Effects of maternal hydration status on the osmolality of maternal milk

Background: It has been demonstrated that human milk osmolality (Mosm) is regulated within an established range, typically 290 to 300 mOsm/kg, and appears to be resistant to effects of maternal dehydration, as reflected by high urinary osmolality (Uosm). Objective: To determine the degree of association between Mosm and Uosm at a common point in time, as well as the reproducibility of both measures over a one-week interval of sampling. Methods: Mosm and Uosm were measured with a Vogel Loser 450 osmometer on samples of the respective biological fluids collected concurrently in 31 lactating women, with infants aged between 30 and 340 days. In the first 15 women recruited, collections were repeated 7 days after the initial ones. Results: The median Mosm for the 46 samples collected was 308 mOsm/kg with a range from 288 to 448 mOsm/kg. The corresponding values for Uosm were 598 mOsm/kg with a range from 93 to 1,678 mOsm/kg. The Spearman rank-order correlation coefficient for within-individual association of Mosm and Uosm was r = 0.214 (p = 0.153). The median Mosm for the 15 repeat-subjects was 309 mOsm/kg on both occasions, with a within-individual Spearman coefficient of r = 0.326 (p = 0.118). By contrast, for the Uosm, the within-subject association was much stronger, with r = 0.699 (p = 0.002). Conclusions: The osmometry technique proved to be a highly stable and reproducible measurement technique. Mosm and Uosm are not significantly associated at a point in time. Intra-subject Mosm varies more across time than intra-subject Uosm.