Effects of dehydration and hyperthermia on brain and muscle blood flow

Dehydration, the process of losing body water through thermoregulatory sweating, can lead to marked alterations in physiological function and decrements in athletic performance during training and competition in temperate-to-hot environments. Impaired endurance capacity in the dehydrated athlete is often associated with significant cardiovascular strain, typified by gradual reductions in cardiac output, mean arterial pressure and blood flow to exercising limb muscle and skin during prolonged exercise in temperate and warm environments (Sawka et al. 1979; Hamilton et al 1991; Montain & Coyle, 1992; González-Alonso et al. 1995, 1998, 2008). Blood flow to the brain might also decline with dehydration since reductions in blood flow to active muscle and skin only account for two-thirds of the fall in cardiac output during prolonged exercise in the heat (González-Alonso et al. 1998). The first study of this project systematically investigated the responses of the cerebral circulation and metabolism to progressive dehydration during submaximal and maximal aerobic exercise in the heat and the benefits of maintaining a normal hydration status (euhydration) via fluid ingestion.

A widely held concept in cardiovascular physiology is that blood flow to the muscles of the arm and legs remain unchanged in humans exposed to severe environmental heat stress, despite cardiac output increasing by up to 5-6 L/min compared to control resting conditions (Rowell 1974). This has been interpreted to mean that muscle blood flow regulation is insensitive to increases in tissue temperature. Findings from our laboratory, however, challenge this long-standing view by showing that the heat stress-mediated increases in leg blood flow (up to 1.1 L/min above baseline) are accompanied by proportional reductions in muscle blood oxygenation indicative of significant increases in leg muscle blood flow with elevations in temperature (Pearson et al. 2011). Because both blood and muscle temperatures increase rapidly at onset of exercise (González-Alonso et al., 1999; González-Alonso & Calbet 2003), our recent observations raise important questions regarding the role of blood and tissue temperature on the regulation of muscle, brain and systemic blood flow in exercising humans. A second aim of this project was therefore to further investigate the responses and temperature-sensitive mechanisms controlling limb tissue hyperaemia in the heat stressed human.

Research Studies

This project consisted of three different studies. The two major aims of these studies were (1) to characterise the impact of hydration and heat stress on brain and active muscle blood flow and metabolism during submaximal and maximal aerobic exercise and (2) to provide insight into the temperature-sensitive mechanisms controlling local tissue blood flow with hyperthermia. Detailed information about the experimental designs employed for each of the studies can be found in the research outputs reported below.

Study 1: Brain circulation and metabolism during prolonged submaximal and incremental maximal aerobic exercise with exercise-induced dehydration and hydration.

Study 2: Brain and muscle blood flow and metabolism during incremental cycling exercise with and without heat stress

Study 3: Temperature-sensitive mechanism controlling limb tissue hyperthermia in the heat stressed-human.

Knowledge of how vital organs such as the brain, heart and active muscles respond to strenuous exercise is paramount to advance our understanding of exercise fatigue mechanisms and thus optimise human performance. This project characterized the brain, muscle and systemic blood flow and metabolic responses to strenuous submaximal and maximal aerobic exercise in conditions commonly experienced by elite and recreational athletes in warm and hot environments. A close association between the development of fatigue and reductions in cerebral and muscle blood flow and oxygen delivery was observed, particularly when fatiguing sooner whilst experiencing significant dehydration and hyperthermia.

Whilst the development of dehydration and hyperthermia exacerbates the cerebrovascular strain and accentuates the fall in cerebral blood flow, the global brain aerobic metabolism is preserved due to compensatory elevations in oxygen and substrate extraction from the circulation. Thus, reduced brain aerobic metabolism is unlikely to contribute to the fall in maximal aerobic power and endurance performance in the dehydrated and hyperthermic athlete. Rather, the early fatigue experienced by the dehydrated athlete in these conditions is coupled to high levels of core hyperthermia and alterations in muscle substrate, glycogen and oxygen utilization and increases in muscle anaerobic metabolism and neural activation, possibly reflecting a mismatch between energy demand and production.

