Associations Between Prolonged Breast-Feeding, Formula Supplementation, and Weight-for-Age Trajectories Through Five Years

Purpose The associations of prolonged breastfeeding beyond one year on infant growth rates remain inconclusive as are those of supplemental formula use. This study aimed to monitor and describe the growth patterns of infants breastfed for more than one year without formula supplementation compared to those with formula supplementation. Methods Data from 252 toddlers registered and followed at a well-baby clinic between 2022 and 2024 were analyzed. Included were 174 infants who were breastfed for at least 12 months without formula supplementation and 78 infants who were breastfed with added formula. Because feeding modality was not randomized, parental choice determined group allocation. Growth, demographic, and medical data were collected every three months for up to five years of age, as was breastfeeding history. Results The body weight of breastfed male infants without formula supplementation was significantly lower at 13 to 18 months of age than that of breastfed male infants with formula supplementation [10.20 kg (9.46-11.00) vs. 10.97 kg (10.35–11.90), respectively, p  < 0.001]. A similar trend was observed among female infants [9.765 kg (9.284–10.70) vs. 10.30 kg (9.52-11.00), respectively, p  = 0.08]. No significant differences in weight were observed between the two groups after 18 months of age, and this trend persisted up to 5 years. Conclusion Prolonged exclusive breastfeeding beyond one year was associated with lower weight in male infants that normalized by 18 months. Future randomized or propensity-matched studies are needed to confirm these findings.