The body loses up to three quarts of water a day through perspiration, urination, feces, and evaporation from the lungs. On days that you exercise, you may need twice that amount. When exercising, the body is not able to take in water at the same rate as it expends it. It is very important to drink increased amounts of water prior to exercise to assure adequate hydration and after exercise to replenish water lost to perspiration. Water can be consumed during exercise, however.
Perspiration expends water which comes from body tissues and, during long exercise bouts, the blood.
This causes a slight thickening of the blood which increases the workload on the heart. Dehydration may cause you to feel nausea, cramping or overall weakness. Unfortunately, the human thirst mechanism is a poor indicator of adequate hydration. Thirst is easily quenched before a body is sufficiently dehydrated after losing water. It is therefore important to pay attention to the amount of fluids consumed each day to ensure proper hydration.