Thirst shows up after dehydration has already started

Waiting until you feel thirsty isn't a reliable hydration strategy

Thirst is regulated by osmoreceptors in the brain that respond to rising concentration of substances in the blood from fluid loss. The problem is this mechanism is inherently delayed by nature: by the time thirst becomes noticeable, the body has already lost a meaningful amount of fluid — various estimates put the noticeable-thirst threshold at roughly 1–2% of body weight lost as water, which is already enough to measurably reduce physical and mental performance.

With age, sensitivity of the thirst mechanism declines further, which is why older adults are at greater risk of dehydration at the same fluid-loss level — they simply don't feel the signal as strongly as they did when younger.

Practical takeaway for anyone training: during a long or intense session, don't wait for a clear feeling of thirst to start drinking — by then, performance has already partly declined. It's smarter to drink small amounts regularly throughout the session and use urine color afterward as a guide (pale yellow is a good sign of adequate hydration) rather than relying only on the subjective urge to drink.