It depends on your core temperature.
鈥淚f you exercise in the cold and your core temperature is well-maintained, then thirst is very likely to be reliable,鈥 says Dr. 脡ric Goulet, co-author of on thirst and running performance published in the European Journal of Applied Physiology.
In the study, 10 runners ran two treadmill half marathons. 聽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.聽
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.
鈥淚f you are getting cold, and your core temperature decreases,鈥 Goulet says, 鈥渢hirst sensation will decrease.鈥 Your body moves blood out of the periphery and into your core when it鈥檚 cold, increasing central blood volume. When your core blood volume increases, the brain thinks you鈥檙e well hydrated, so it won鈥檛 trigger thirst.
So if you鈥檙e 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: and his colleagues found that a 3 percent loss of bodyweight due to dehydration does not hurt health or performance. That鈥檚 a whopping 4.5 pounds in a 150-pound athlete, so you have some room for error.)
The bottom line: If you鈥檙e 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.