Aging and Heat Stress: From Rest to Exercise
Creators: name: Stapleton, Jill
Publisher: name: Université d'Ottawa / University of Ottawa
Year: 2015
Subjects: Aging
Calorimetry
Heat stress

description: The current thesis examined local and whole-body heat loss responses during heat stress at rest, exercise and/or pharmacological stimuli as a function of increasing age. The first study examined the effects of age on whole-body heat loss and heat storage during passive exposure to conditions representative of the upper temperature extremes in Canada. The results demonstrate that the cumulative change in body heat content after 2 h of rest was significantly greater in older adults in a hot-dry (older: 212±25; young: 131±27 kJ, P=0.018) and hot-humid (older: 426±37; young: 317±45 kJ, P=0.037) condition. The second study evaluated the maximal capacity of whole-body evaporative heat loss as a function of age and aerobic fitness. The findings demonstrate that whole-body evaporative heat loss was significantly lower in middle-aged untrained (Ex2: 426±34; Ex3:497±17 W) and older (Ex2: 424±38; Ex3: 485±44 W) compared to young (Ex2: 472±42; Ex3: 558±51 W) and middle-aged trained (Ex2: 474±21; Ex3: 552±23 W) males at the end of the last two exercise bouts (P&lt;0.05). The third study assessed the maximal capacity of whole-body evaporative heat loss in females and found that whole-body evaporative heat loss was significantly lower (P=0.002) in the older (Ex2: 343±39 W; Ex3: 389±29 W) compared to the young (Ex2: 383±34 W; Ex3: 437±36 W) females at the end of the second and third exercise-induced heat loads of 325 and 400 W, while no differences were observed during recovery (P=0.693). The fourth study examined nitric oxide-dependent sweating during exercise/rest cycles in young and older adults. We showed that nitric oxide-dependent sweating during short bouts of exercise in the heat is observed in young males, but not in older adults. The fifth study examined: 1) the extent to which peripheral factors (i.e., sweat gland and skin vasodilatory function) contribute to the postexercise suppression of heat loss; and 2) whether age-related differences exist in the mechanisms modulating postexercise heat loss. The findings demonstrate that there were no differences in sweat rate between the no exercise resting condition and a postexercise condition at either an acetylcholine (ACh) or methacholine (MCh) site for the young (ACh: P=0.992 and MCh: P=0.710) or older (ACh: P=0.775 and MCh: P=0.738) adults. However, older adults had a lower sweating response for both the no exercise resting condition (ACh: P=0.049 and MCh: P=0.006) and postexercise condition (ACh: P=0.050 and MCh: P=0.029) compared to their younger counterparts. Taken together, the current thesis shows true age-related impairments in the ability to dissipate heat exist during both a passive and exercise-induced heat stress over a certain heat load threshold. Specifically, older adults have an impaired ability to dissipate heat compared to young adults during rest in hot-dry and hot-humid conditions. Additionally, middle-aged untrained and older adults have a reduced capacity to dissipate heat at an exercise-induced heat load of ≥400 W for males and ≥325 W for females, which becomes more pronounced at as the level of heat load increases. However, the age-related impairment in heat loss can be attenuated by maintaining a high level of aerobic fitness. Furthermore, age-related impairments in sweating may be associated with age-related reductions in nitric oxide-mediated sweating. However, there does not appear to be age-related differences in the modulation of heat loss postexercise.