Short-term oral liquid ingestion decreases human milk osmolality

Introduction: previous studies have reported stability of human milk osmolality in situations of liquid restriction and a decrease in cow milk osmolality in conditions of controlled water ingestion. Objective: To confirm any possible influence of shortterm water ingestion and restriction in human milk osmolality. Method: 26 mothers of breastfed infants aged 3-6 months were invited to come twice and stay for 90-min in an educational meeting with nutritionists from CeSSIAM. Treatment-1 (tx-1) consisted in consuming 1l of water over 90-min; treatment-2 (tx-2) consisted in being abstained from fluid intake for 90-min. Breastmilk and urine samples were obtained before and after each treatment and measured to determine volume; subsequently, two aliquots were stored at -20oc prior to analysis. Urinary (Uosm) and milk (Mosm) osmolality were measured in a Voguel Loser 815 osmometer. Statistical analyses were performed in SPSS version 20. Results: Maternal mean age was 24 y, 42% of the babies were girls. Median baseline and final (90-min) Uosm for Tx-1 were 628 and 248 mOsm/kg (p<0.001), corresponding values for Mosm, 284 and 276 mOsm/kg (p<0.001). Median baseline and 90-min Uosm for Tx-2 were 632 and 655 mOsm/kg (p=0.320), corresponding values for Mosm, 282 and 280 mOsm/kg (p=0.325). There was significant different in the change of Mosm (p=0.013) as well as in the change of Uosm (p<0.001) between treatments. Conclusions: We found asymmetrical change in breast milk osmolality after water consumption and water restriction; while fluid restriction does not influence Mosm and Uosm, liquid ingestion impacts human milk and urine by decreasing osmolality.