According to Carmichael Training System, your body begins to learn how to regulate your body temperature as you gain fitness. Your sweat response kicks in much sooner because it knows you mean business when you’re working out. Your body begins to regulate your core temperature early, so you can work out longer. You might also notice that you’re sweating more during your workout than your less fit friends because your body has become more effective at keeping your internal temperature cooler.
Your sweat will also be different as you become more fit. Typically, you lose water, sodium, potassium, and other electrolytes through your sweat. As your fitness improves, your body gets better at retaining the important electrolytes to help keep your fluid levels balanced. It’s not that athletes need less hydration; athletes can sweat off up to 6% of their body weight during a workout (via Piedmont Healthcare). That’s 9 pounds for a 150-pound athlete.