Water Turnover Across The Spectrum Of Physical Activity: Implications For Water Intake Recommendations

There is considerable variation in water turnover (WT) with lifestyle and environmental factors. However, there is currently little information on WT in weight-stable individuals habitually performing large volumes of physical activity (PA). The purpose of the present study was to determine the correlates of and variability in WT across the PA spectrum. PURPOSE: To test the hypothesis that WT is increased in individuals habitually engaged in large volumes of PA. METHODS: Fifty-six healthy, weight-stable adults (27 females and 29 males; 20-58 yrs) with PA levels ranging from sedentary to ultra-endurance training were recruited. WT (L/day) was measured via the doubly labeled water method. Following oral dosing, 2H2 and O18 were measured in urine samples collected daily for 14 days using an Isotopic Liquid Water Analyzer. Body composition was measured by DXA before and after 14 days. Mileage per week and PA were quantified using self-report and accelerometry, respectively. VO2max was measured using indirect calorimetry during treadmill running to exhaustion. We conservatively estimated water intake from beverages as 40% of what remained after subtracting 10% for metabolic water from WT. RESULTS: Males tended to have higher WT than females (5.94 ± 1.34 vs. 4.89 ± 1.03 L/d, p = 0.002). WT was correlated with body mass (r = 0.43, p < 0.001), fat-free mass (r = 0.56, p < 0.001), counts/min/day (CPM) (r = 0.46, p < 0.001), self-reported weekly mileage (r = 0.46, p < 0.001), and VO2max (r = 0.53, p < 0.001). Using General Linear Model with step backward deletion, WT = -5.39734 - 0.465133*Sex (1 if male, -1 if female) + 0.0812205*VO2max + 0.00152379*CPM/d + 0.0800099 *BM (SEE = 0.91, Adj r2 = 51.25). Ultra-endurance runners reporting ≥45 miles/week compared with sedentary individuals at <10 miles/week had higher WT (6.1 ± 1.4 vs. 4.3 ± 1.0 L/d, p = 0.003). Estimated water intake from beverages ranged from 0.9 to 3.3 L/d in the entire sample and was higher in ultra-endurance runners compared with sedentary individuals (2.2 ± 0.5 vs. 1.5 ± 0.3 L/d, p = 0.002). CONCLUSION: The highest levels of WT and estimated water intake were observed in ultra-endurance runners engaged in ≥45 miles of running per week with correspondingly high VO2max values. These findings may have important implications for water intake recommendations across the PA spectrum. KRH: VT Translational Obesity Research Interdisciplinary Graduate Education Program Predoctoral Fellowship and R01DK135410; OPN: VT Presidential Postdoctoral Fellowship; GZR: NextGeneration EU funds Margarita Salas Postdoctoral Fellowship.