Abstract
Studies have reported that older females have impaired heat loss responses during work in the heat compared to young females. However, it remains unclear at what level of heat stress these differences occur. Therefore, we examined whole-body heat loss [evaporative (HE) and dry heat loss, via direct calorimetry] and changes in body heat storage (∆Hb, via direct and indirect calorimetry) in 10 young (23±4 years) and 10 older (58±5 years) females matched for body surface area and aerobic fitness (VO2peak) during three 30-min exercise bouts performed at incremental rates of metabolic heat production of 250 (Ex1), 325 (Ex2) and 400 (Ex3) W in the heat (40°C, 15% relative humidity). Exercise bouts were separated by 15 min of recovery. Since dry heat gain was similar between young and older females during exercise (p=0.52) and recovery (p=0.42), differences in whole-body heat loss were solely due to HE. Our results show that older females had a significantly lower HE at the end of Ex2 (young: 383±34 W; older: 343±39 W, p=0.04) and Ex3 (young: 437±36 W; older: 389±29 W, p=0.008), however no difference was measured at the end of Ex1 (p=0.24). Also, the magnitude of difference in the maximal level of HE achieved between the young and older females became greater with increasing heat loads (Ex1=10.2%, Ex2=11.6% and Ex3=12.4%). Furthermore, a significantly greater ∆Hb was measured for all heat loads for the older females (Ex1: 178±44 kJ; Ex2: 151±38 kJ; Ex3: 216±25 kJ, p=0.002) relative to the younger females (Ex1: 127±35 kJ; Ex2: 96±45 kJ; Ex3: 146±46 kJ). In contrast, no differences in HE or ∆Hb were observed during recovery (p>0.05). We show that older habitually active females have an impaired capacity to dissipate heat compared to young females during exercise-induced heat loads of ≥325 W when performed in the heat.
Highlights
IntroductionA number of studies have shown that thermoregulatory function (i.e., heat loss through sweating and skin blood flow) during exercise is compromised in older compared to younger males [1,2,3,4,5,6,7]
A number of studies have shown that thermoregulatory function during exercise is compromised in older compared to younger males [1,2,3,4,5,6,7]
No differences were observed for the rate of dry heat exchange at baseline or during exercise (p = 0.52) between groups, but did change over time (p = 0.004) such that the rate of dry heat gain was greater at the end of Ex2 compared to Ex1 for the older females (p
Summary
A number of studies have shown that thermoregulatory function (i.e., heat loss through sweating and skin blood flow) during exercise is compromised in older compared to younger males [1,2,3,4,5,6,7]. A decrease in thermosensitivity and/or delay in the onset threshold, as previously shown to occur in older adults [5,6,12,13], would increase the duration of this heat imbalance resulting in a greater amount of heat stored in the body. This is consistent with a recent study by Larose et al [8] who observed marked reductions in whole-body evaporative heat loss in older compared to young females after the first 10 min of exercise. This resulted in greater heat storage during four short intermittent (i.e., 15 min) exercise bouts performed at a fixed rate of metabolic heat production of 300 W [equivalent to ~44% peak oxygen uptake (VO2peak)] in the heat [35°C and 20% relative humidity (RH)] [8]
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