by Judy Heller
Printed March 2001, ADA marathon training newsletter
Water balance is essential to our well being, especially when exercising. The muscles are 75 percent water, the brain, 70 percent, the blood, 85 percent. During exercise, muscle activity produces heat which raises your body temperature. As the body temperature rises, blood flows to the surface capillaries of the skin to release the heat.
The primary means for the body to cool during exercise is perspiration as "sweat" and evaporation of perspiration. The amount of evaporation and heat loss depend on the air temperature, humidity, and wind speed. When humidity is high and the air is still, evaporation will diminish or cease completely.
Your heart rate increases as the heart must beat harder in an attempt to provide blood to both the skin surface and the exercising muscles. Another factor affecting heart rate is the loss of water through your sweat glands. As the blood volume decreases, the heart has less blood to work with which forces it to beat harder still. Anyone who exercises in the heat runs the risk for heat illness. Injuries from heat occur in three forms, heat cramps, heat exhaustion, and heat stroke.
Water is considered the most optimal fluid to drink for events lasting less than two hours. It is needed for the delivery of oxygen and nutrients to muscles being used, and to transfer heat from the muscles to the skin to evaporate and cool the body. Being well hydrated means your urine is clear and lots of it. For events lasting longer than two hours, however, studies show that liquid carbohydrate sport drinks are a better nutrient providing carbohydrates and electrolytes.
The thirst mechanism is a poor indicator of when your body needs fluids, especially as you age. Even minor dehydration (1% loss) interferes with strength, stamina, coordination, and reaction time. The golden rule for rehydration during exercise is taking small quantities at frequent intervals every 10-15 minutes. This is especially important when training longer than one hour. After exercise drink more than you think you need.
There are a variety of carriers available: the waist belt carrier with single or double bottle; the Kamel pak which is like a small back pack with rubber tubing. It is a very important part of your training to get into the habit of drinking every 10-15 minutes. Just as your running or walking is training your muscles and connective tissue for completion of the half or whole marathon, drinking water or sport drinks are a training for your system to absorb and utilize all that the fluid offers. If you don't already have a convenient water carrier, now is the time to get one. If you do have one, now is the time to start using it, if you are not.