Change in hydration indices associated with an increase in total water intake of more than 0.5L/day, sustained over 4weeks, in healthy young men with initial total water intake below 2L/day.

Jodi D Stookey,David W Killilea,Janice Hamer

doi:10.14814/phy2.13356

Abstract

This secondary data analysis addressed gaps in knowledge about effects of chronic water intake. Longitudinal data from the Adapt Study were used to describe effects of prescribing a sustained increase in water intake relative to baseline, for 4 weeks, on multiple indices of total body water (TBW) flux, regulation, distribution, and volume in five healthy, free‐living, young men, with mean total water intake initially below 2 L/day. Indices were measured weekly. Within‐person fixed effect models tested for significant changes in indices over time and associations between changes in indices. Agreement between indices was described. Mixed models tested if baseline between‐person differences in hydration indices modified changes in indices over time. Body water flux: The half‐life of water in the body decreased significantly. Body water regulation: Serum osmolality decreased significantly. Urine anti‐diuretic hormone, sodium, potassium, and osmolality decreased significantly. Plasma aldosterone and serum sodium increased significantly. Body water distribution: No significant changes were observed. Body water volume: Saliva osmolality decreased significantly. Body weight increased significantly by a mean ± SEM of 1.8% ± 0.5% from baseline over 4 weeks. Changes in indices were significantly inter‐correlated. Agreement between indices changed over 4 weeks. Baseline saliva osmolality significantly modified responses to chronic water intake. The results motivate hypotheses for future studies: Chronic TBW deficit occurs in healthy individuals under daily life conditions and increases chronic disease risk; Sustained higher water intake restores TBW through gradual isotonic retention of potassium and/or sodium; Saliva osmolality is a sensitive and specific index of chronic hydration status.

Highlights

The Institute of Medicine set Adequate Intake (AI) recommendations for drinking water to prevent adverse acute effects of dehydration (IOM, 2004)
The recommendations are, explicitly, not designed to lower risk related to chronic effects of dehydration (IOM, 2004, p4): “ a low intake of total water has been associated with some chronic diseases, this evidence is insufficient to establish water intake recommendations as a means of reducing the risk of chronic disease
This study addressed gaps in knowledge about the effects of chronically increased water intake on hydration indices in healthy, young men, under daily life conditions

Summary

Introduction

The Institute of Medicine set Adequate Intake (AI) recommendations for drinking water to prevent adverse acute effects of dehydration (IOM, 2004). The 7-day mean total intake of energy, protein, carbohydrate, and sodium did not vary significantly over the study period (Stookey et al 2013). The mean Æ SEM first morning saliva cortisol was 29.0 Æ 6.9 nmol/L, at baseline, and decreased significantly to 17.7 Æ 4.7 over the periods of higher water intake. After overnight food and water restriction, at baseline, the mean Æ SEM urine osmolality was 910 Æ 58 mmol/kg (range 819–1137 mmol/kg). Within 60 min after intake of 750 mL drinking water, the mean Æ SEM decrease in urine osmolality was À737 Æ 58 mmol/kg (range À616 to À944 mmol/kg)

Methods

Results

Discussion

Conclusion