When you run, you lose water through sweat. The amount you sweat is dependent on genetics (some people simply sweat more than others when making the same effort), the temperature you are running in, and the intensity you are running at. The harder you run, the higher your body temperature rises, activating your sweat glands to release sweat to help you cool down. This loss of body water then reduces blood volume and muscle blood flow, which contributes to reduced performance. Dehydration is commonly recognised as a cause of fatigue in exercise.