These findings provide physiologists, sport scientists, coaches and athletes with evidence to explain how dehydration and/or hyperthermia impact brain and muscle blood perfusion and metabolism during exercise and heat stress and ultimately human performance. The data further substantiate the use of fluid replacement during exercise as a means of delaying the reductions in brain, skeletal muscle and systemic circulation during exhaustive exercise in temperate to hot environments and thus optimise performance in the endurance athlete. 

Research Outputs

Trangmar SJ, Chiesa ST, Stock CG, Kalsi KK, Secher NH & González-Alonso J (2014). Dehydration affects cerebral blood flow but not its metabolic rate for oxygen during maximal exercise in trained humans, J Physiol 592, 3143-3160.

Trangmar SJ, Chiesa ST, Llodio I, Garcia B, Kalsi K, Secher NH & González-Alonso J (2015). Dehydration accelerates reductions in cerebral blood flow during prolonged exercise in the heat without compromising brain metabolism. Am J Physiol Heart Circ Physiol 309,H1598-H1607.

Trangmar SJ, Chiesa ST, Kalsi K, Secher NH & González-Alonso J (2017). Whole body hyperthermia, but not skin hyperthermia, accelerates brain and locomotor limb circulatory strain andimpairs exercise capacity in heat stressed humans. Physiol Rep 5 (2), e13108.

Trangmar SJ & González-Alonso J (2017). New insights into the impact of dehydration on blood flow and metabolism during exercise. Exerc Sport Sci Rev 45, 146-153.

Chiesa ST, Trangmar SJ, Kalsi K, Rakobowchuk M, Banker DS, Lotlikar MD, Ali L & González-Alonso J (2015). Local temperature-sensitive mechanisms are important mediators of limb tissue hysperemia in the heat-stressed human at rest and during small muscle mass exercise. Am J Physiol Heart Circ Physiol 309, H369-H380.

Related Papers

Trangmar SJ, González-Alonso J (2019). Heat, hydration and the human brain, heart and skeletal muscles. Sports Med 49 (Suppl 1):S69–S85.

Chiesa ST, Trangmar SJ, Watanabe K & González-Alonso J (2019). Integrative human cardiovascular responses to hyperthermia. In: J.D. Périard JD & Racinais S (eds.) Heat Stress in Sport and Exercise. Springer, Cham

Kalsi KK, Chiesa ST, Trangmar SJ, Lotlikar MD, Ali L, González-Alonso J (2017). Mechanisms for the control of local tissue blood flow during thermal interventions: influence of temperature-dependent ATP release from human blood and endothelial cells. Exp Physiol 102, 228-244.

González-Alonso J, Calbet JAL, Boushel R, Helge JW, Søndergaard H, Munch-Andersen T, van Hall G, Mortensen SP & Secher NH (2015). Blood temperature and perfusion to exercising and non-exercising human limbs. Exp Physiol 100, 1118-1131.

Kalsi KK & González-Alonso J(2012). Temperature-dependent release of ATP from human erythrocyles: mechanism for the control of local tissue perfusion. Exp Physiol 97, 419-432.

