A human body consists mainly of water. During intensive exercise, sweating is the body’s defense reaction to increased heat generation in muscles. The rate of sweating during marathons in unfavorable conditions (no wind, high temperature, and humidity) can reach even more than 3l/hour. Dehydration effects: 1% - Capacity reduction, deterioration of the cardiovascular system 3% - Critical capacity reduction 5-10% - Headaches, drowsiness, concentration disorders, nausea, accelerated breathing, muscle spasms >10% - Life threat, fainting, visual disturbances