Hypohydration does not Exacerbate Age‐related Impairments in Whole‐body Heat Loss during Exercise in the Heat.

In young adults, hypohydration elicits impairments in whole‐body heat loss that are greater during moderate vs. high exercise‐heat loads. However, it is unknown if the effect of hypohydration on heat dissipation is exacerbated in older adults who demonstrate reduced heat loss capacity compared to their younger counterparts, even when euhydrated. On two separate occasions, six young (25 years [SD 3]) and six older (58 years [SD 4]) males performed a dehydration protocol consisting of light‐to‐moderate intensity intermittent treadmill walking (~150 min) in the heat (40°C) while wearing impermeable clothing. Thereafter, participants rested in a thermoneutral environment (~21°C) for ~240 min during which time they received either 1) a volume of intravenous saline (0.9%) equal to the fluid lost (euhydrated condition; EUH) or 2) no infusion (hypohydrated condition; HYP). Participants then performed a heat stress test (semi‐nude) consisting of two 30‐min bouts of cycling at moderate (175 W·m 2 ) and higher (275 W·m 2 ) rates of metabolic heat production, each followed by a 15‐min recovery, in hot, dry conditions (40°C, ~15% relative humidity). Fluid loss was estimated from changes in body mass and plasma volume from pre‐trial values. Metabolic heat production and whole‐body heat loss were measured during each heat stress test via indirect and direct calorimetry, respectively. Body heat storage was calculated as the temporal summation of heat production and loss. In both age groups, body mass was reduced (mean ± 95% confidence interval) from pre‐trial values prior to the start of the the heat stress test in HYP (young: −5.0 ± 0.5%; older: −4.1 ± 0.8%; both P≤0.03) but not EUH (young: −0.3 ± 0.6%; older: 0.1 ± 0.3%; both P≥0.38). Likewise, plasma volume was attenuated from pre‐trial values prior to the heat stress test in both groups in HYP (young: −11 ± 3%; older: −9 ± 6%; both P≤0.03), whereas no changes in plasma volume were seen in EUH (young: 3 ± 5%; older: 2 ± 4%; both P≥0.23). The alterations in body mass and plasma volume were not different between age groups in either condition (all P≥0.59). In the young, whole‐body heat loss was similar between hydration conditions during the moderate exercise bout (EUH: 352 ± 22 W vs. HYP: 324 ± 33 W; P=0.36) but diminished in HYP (461 ± 34 W) relative to EUH (516 ± 37 W) at the higher work rate (P=0.04). In the older adults however, no influence of hydration on heat loss was noted during either the moderate (EUH: 346 ± 27 W vs. HYP: 331 ± 26 W; P=0.54) or higher (EUH: 445 ± 33 W vs. HYP: 440 ± 32 W; P=0.98) work rates. When participants were EUH, heat loss was comparable between age‐groups during the moderate work bout (P=0.32) but reduced in the older relative to young adults during exercise at the higher work rate (P=0.03). By contrast, similar rates of heat loss were observed in the young and older adults during both exercise bouts when participants were HYP (P=0.40). In the moderate exercise bout, there were no differences in body heat storage as a function of age or hydration status (all P≥0.22). However, during the higher work rate, body heat storage in the young was elevated in HYP (174 ± 17 kJ) relative to EUH (117 ± 30 kJ; P=0.04) to a level comparable to that observed in the older adults during both EUH (188 ± 57; P=0.68) and HYP (184 ± 51 kJ; P=0.40). In summary , we demonstrate impairments in whole‐body heat loss at higher work rates in young but not older adults matched for fluid loss, indicating that hypohydration does not exacerbate the age‐related impairments in heat dissipation. Support or Funding Information The Ministry of Labour (Ontario) and Natural Sciences and Engineering Research Council of Canada. This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal .