When you run, muscle contraction generates energy and heat. The body evacuates this heat by perspiring, regulating the body's heat. Runners lose water when they sweat, and sometimes in large quantities: a marathoner loses an average of 4 litres of water during a race! Water loss varies between 0.5 and 1 litre per hour during a running session. But that can reach 2 litres depending on the intensity and weather conditions, causing dehydration.
If you don't compensate for the water loss, your performance will suffer: the loss of 1% of your body weight in water decreases your physical capacity by 10%! This can also have other negative effects on your body: increased heart rate, fatigue, headache, dizziness, muscular damage, tendinitis etc.
Advice by Sport