Comparative evaluation of the performance of urinary biomarkers of hydration status among lactating women in the Western Highlands of Guatemala

Background Total daily water requirements rise by 1 L while food taboos and altered feeding practices may intercede in lactation. In fact, our group's studies on hydration status in lactation in the Western Highlands of Guatemala were inferior to those of non‐lactating controls.research professionals. Presented elsewhere (Diaz‐Jereda et al. 2017) we document water‐intake deficits in of over a liter compared to the Adequate Intake recommendations for lactation (IOM, 2005). These convergent findings raise concerns about hydration status during lactation in Guatemala and all valid options for assessing hydration status should be available to researchers and public health personnel. Objectives To compare 4 urinary hydration biomarkers ‐‐ osmolality (U OSM ), specific gravity (USG), volume (U VOL ) and color (U COL ) among lactating women in the Western Highlands of Guatemala. Methods Lactating women (n=13) with infants aged 3–6 months came in twice as part of an interventional, cross‐over study on the modifiability of breastmilk osmolality.. Baseline and endline urine specimens were collect in both Treatment 1 (consisted of 1 L water ingestion within 90 min), and Treatment 2 (90‐min fluid abstention), resulting in 4 samples per woman (pooled n=52). Urine was measured for volume, aliquoted and stored at −20°C. At analysis, samples were thawed to room temperature and mixed by agitation prior to measuring U OSM and USG. U COL was read by one rater using an 8‐scale urine color chart. Hydration status was estimated using only baseline samples (n=26), according to cut‐off values recently validated for lactating women (McKenzie et al., 2015). Hypohydration criteria: U OSM >500 mOsm/kg; USG >1.010; and U COL >4. We used untimed spot‐urine volume for U VOL , as the rate expression of U VOL /h could not be calculated for baseline. Results Respective medians of the urinary biomarkers were: U OSM 618 mOsm/kg; USG 1.015; U VOL 75 mL; and U COL 3. Spearman's correlation coefficient (r) showed significant inter‐correlations (p<0.01 2‐tailed) for all biomarkers: U OSM − U COL (r=0.933); U OSM − USG (r=0.880); USG − U COL (r=0.851); U OSM − U VOL (r=−0.756); U VOL − U COL (r=−0.742); and U VOL − USG (r=− 0.664). The classification prior to Treatments 1 and 2, respectively, revealed 69 and 77% (U OSM ), 92 and 84% (USG), and 46 and 62% (U COL ) as hypohydrated. Sensitivity for hypohydration at >500 mOsm/kg was 100% with USG values and 80% with U COL . Specificity for <500 mOsm/kg was 100% for U COL . Conclusion The hydration biomarkers indicate a state of hypohydration among at least half of the lactating women on two separate occasions. A firm generalization however requires a larger and representative sample. All urinary biomarkers show high correlation with one another, while the classification based on the current cut‐off values show differences between methods. USG has the highest specificity whereas U COL has the highest sensitivity against U OSM . In the field setting, USG and U COL analysis are an inexpensive, easy and accurate hydration‐status option.