Initial Hydration Status and Fluid Balance during Recreational Exercise in Physically Active Adults

Abstract

Athletes typically fail to consume sufficient fluids during exercise to replace losses, whilst many arrive for exercise already dehydrated. However, very little is known about the pre-exercise hydration status and fluid balance of recreational exercisers, which may influence exercise adherence and overall wellbeing. PURPOSE: To investigate initial hydration status and fluid balance during recreational exercise in physically active adults. METHODS: Fifty-two physically active adult male (n = 31) and female (n = 21) subjects aged 36 ± 12 years (mean ± SD); body mass 80.7 ± 15.9 kg; volunteered to participate at a fitness centre. Urine samples were collected upon arrival. Nude body mass was recorded before and after exercise. Subjects performed a freely chosen gymnasium-based exercise session in mean ambient conditions of 21 oC and 55% relative humidity, with ad libitum access to water. Sweat loss was estimated from the change in body mass after correction for fluid intake and urinary losses. RESULTS: Mean ± SD (range) exercise duration was 64 ± 22 mins (15 - 125 mins). Pre-exercise urine osmolality was 698 ± 288 mOsmol.kg-1 (166 - 1245 mOsmol.kg-1) and was not different for gender (p = 0.915). Urine osmolality was above 900 mOsmol. kg-1 (used as a threshold for dehydration) in 38% of participants; 32% of males and 43% of females. The change in body mass during exercise was −0.51 ± 0.32 kg (−1.30 - 0.25 kg) and was greater in males than females (−0.59 ±0.35 vs. −0.39 ± 0.23 kg; p< 0.05). This was equivalent to a percentage change in pre-exercise body mass of-0.62 ± 0.2% (−1.63 - 0.40%) which was not different for gender (p = 0.258). Sweat loss was 794 ± 391 ml (50 - 1656 ml) and was greater in males than females (938 ± 390 vs. 556 ± 264 ml; p < 0.01), whilst fluid intake was 390 ± 298 ml (0 - 1610 ml) and was also greater in males than females (468 ± 329 vs. 275 ± 200 ml; p < 0.05). CONCLUSION: Although there was large inter-individual variability, male and female recreational exercisers generally became modestly dehydrated during a typical exercise session, as insufficient fluid was consumed to replace sweat losses. Furthermore, for many adults this was superimposed on existing dehydration because a large proportion of participants arrived to take part in exercise in a dehydrated state.