Water is an essential component for the survival of all living beings on this planet. In the case of the human being when performing physical activities the need is even greater, because with perspiration and more breathy breathing there will be greater loss of fluids and the replacement should be more frequent and greater, that is, if we should already ingest the adequate amount of water, when doing physical exercises can not be different.
Sweat losses can be substantial, in addition to containing water, sweat contains electrolytes that will be lost. If not properly replaced, imbalances of water and electrolytes cause a considerable loss of performance, that is, in addition to all the risks that dehydration already causes in our body, it can still decrease our performance.
According to the Food and Nutrition Board of the Federal Institute of Medicine of the United States an adult man should ingest about 4l of water daily and a woman about 3l of water. This quantity is given for the proper functioning of the human body without necessarily the practice of physical exercise.
If you do physical exercises you should drink water whenever you feel thirst because the body gives indications of this need.