Aaron Shelly
Director of Sports Performance Nutrition
Texas Tech University

Whenever your body is short of water, performance bombs. Why you ask ? Exercise increases body temperature in direct proportion to the exercise load. Your body tries to maintain its resting temperature of 98.6 F, by moving the extra heat to the skin via the blood. There it dissipates into the air, mainly by evaporation of sweat. But your blood must also carry oxygen and nutrients to the muscles and remove the wastes of muscle metabolism. Available blood is shared between all these tasks. The higher your core temperature rises, the more blood is used for cooling and less is available for muscles. So, the cooler you stay during exercise, short of being cold, the better your muscles will function.

Outside the narrow range of 98 - 100 F, your body will always sacrifice muscle function for temperature regulation, because a decline in muscle functions, even to complete immobility, is not life-threatening. But, if body core temperature rises a mere 9 F, normal biochemistry ceases and you die.

Heavy exercise ( equivalent to sports of more than 1 hour continuous duration ) can increase heat production in muscles to more than 20 times their normal resting rate. Even with optimum hydration and a cool environment, this heat can raise your core temperature to 103 F within 15 minutes. Studies show that you can still perform well at this temperature, but probably not at your best.

So, how do we combat the above metabolism? Drink all liquids as cold as you can stand them, to give a reservoir of cold in the gut.
Why ? Cold water, below 50 F is absorbed faster than room temperature water. As a bonus, it supplies a reservoir of cold in the stomach that will absorb considerable body heat. Sip, don't gulp. Gulping swallows air, which disturbs stomach function and slows absorption.

The same applies to carbonated drinks. The gas slows absorption. Avoid them. In fact, almost anything added to water slows absorption. The walls of your intestines are semi-permeable membranes like very fine mesh. Water passes through easily but, most particles do not. So pure water containing no particles is absorbed rapidly. As soon as you dissolve anything in water, say sugar, absorption slows.

Many commercial sport drinks contain high levels of glucose or sucrose or similar simple sugars. They inhibit absorption. Don't use them during exercise. Carbonated drinks are worse. Typically, they are over 10% simple sugars. If you drink 12oz. of plain water, 8oz. of it will empty from your stomach within 15 minutes. If you drink 12oz. of a 10% sugar solution, less than 1% will empty in the same period.

But a lesser level of sugar can be helpful. Simple glucose at 1-5% hardly inhibits stomach emptying at all and does provide a boost to blood glucose. Fructose at 2% enhances stomach emptying causes less insulin burst and helps restore liver glycogen.

You have just finished a tough workout ( equivalent to sports of more than 1 hour in duration ). How do you get your body back to normal ? First and most important, you are dehydrated. Second, your stomach is in a highly acidic condition and almost empty. Third, your muscles are loaded with debris of metabolism. Fourth, your glycogen reserves are depleted. Fifth, you are in electrolyte overload, because the percentage of body water lost is much greater than the percentage of body minerals lost.

Re-hydrate immediately by drinking plain cold water. Sip, don't gulp. Coax yourself, because the thirst response is still inhibited after performance. Avoid juices, especially citrus juices, which inhibit re-hydration because of their high sugar content and only add to stomach acidity, promoting cramps and nausea. Plain water is the only story.

Don't sit down or lie down right after an event, no matter how tough. Muscle cramps and post-event injuries often occur because insufficient blood gets to the fatigued muscles to remove wastes. A lot of the force for blood circulation comes NOT from the heart, but from working muscles. Keep drinking and walking.

Until you are four 8oz. glasses ahead, avoid any food to ensure quick water absorption. Once your stomach is bathed with water, then food is fair game. When I say food, I mean complex carbohydrates, not sodas, beer or candy bars. Carbohydrate loading drinks are good as well. After digestion, carbohydrates are converted to glycogen and stored in your muscles and liver for fuel. But, in order to store each gram of glycogen, the body also has to store 2.7 grams of water. Basic biochemistry, but good news for you as an athlete.

Careful carbohydrate loading can double your usual glycogen store. For example, in a 150lb athlete, total glycogen increases from 400 grams (14oz.) to about 800 grams (28oz.). The extra glycogen pulls in 400 x 2.7 grams of water (36oz.), more than two pints.

Then the metabolism of glycogen during exercise forms another 0.6 grams of water per gram of glycogen used for energy. So using the extra 400 grams you have loaded, yields a further 10oz. of water. That yields a total of 46oz of extra water, making carbohydrate loading as important to fluid status as it is to glycogen status.

Continue drinking for the next 12 hours. A common problem among many athletes is chronic partial dehydration. They never drink enough to completely re-hydrate. Usually, they are back to training the day after a workout/game, depleting their body of water again. A good rule of thumb is to drink 15 glasses of water over the next 12 hours following a workout to ensure proper re-hydration