In humans, hypohydration attenuates sweat secretion and attenuates whole‐body heat loss, probably to mitigate further fluid losses and thereby support blood pressure regulation.
Recently, however, we demonstrated that the hypohydration‐mediated reduction in net whole‐body heat exchange (evaporative heat loss – dry heat gain) was blunted in middle‐aged compared to younger men during moderate exercise in dry heat; albeit, the underpinning mechanisms could not be determined.
Here we evaluated the hypothesis that those findings stemmed from a diminished influence of extracellular hyperosmolality on net whole‐body heat exchange in middle‐aged‐to‐older compared to young men.
Consistent with that hypothesis, extracellular hyperosmolality induced by an intravenous infusion of hypertonic saline (3% NaCl) reduced net heat exchange and augmented rectal temperature to a greater extent in the young compared to middle‐aged‐to‐older men.
Thus, age‐related differences in the influence of hypohydration on thermoregulatory function appear to be due to blunted sensitivity to hyperosmolality with ageing.
..Our findings indicate that ageing blunts the effect of extracellular hyperosmolality on thermoregulatory function during exercise‐heat stress