Mikell L. Hammer, Evan C. Johnson, Ainsley Huffman, Thomas Vidal, LynnDee Summers, D. Layne Nixon & Stavros A. Kavouras, FACSM

Human Performance Laboratory, Department of Health, Human Performance and Recreation, University of Arkansas, Fayetteville, Arkansas

Proper hydration and fluid replacement are important for optimal health and exercise performance. Higher levels of physical activity requires increased fluid intake due to increased water losses via sweating. However, there is little research on the effect of physical activity on hydration status in free-living conditions. PURPOSE: To determine the relationship between hydration status, physical activity, and body weight. METHODS: This study involved 9 visits to the Human Performance Laboratory over 22 days. Body weight and urine measurements were taken at all visits. Physical activity was assessed by the International Physical Activity Questionnaire (IPAQ) and hydration status by plasma osmolality (POsmo), urine osmolality (UOsm), urine specific gravity (USG), and urine color (UC). From the IPAQ subjects were classified as low, moderate and high physical activity levels and the total amount of physical activity was expressed as MET-minw-1. All values represent means across 22 days of measurements except physical activity, which was a mean of three measurements. Participants were excluded if they exercised more than 4 hours a week or if they were on medications that effected fluid balance. One-way analysis of variance was computed to determine differences in hydration status between groups. Additionally, 2-tailed Pearson correlations were computed to determine relationships between all study measurements while using physical activity as a continuous variable (i.e., met-minw-1). RESULTS: 44 participants (23 males, 32±11 y, 1.70±0.07 m, 28.7±12.0 %BF 25.7±6.5 kgm2) completed the study; n= 7 for low activity (1,021±451 MET-minw-1), n= 30 for moderate activity (1,322±489 MET-minw-1), n= 7 for high activity (4,172±1,675 MET-minw-1). Hydration status did not differ across all levels of physical activity; UOsm: 736±150, 659±187, 538±286; POsm: 287±3, 287±4, 287± 4; USG: 1.019±0.005, 1.017±0.005, 1.014±0.008; and UC: 3.3±0.6, 3.2 ±0.6, 2.6±0.8, for low, moderate, and high physical activity levels, respectively (all p≥0.091). Average body weight was significantly correlated with UOsm (R=0.439, p=0.003) and USG (R=0.375, p=0.012) but not POsm (R=0.055, p=0.723). Lastly, body weight was significantly correlated with physical activity MET-minw-1 (R=0.325, p=0.031). CONCLUSION: No significant differences in hydration status were observed across groups. However, the small numbers of participants in the low and high physical activity levels may have made this comparison difficult. The results show that weight is associated with hydration status and physical activity.

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