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For decades, endurance athletes have been encouraged to determine their sweat rate during training so they can drink at an equal rate to prevent dehydration. The standard method is simple. Weigh yourself without clothes immediately before a run. Run for one hour without drinking. Weigh yourself again immediately after the run. The amount of weight you’ve lost equals your sweat rate per hour.
Recent research has called into question both the validity of this procedure and the usefulness of knowing one’s sweat rate. Studies performed by Timothy Noakes at the University of Cape Town, South Africa, have shown that not all weight loss during exercise is accounted for by body water. So, if you drink to completely offset weight loss during running, you’re actually taking in more water than you’re losing.
Other studies have shown that runners neither perform better nor keep their core body temperatures lower when they drink to completely offset weight loss than when they just drink according to their thirst (which typically results in only 65-70 percent replacement of weight loss).
Maybe you don’t need to sweat all over your scale after all.