For some people, summer is the time to head indoors to exercise. But others welcome the heat as a way to sweat more and get a better workout. Indeed, I've long regarded the sweatiness of my exercise sessions as a sign of how hard I was pushing myself. But it turns out I've been wrong: How much you sweat doesn't necessarily correlate with how intense your workout is or how many calories you burn. When your body temperature rises, your eccrine glands secrete sweat, and the evaporation of moisture from your skin helps you cool off. Of course, sweating can occur for other reasons, such as stress or fear. That type of sweat comes from the apocrine glands, which are located mainly in the underarm and groin. How much we sweat during exercise is due to a number of factors, including gender (men tend to sweat more than women) and age (younger people sweat more than older people) as well as genetics, temperature and humidity. Weight plays a role as well. Larger people tend to sweat more, because their bodies generate more heat. Another contributor is fitness level. Surprisingly, fit people tend to sweat sooner during exercise and more copiously than those who are less fit. Research suggests that as your fitness level improves, your body's heat-regulating system becomes more efficient, cooling you down faster and allowing you to work harder. Don't be misled by the loss of a few pounds after a high-sweat workout. This is simply water weight that you gain back when you rehydrate and doesn't necessarily mean you've burned lots of calories.