It depends on your core temperature.
“If you exercise in the cold and your core temperature is well-maintained, then thirst is very likely to be reliable,” says Dr. Éric Goulet, co-author of a new study on thirst and running performance published in the European Journal of Applied Physiology.
In the study, 10 runners ran two treadmill half marathons. During one half marathon, the runners drank water according to thirst. During the other, they drank water to maintain a bodyweight within two percent of their starting weight.
The thirst-driven group had no reduction in performance despite drinking four times less than the runners who drank to maintain their weight. But all of the runners in this study were exercising in 86-degree weather and maintained core temperatures around 100 degrees. A dropping core temperature changes the game.
“If you are getting cold, and your core temperature decreases,” Goulet says, “thirst sensation will decrease.” Your body moves blood out of the periphery and into your core when it’s cold, increasing central blood volume. When your core blood volume increases, the brain thinks you’re well hydrated, so it won’t trigger thirst.
So if you’re cold, old school hydration rules apply: Calculate your sweat rate, then make sure to take in enough water or sports drink during your workout to make up for most of the fluid loss. (Note: Dr. Goulet and his colleagues found that a 3 percent loss of bodyweight due to dehydration does not hurt health or performance. That’s a whopping 4.5 pounds in a 150-pound athlete, so you have some room for error.)
The bottom line: If you’re cold, thirst may be an unreliable indicator of hydration. Try calculating your sweat rate, then aim to replace most of that fluid during your workout.