Fluid Balance Before and During Exercise and the Effects of Exercise-Induced Dehydration on Physiological Responses, Substrate Oxidation, Muscle Metabolism, and Performance
This thesis set out to answer 4 major questions: 1) Do elite hockey players arrive for a game hydrated and do they consume enough fluid to prevent dehydration over the course of a game? 2) Is hydration status repeatable between days and can an athlete who arrives dehydrated prior to training or competition become hydrated in the time before the start of activity? 3) What is the extent of dehydration (%body mass (BM) loss) necessary to change substrate oxidation and skeletal muscle metabolism during exercise in male and female subjects? 4) Will progressive dehydration have a negative effect on endurance performance? The first study evaluated the pre-game hydration status, sweat loss and fluid intake patterns of elite male junior ice hockey players during a game. Sweat loss was 3.2 ± 0.2L and exceeded net fluid intake (2.1 ± 0.1L). Mean BM loss was 1.3 ± 0.3%, but 8 out of 24 players lost between 1.8 - 4.3% BM. Despite abundant opportunities to hydrate during a hockey game, 33% of players did not drink enough to prevent sweat losses of ≥2% BM. The second study investigated 1) the day-to-day variability of morning urine specific gravity (USG) and consuming 600mL of water on the hydration status of hydrated and dehydrated (USG>1.020) subjects, and 2) the effects of consuming water or carbohydrate electrolyte solutions (CES) on hydration status of dehydrated subjects. Morning USG and hydration responses to the ingestion of 600mL of water were repeatable and mildly dehydrated subjects could reach euhydration within 45min after ingesting any type of fluid with no added effect of a CES. The next two studies (3 & 4) investigated the effects of mild progressive dehydration during 120min of exercise at ~65% VO2peak on whole body substrate oxidation and skeletal muscle metabolism, as well as cardiovascular, thermal, and mental responses in recreationally active, hydrated females and males. In both studies, muscle glycogenolysis was increased in the initial 60min of exercise in the dehydrated state when BM loss were ≤1%. Increased glycogenolysis appeared due to increases in core temperature during progressive dehydration as there were no differences in plasma epinephrine or the energy status of the cell (free ADP or AMP) between trials. Normal changes in physiological parameters accompanying exercise in a hydrated state were exacerbated with progressive mild dehydration. The final study determined the impact of dehydration on cycling performance. Active males cycled at ~65% VO2peak for 90min followed by a time trial (TT: 6 kJ/kg BM) with fluid to replace sweat losses (HYD) or without fluid (DEH). DEH subjects began the 90 min trial 0.6% dehydrated and progressively became more dehydrated with a BM loss of 1.4% at 45min, 2.3% at 90min, and 3.1% post-TT. TT performance was significantly compromised with ~2-3% BM loss (HYD 32 ± 4 vs. DEH 36 ± 3 min).