Blunted Effects of Elevated Serum Osmolality on Whole‐body Heat Loss and Rectal Temperature in Middle‐aged‐to‐older Men Exercising in Dry Heat
Authors: Meade Robert D. , Notley Sean R. , D'Souza Andrew W. , Rutherford Maura M. , Binet Emileigh R. , Boulay Pierre , Kenny Glen P. 
Published: April 2020

We recently demonstrated that sweating‐induced hypohydration attenuated whole‐body heat loss (via reduced sweat evaporation) in young but not middle‐aged men during exercise‐heat stress. In that work, we reasoned that this divergent response may stem from an attenuated influence of intravascular osmolality (elevated during hypohydration) on heat dissipation in the middle‐aged. This hypothesis is supported by previous studies showing i) increased osmolality attenuates sweating in young adults and ii) fluid‐regulatory responses to osmotic stimuli are blunted with aging. However, the independent effect of intravascular osmolality on whole‐body heat loss and thermal strain in middle‐aged men during exercise has not been directly evaluated. On two separate occasions, six young (25 years [SD 4]) and eight middle‐aged‐to‐older (61 years) men underwent a 90‐min intravenous infusion of either 0.9% saline (200 [5] μmL·min−1·kg−1; ISO) or 3.0% saline (125 μmL·min−1·kg−1; HYP). Participants then performed 60 min of cycling at a fixed metabolic heat production of 250 W·m−2 in dry heat (40ºC, ~15% relative humidity). Serum osmolality and plasma volume responses were determined from venous blood samples procured at baseline (prior to infusion) and end‐exercise (60‐min). Heat production and whole‐body heat loss (evaporative + dry heat loss) were measured via indirect and direct calorimetry, respectively. Rectal temperature was monitored continuously. End‐exercise serum osmolality was elevated in HYP compared to ISO in the young (309 [4] vs 299 [3] mOsm·kg−1; p<.001) and middle‐aged‐to‐older (312 [2] vs 300 [4] mOsm·kg−1; p<.001) men and was similar between age‐groups (both p≥.07). In the young, the reduction in plasma volume from baseline at end‐exercise was greater in ISO relative to HYP (−8 [3] vs −4 [3]%; p=.014). By contrast, no differences in the plasma volume response were detected between ISO and HYP in the older group (−9 [4] vs −4 [6]%; p=.14). Changes in plasma volume were similar between age‐groups in each condition (both p≥.75). Whole‐body heat loss was attenuated in HYP compared to ISO in the young (233 [13] vs 251 [15] W·m−2; p=.012), whereas no effect of osmolality was observed in the middle‐aged‐to‐older men (225 [14] vs 227 [16] W·m−2; p=.74). Further, heat loss was attenuated in the middle‐aged‐to‐older vs young in ISO (p=.017) but was comparable between groups in HYP (p=.26). Similarly, a greater end‐exercise rectal temperature was noted in HYP vs ISO in the young (38.2 [0.3]°C vs 37.9 [0.3] °C; p=.037) but not the middle‐aged‐to‐older men (38.3 [0.3]°C vs 38.3 [0.3]°C; p=.88). The rectal temperature response was greater in the middle‐aged‐to‐older during ISO (p=.013) whereas no difference between age‐groups was observed in HYP (p=.59). Our findings indicate that whole‐body heat loss and rectal temperature responses to exercise in dry‐heat are not appreciably modified by increased serum osmolality in middle‐aged‐to‐older men. An age‐related reduction in osmotic sensitivity may therefore explain the blunted effect of hypohydration on heat loss previously observed in this population.

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