Clothes that don’t allow much ventilation make you sweat more; so when you step on the scale after a run, you may find your weight is down—but the loss is water weight, not fat. Any wrestler who’s ever had to make a weigh-in before a match can tell you that.
Even more important, aiming to sweat excessively during training actually hurts your ability to lose fat, because it dehydrates you, so you can’t go as hard. A recent study review in the Journal of Sport and Health Science concluded that dehydration negatively impacted performance in activities that lasted more than 30 seconds.
Interestingly, there was no significant negative effect on activities lasting less than 15 seconds—which may mean you can lift heavy without adequate hydration. But we recommend always drinking enough to keep your urine clear (the best indicator of good hydration).