The effect of fluid intake during exercise on aerobic exercise performance and post-exercise recovery
thesisposted on 22.05.2020, 10:46 by Mark Funnell
Athletes exercising in temperate and warm conditions have elevated sweat rates, and ad-libitum fluid intake is typically in-sufficient to adequately replace sweat losses, resulting in hypohydration. Hypohydration of >2% body mass impairs aerobic exercise performance in temperate and warm conditions, with the impairment in performance likely due to a combination of physiological and perceptual mechanisms.
The methods used in previous research to induce hypohydration (i.e. fluid restriction, heat exposure, diuretic use) have meant subjects were aware of manipulations to their hydration status. More recent studies have aimed to blind subjects to manipulations in hydration status via intravenous infusion or intragastric infusion of fluids. While these studies were successful in blinding subjects to manipulations in hydration status, they could not ascertain whether knowledge of hydration status contributed to the negative effect of hypohydration on exercise performance. Thus, the first aim of this thesis was to determine whether knowledge of hypohydration influenced exercise performance (Chapter 3). Changes typical of hypohydration (i.e. increased heart rate, rating of perceived exertion, gastrointestinal temperature, serum osmolality and thirst, and decreased plasma volume) were apparent with hypohydration at the end of 2 h of steady-state cycling and did not differ between blinded and unblinded groups. Hypohydration of ~3% body mass similarly impaired cycling time trial performance (-10% unblinded vs -11% blinded) in the heat in trained cyclists, regardless of knowledge of hydration status.
There is a lack of research assessing exercise-induced hypohydration on running performance in a temperate environment, a common scenario for intermittent games players and endurance athletes that regularly exercise in temperate environments with insufficient fluid intake. Given the weight-bearing nature of running, the negative effects of hypohydration might be offset by the positive effects of body mass loss. Therefore, the purpose of Chapter 4 was to investigate the effect of hypohydration on 3 km running performance in a temperate environment. Despite the body mass loss (~1.7 kg) associated with a negative fluid balance, hypohydration of ~2% body mass impaired 3 km running time trial performance by ~6%.
Although the negative consequences of hypohydration on exercise performance are well-known, endurance athletes regularly finish training sessions/events with significant hypohydration, and field studies have shown an inverse relationship between body mass loss and endurance performance (i.e. the greater the exercise-induced body mass loss, the quicker the finishing time). This led to the hypothesis that a familiarisation/adaptation to exercise-induced hypohydration is conceivable. Thus, the third research question of this thesis was proposed, can athletes familiarise to a limited number of repeated exercise-induced dehydration exposures to reduce the negative effect of hypohydration on performance (Chapter 5). Five repeated exposures to exercise-induced dehydration (~2% body mass) did not reduce the negative effects of hypohydration on 3 km running performance in intermittent games players.
Research assessing rehydration from exercise-induced dehydration is often poorly controlled. Research typically informs subjects of the purpose of assessing rehydration, or influences behaviour by providing fluids, thus influencing fluid intake behaviours. Therefore, the final aim of this thesis was to determine if athletes rehydrate within 20 h of an intermittent exercise session, with subjects unaware that hydration status was being assessed. Subjects were also permitted to drink ad-libitum water during the exercise session and continue recording dietary intakes in a free-living environment for 20 h post-exercise (Chapter 6). Intermittent games players lost ~2% body mass during an intermittent running session when access to ad-libitum fluid was permitted. A small degree of hypohydration likely remained present 20 h post-exercise, evidenced by decreased body mass and increased urine osmolality (concentration) the morning post-exercise, suggesting specific rehydration strategies may be required for intermittent games players.
From this thesis, it can be concluded that hypohydration (~3% body mass) impaired cycling performance in the heat, regardless of knowledge of hydration status (Chapter 3). Despite the body mass loss associated with a negative fluid balance, hypohydration of ~2% body mass impaired 3 km running time trial performance in a temperate environment (Chapter 4). A small number of repeated exposures to exercise-induced dehydration did not attenuate the performance decrement from hypohydration on running time trial performance (Chapter 5). Rehydration from intermittent exercise was likely not achieved within 20 h post-exercise, suggesting rehydration strategies may be required in team sports where training is completed daily (Chapter 6).
- Sport, Exercise and Health Sciences