Meet the Principal Investigator(s) for the project

Professor José González-Alonso - José González-Alonso is a professor of exercise and cardiovascular physiology in the Division of Sport, Health and Exercise Sciences, Department of Life Sciences. José joined Brunel University London in 2006 and since then has served as director of the Centre for Sports Medicine and Human Performance (2006-2013) and director of the Centre for Human Performance, Exercise and Rehabilitation (2015-2021). Prior to Brunel, he worked for 6 years as a senior researcher at The Copenhagen Muscle Research Centre, Rigshospitalet, a centre of excellence in integrative human physiology led by the late Professor Bengt Saltin, and for 5½ years as a post-doctoral fellow at the then Department of Human Physiology, August Krogh Institute, now Department of Nutrition, Exercise and Sports, University of Copenhagen, Denmark. He completed his MA and PhD in Exercise Physiology at the Department of Kinesiology and Health Education, University of Texas at Austin, USA, under the supervision of Professor Edward F. Coyle, and his undergraduate studies (Licenciado) in Physical Education at the ‘Instituto Nacional de Educación Física de Cataluña,’ University of Barcelona, Spain.  He has authored or co-authored more than 100 original research articles, reviews and perspectives (PubMed; Google Scholar total citations > 16,300 with a Hirsch h-index of 65) and given more than 60 invited presentations at international scientific conferences and meetings including some keynote lectures. His main area of expertise is integrative physiology with particular emphasis on human cardiovascular control and skeletal muscle blood flow regulation during exercise and environmental stress. His research has implications for human performance and health and treatment of some circulatory disorders and/or rehabilitation.  José is currently a member of the European College of Sport Science  (ECSS) Scientific Board, after serving in the Scientific Committee for 4 years. He is also a member of the Editorial Board of the European Journal of Applied Physiology (Advisory Editor). He has served as a member of the UK Research Excellence Framework (REF 2014) sub-panel 26 Sport and Exercise Sciences, Leisure and Tourism and a member of the Editorial Boards of The Journal of Physiology (2010-2013) and Frontiers in Physiology - Exercise Physiology Section (2016- 2018). José is also a member of the Physiological Society (Physoc), the American Physiological Society (APS) and the British Association of Sport and Exercise Sciences (BASES) and a Fellow of the European College of Sport Science.  Articles of general interest Targeting red cell-derived adenosine triphosphate signalling to improve aged muscle circulation. Journal of Physiology 600, 3215-3216, 2022.  Physiological function during exercise and environmental stress in humans: An integrative view of body systems and homeostasis. Cells 11, 383, 2022. New ideas about hydration and its impact on the athlete’s brain, heart and muscles. Sports Sci Exch 29 (196): 1-7, 2019. The brain has a remarkable ability to cope with dehydration during exercise. The Conversation. 31 July 2014. Understanding the vascular and cardiac interactions during endurance exercise  Ultrasound recordings of peripheral blood flow (brachial and common carotid artery blood flow) and cardiac function (4 chamber view) at rest and during prolonged exercise in endurance-trained athletes. The images were obtained by Dr Kazuhito Watanabe during his post-doctoral training at the Centre for Human Performance, Exercise and Rehabilitation between 2016-2018. To find detailed information about this study see: Watanabe K, Stöhr EJ, Akiyama K, Watanabe S, González-Alonso J (2020). Dehydration reduces stroke volume and cardiac output during exercise because of impaired cardiac filling and venous return, not left ventricular function. Physiological Reports 8(11):e14433. Link to video with ultrasound images  https://youtu.be/jQNO5fVonsk Recent interview with Emeritus Professor Glenn McConnell, Victoria University, Australia, for Inside Exercise on the topic of 'Exercise in the Heat, Dehydration, Fluid Ingestion and Circulation'. Spotify: https://open.spotify.com/episode/0wlFp5nisiNXKMwsF0Snbl?si=dh-GkL5KTFKsjerueC97ig&context=spotify%3Ashow%3A3pSYnNSXDkNLH8rImzotgP Apple Podcasts: https://podcasts.apple.com/au/podcast/inside-exercise/id1631457776 YouTube: https://t.co/jsyTZdoao Recent interview with Professor Lars Nybo from the University of Copenhagen for his NYBO&NEXS journal club. The conversation focused a paper which has received >1500 citations. González-Alonso et al. Influence of body temperature on the development of fatigue during prolonged exercise in the heat. J Appl Physiol 86(3): 1032-1039,1999 YouTube: https://www.youtube.com/watch?v=V7FXbT79UaM Tweeter - X: https://twitter.com/NYBO_NEXS_KU_DK

Project last modified 29/04/2022