environmental, maternal, and individual factors on excess weight gain among low-income children during the first year of life.Methods: We analysed data from a prospective birth cohort conducted in Brazil, which followed infants at birth, 3rd, 6th, and 12th month of life (n = 205). The weight-for-age z-score was used to calculate the conditional weight gain (CWG).Direct and indirect effects on CWG were estimated using structural equation modelling.Results: Children's consumption of ultra-processed food (UPF) ≥1 time/day, breastfeeding duration >90 days, and maternal obesity showed a statistically significant direct effect on children's CWG. We observed a positive indirect pathway linking the maternal intake of UPF ≥4 times/day to the CWG, given its direct effect on the infant's UPF consumption. Conclusions:In this low-income population, women who were frequent users of UPF tended to introduce these products more frequently into their children's diets. The early trade-off trend in children's diet between breastfeeding and UPF was the leading cause of excessive weight gain.