Urinary oxytocin levels in one-month-old infants are higher when samples are collected shortly after feeding, irrespective of feeding method.

BACKGROUND: Oxytocin regulates social bonding, lactation, and early caregiver-infant attachment. Breastfeeding promotes parent-infant bonding; however, its direct effect on oxytocin secretion in young infants remains poorly characterized. Urinary measurement offers a non-invasive alternative in early infancy. OBJECTIVE: To examine associations between urinary oxytocin levels in one-month-old infants and elapsed time since the last feeding, feeding method, and maternal postnatal depression. METHODS: In this cross-sectional study (November 2020-April 2022), urine was collected from 90 infants at their one-month health check-up. Creatinine-adjusted oxytocin concentrations were compared by elapsed time since feeding (n = 77 with available timing data) and feeding method (n = 90) using the Kruskal-Wallis test with Steel-Dwass post-hoc comparisons and the Wilcoxon rank-sum test. RESULTS: The median creatinine-adjusted urinary oxytocin concentration was 1109 (IQR 629-2010) pg/mg Cre. Urinary oxytocin concentrations were significantly higher in infants sampled within 2.5 h of the last feeding than in those sampled later (1440 vs. 617 pg/mg Cre; p = 0.0001), irrespective of feeding method. Among the four elapsed-time groups (<1 h, 1-2 h, 2-3 h, ≥3 h), the Kruskal-Wallis test showed a significant overall difference (p = 0.0045), with concentrations being significantly lower in the ≥ 3-h group. No significant associations were found with feeding method, sex, birth weight, or Edinburgh Postnatal Depression Scale scores. CONCLUSIONS: Urinary oxytocin levels were higher in infants sampled shortly after feeding, irrespective of feeding method, suggesting that nutritional stimulation and the caregiving context of feeding may be associated with infant oxytocin levels.