Effects of Water Loading on Observed and Predicted Plasma Sodium, and Fluid and Urine Cation Excretion in Healthy Individuals

Rationale & Objective: The discovery of sodium storage without concurrent water retention suggests the presence of an additional compartment for sodium distribution in the body. The osmoregulatory role of this compartment under hypotonic conditions is not known. Study Design: Experimental interventional study. Setting & Participants: Single-center study of 12 apparently healthy men. Intervention: To investigate whether sodium can be released from its nonosmotic stores after a hypotonic fluid load, a water-loading test (20 mL water/kg in 20 minutes) was performed. Outcomes: During a 240-minute follow-up, we compared the observed plasma sodium concentration ([Na + ]) and fluid and urine cation excretion with values predicted by the Barsoum-Levine and Nguyen-Kurtz formulas. These formulas are used for guidance of fluid therapy during dysnatremia, but do not account for nonosmotic sodium stores. Results: 30 minutes after water loading, mean plasma [Na + ] decreased 3.2 ± 1.6 (SD) mmol/L, after which plasma [Na + ] increased gradually. 120 minutes after water loading, plasma [Na + ] was significantly underestimated by the Barsoum-Levine (−1.3 ± 1.4 mmol/L; P = 0.05) and Nguyen-Kurtz (−1.5 ± 1.5 mmol/L; P = 0.03) formulas. In addition, the Barsoum-Levine and Nguyen-Kurtz formulas overestimated urine volume, while cation excretion was significantly underestimated, with a cation gap of 57 ± 62 (P = 0.009) and 63 ± 63 mmol (P = 0.005), respectively. After 240 minutes, this gap was 28 ± 59 (P = 0.2) and 34 ± 60 mmol (P = 0.08), respectively. Limitations: The compartment from which the mobilized sodium originated was not identified, and heterogeneity in responses to water loading was observed across participants. Conclusions: These data suggest that healthy individuals are able to mobilize osmotically inactivated sodium after an acute hypotonic fluid load. Further research is needed to expand knowledge about the compartment of osmotically inactivated sodium and its role in osmoregulation and therapy for dysnatremias. Funding: This investigator-initiated study was partly supported by a grant from Unilever Research and Development Vlaardingen, The Netherlands B.V. (MA-2014-01914).