Effects of diet, habitual water intake and increased hydration on body fluid volumes and urinary analysis of renal fluid retention in healthy volunteers.

PURPOSE: To increase our knowledge about the causes and physiological consequences of concentrated urine, the relevance of which in the general population is uncertain.
METHODS: Twenty healthy volunteers (mean age 42 years) recorded all intake of food and water for 2 weeks. During the 2nd week, they increased their daily consumption of water by 716 mL (32%). The volunteers delivered a 24-h and a morning urine sample for analysis of osmolality and creatinine during the first 4 days of both weeks, and a sample each time they voided on the other days. The water content of food and liquid was calculated and the body fluid volumes were measured by bioimpedance. Haemodynamic stability was assessed with the passive leg-raising test.
RESULTS: There was a curvilinear correlation between the daily intake of water and biomarkers measured in the 24-h collection of urine (coefficient of determination 0.37-0.70). Habitual low intake of water was associated with larger body fluid volumes. The increased fluid intake during the 2nd week was best reflected in the 24-h collection (-15 and -20% for the osmolality and creatinine, respectively, P < 0.002), while morning urine and body fluid volumes were unchanged. Increased fluid intake improved the haemodynamic stability in volunteers with a low intake of water (< median), but only in those who had minimally concentrated morning urine.
CONCLUSIONS: The 24-h collection reflected recent intake of fluid, whereas the morning urine seemed to mirror long-term corrections of the fluid balance. Concentrated urine was associated with larger body fluid volumes.