Milk Osmolality and Body Hydration Status in Lactating Mothers from the Western Highlands of Guatemala

Background: Human plasma osmolality is tightly regulated in the 275 – 295 mOsm/kg range and breast milk osmolality (Mosm) oscillates through a slightly higher range. Studies in dairy cows demonstrate an influence of systemic hydration on bovine Mosm. Objective: To describe hydration status in lactating women and explore any relation with Mosm. Methods: 31 low- and middle-income women gave a full-breast expression after 90 min of nursing abstinence along with a concurrent spot sample of urine, 15 of whom repeated ~7 days later. Milk volume (Mvol) was measured and aliquots of urine and milk stored from 1 to 12 wks. Osmolality was determined in both fluids using a Vogel, Loser 815 osmometer expressed as mOsm/kg. Data were analyzed using SPSS version 20. Results: The Mvol ranged from 2-100 mL, and Mosm from 288-448 mOsm/kg, whereas Uosm ranged from 93-1678 mOsm/kg. Nevertheless, the Spearmann rank-order correlation coefficient was r=0.214 (p=0.153) [n=46]; Mosm was not associated with maternal age r=0.243 (p=...