Human Fetuses Accrue Osmotically Inactive Sodium Stores in Anticipation of Birth

Abstract

BackgroundSodium (Na) is stored in the skin and muscle in a non‐osmotic manner. It has been postulated that in adults, these stores protect from water loss and regulate immune function. In post‐weanling rats, osmotically inactive Na pools are mobilized to support growth. The presence of and role for such stores earlier in development have not been explored.ObjectiveDetermine if osmotically inactive Na pools exist in human infants.MethodsWe utilized literature providing human fetal body composition (Ziegler, EE, 1976) and water compartment distribution (Friis‐Hansen B, 1983) data to calculate changes in total body stores of Na, potassium (K) and chloride (Cl) during fetal development (24–40 weeks gestation). Total body mineral content was estimated by two methods, with calculations based upon 1) body water compartments (BW) and 2) reported mineral content per 100 g fat free mass (MC). Differences between these calculations were interpreted to represent osmotically inactive stores. The content of Na, K and Cl within water compartments were estimated to be 140, 4, and 110 mEq/L (extracellular) and 4, 150, and 10 mEq/L (intracellular), respectively.ResultsEstimates of total body K were nearly identical at each gestational time point (<10% variance; (Kmc−Kbw)/Kmc). Estimated total body Cl by MC exceeded that using BW compartments at all time points, with a maximum difference at 40 weeks gestation of 40 mEq (23% variance). In contrast, total body Na by MC greatly exceeded that calculated based upon BW compartments, with differences increasing from 17 mEq at 24 weeks gestation (variance of 25%) up to 100 mEq at 40 weeks gestation (variance of 35%).ConclusionThe disproportionate differences in total body mineral content between the two methods coupled with increasing variances at advancing gestational ages suggest that Na, and to a lesser extent Cl, are stored in osmotically inactive pools within the fetus. These pools increase in size with advancing gestational age. As breastmilk is relatively Na‐deficient (10 mEq/L), we speculate the fetal osmotically inactive Na pool is vital to maintain a sufficient total body Na content that is necessary for optimal postnatal growth.