Endure: Mind, Body, and the Curiously Elastic Limits of Human Performance

by Alex Hutchinson

it’s worth considering what thirst is for. The simplest explanation is that it’s the body’s way of ensuring that you keep your fluid levels topped up. In this picture, voluntary dehydration is a failure of the system: it indicates that your thirst sensation isn’t very good at its job, because it fails to notice that you’re losing fluids. But physiologists have shown that this isn’t how thirst works. Instead of monitoring fluid levels, your body monitors “plasma osmolality,” which is the concentration of small particles like sodium and other electrolytes in your blood.25 As you get dehydrated, your blood gets more concentrated, and your body responds by secreting an antidiuretic hormone that causes your kidneys to start reabsorbing water, and by making you thirsty. Unlike your body’s fluid levels, plasma osmolality is very tightly regulated: when you’re looking at the right variable, your thirst sensation (along with other homeostatic mechanisms like antidiuretic hormone) doesn’t make mistakes. This means that what looks like a potential problem—voluntary dehydration—may actually be completely normal from the body’s perspective. In a 2011 study, eighteen South African Special Forces soldiers undertook a sixteen-mile march carrying 57-pound packs, including rifles and water supplies, in temperatures that peaked at 112 degrees Fahrenheit.26 The soldiers were permitted to drink as much water as they wanted, but—as expected—they nonetheless lost an average of six pounds, corres...