Steven J. Trangmar · José González‐Alonso
Department of Life Sciences, University of Roehampton, London, UK – Centre for Human Performance, Exercise and Rehabilitation, College of Health and Life Sciences, Brunel University London, Uxbridge UB8 3PH, UK – Division of Sport, Health and Exercise Sciences, Department of Life Sciences, College of Health and Life Sciences, Brunel University London, Uxbridge, UK
Published online: 22 January 2019
© The Author(s) 2019
People undertaking prolonged vigorous exercise experience substantial bodily fuid losses due to thermoregulatory sweating. If these fluid losses are not replaced, endurance capacity may be impaired in association with a myriad of alterations in physiological function, including hyperthermia, hyperventilation, cardiovascular strain with reductions in brain, skeletal muscle and skin blood perfusion, greater reliance on muscle glycogen and cellular metabolism, alterations in neural activity and, in some conditions, compromised muscle metabolism and aerobic capacity. The physiological strain accompanying progressive exercise-induced dehydration to a level of~4% of body mass loss can be attenuated or even prevented by: (1) ingesting fluids during exercise, (2) exercising in cold environments, and/or (3) working at intensities that require a small fraction of the overall body functional capacity. The impact of dehydration upon physiological function therefore depends on the functional demand evoked by exercise and environmental stress, as cardiac output, limb blood perfusion and muscle metabolism are stable or increase during small muscle mass exercise or resting conditions, but are impaired during whole-body moderate to intense exercise. Progressive dehydration is also associated with an accelerated drop in perfusion and oxygen supply to the human brain during submaximal and maximal endurance exercise. Yet their consequences on aerobic metabolism are greater in the exercising muscles because of the much smaller functional oxygen extraction reserve. This review describes how dehydration differentially impacts physiological function during exercise requiring low compared to high functional demand, with an emphasis on the responses of the human brain, heart and skeletal muscles.
Dehydration and hyperthermia, which are frequently experienced by humans exercising in hot environments, are major physiological stressors that can severely hinder general physiological function and cognitive and athletic performance during endurance events (i.e., long distance running, cycling or swimming, triathlon and team sports). The interaction among the type, duration and intensity of exercise and the environmental conditions determines the body’s overall functional demand. Dehydration poses additional stress on the human body regulatory systems, and its interaction with the overall functional demand will dictate the extent of the physiological and perceptual strain and thus whether physical performance is compromised. The endurance athlete’s training and heat acclimation status would also have an influence. In this light, the severely dehydrated, heat unacclimated and unfit endurance athlete, who is training or competing in a hot and humid environment, will likely experience the most deleterious physiological and performance effects. The mildly dehydrated, highly ft endurance athlete who is training and competing in a cold environment will be at the opposing end of the spectrum.