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, Löser 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=0.104), nor with lactation age r= ‐0.280 (p=0.060). Pearson correlation for repeat Mosm was r=0.204 (p=0.233) and for Uosm was r=0.642 (p=0.005) [n=15] Conclusions Lactating women have higher Uosm than other sectors in Western Guatemala. Mosm is not associated with systemic hydration as reflected by Uosm. There is a high association of systemic hydration status on repeated measurement Supported by The European Hydration Institute, Madrid, Spain; Fundación Iberoamericana de Nutricion, Granada, Spain; and Hildegard Grunow Foundation, Munich, Germany