Fluid replacement is probably the most important nutritional concern for athletes.
As you exercise, fluid is lost through your skin as sweat and through your lungs when you breathe. If this fluid is not replaced at regular intervals during exercise, you can become dehydrated. When you are dehydrated, you have a smaller volume of blood circulating through your body. Consequently, the amount of blood your heart pumps with each beat decreases and your exercising muscles do not receive enough oxygen from your blood. Soon exhaustion sets in and your athletic performance suffers.