Total Body Water Via Bioelectrical Impedance Analysis in Relationship to Plasma and Urine Sodium Concentrations

Abstract

Research suggests that acute changes in plasma sodium concentration ([PNa]), especially a decrease, may negatively affect bioelectrical impedance analysis (BIA) by either increasing or decreasing an individual's resistance to the outputted frequency. While research is conflicting, there is limited research utilizing the more practical and cost-effective lower body BIA when determining relationships between sodium balance and total body water (TBW) calculations. PURPOSE: To examine the relationship of calculated percent hydration (%HY) by TBW measures to [PNa] and urine sodium concentration ([UNa]). METHODS: Participants included 16 male and female recreational athletes (age=28.13+6.82y, height=171.2+12.16cm, weight=73.81+15.21kg). As part of a larger study we collected pre and post-exercise measures of [PNa], [UNa] and TBW (measured by the Tanita TBF-300A, Tanita Corporation, Tokyo, Japan). TBW measures were used to calculate %HY using a previously developed formula and categorized as: 1) hyperhydrated, 2) euhydrated-well hydrated, and 3) slightly-extremely hypohydrated. [PNa] and [UNa] measures were categorized as: 1) hyponatremic/sodium conservation, 2) normonatremic/normal sodium, and 3) hypernatremic/excessive sodium. RESULTS: We found no relationship between %HY and [UNa]. There was a relationship between %HY and [PNa] overall χ24(n=157)=14.329, p=0.003 and at post-exercise χ24(n=65)=9.079, p=0.029, but not at pre-exercise. Overall 10.2% of the participants were hyponatremic and 2.5% of these were euhydrated-well hydrated and 5.1% were slightly-extremely hypohydrated. Percentages were similar at post exercise 16.9% were hyponatremic, 10.8% of those were slightly-extremely hypohydrated, and 3.1% were euhydrated-well hydrated. CONCLUSION: Regardless of hydration status, active individuals may present with low [PNa]. Our findings suggest that %HY calculations from TBW measures are affected by [PNa] and may not be accurate when an individual is either euhydrated or hyperhydrated. An increase in water content decreases impendence, which may skew TBW calculations taken by a lower body BIA.