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Acta Physiol (Oxf). 2019 May;226(1):e13226. doi: 10.1111/apha.13226. Epub 2018 Dec 16.

Interactive effects of age and hydration state on human thermoregulatory function during exercise in hot-dry conditions.

Author information

Human and Environmental Physiology Research Unit, School of Human Kinetics, University of Ottawa, Ottawa, Ontario, Canada.
Faculty of Physical Activity Sciences, University of Sherbrooke, Sherbrooke, Quebec, Canada.
Departments of Medicine, Cardiac Sciences and Community Health Sciences, Faculties of Medicine and Kinesiology, University of Calgary, Calgary, Alberta, Canada.
Clinical Epidemiology Program, Ottawa Hospital Research Institute, Ottawa, Ontario, Canada.



Ageing and hypohydration independently attenuate heat dissipation during exercise; however, the interactive effects of these factors remain unclear. We assessed the hypothesis that ageing suppresses hypohydration-induced reductions in whole-body heat loss during exercise in the heat.


On two occasions, eight young (mean [SD]: 24 [4] years) and eight middle-aged (59 [5] years) men performed 30-minute bouts of light (heat production of 175 W m-2 ) and moderate (275 W m-2 ) cycling (separated by 15-minute rest) in the heat (40°C, 15% relative humidity) when euhydrated and hypohydrated (~4% reduction in body mass). Heat production and whole-body net heat exchange (evaporative heat loss + dry heat gain) were measured via indirect and direct calorimetry (respectively) and heat storage was calculated via their temporal summation.


Net heat exchange was reduced, while heat storage was elevated, in the middle-aged men during moderate exercise when euhydrated (both P ≤ 0.01). In the young, evaporative heat loss was attenuated in the hypohydrated vs euhydrated condition during light (199 ± 6 vs 211 ± 10 W m-2 ; P ≤ 0.01) and moderate (287 ± 15 vs 307 ± 13 W m-2 ; P ≤ 0.01) exercise, but was similar in the middle-aged men, averaging 223 ± 6 and 299 ± 15 W m-2 , respectively, across conditions (both P ≥ 0.32). Heat storage was thereby exacerbated by hypohydration in the young (both P < 0.01) but not the middle-aged (both P ≥ 0.32) during both exercise bouts and, as a result, was similar between groups when hypohydrated (both P ≥ 0.50).


Hypohydration attenuates heat loss via sweating in young but not middle-aged men, indicating that ageing impairs one's ability to mitigate further sweat-induced fluid loss during hypohydration.


ageing; dehydration; fluid regulation; heat stress; sweating; thermoregulation


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