To me, the primary message is that, like oxygen and heat and (as we’ll discover) fuel, the loss of fluids first makes itself felt via the brain. Thirst, not dehydration, increases your sense of perceived effort and in turn causes you to slow down. Eventually, the physiological consequences of dehydration assert themselves, increasing the strain on your cardiovascular system and pushing your core temperature up as the volume of blood in your arteries decreases. But that only happens if you’ve already ignored the signs of thirst.
