Last week, I emphasized the evaporation of sweat as the primary cooling mechanism for the body when exercising in hot weather. Water is an essential component of the body and there is a delicate water balance that must be maintained. That’s why replacing fluid lost as sweat is critical.

At one time, water was the only fluid replacement drink. This changed when scientists at the University of Florida created Gatorade, which mimicked what was lost in sweat. Gatorade was launched with a marketing blitz and coaches were bombarded with information on the need to avoid dehydration. This was a huge breakthrough, because many coaches then believed in withholding water during practice to make players tougher.

Water versus fluid-replacement drinks

The incidence of heat stroke deaths plummeted in the years after Gatorade was introduced. Was this because Gatorade is wonderful? Or because, in order to sell Gatorade, the producers had to convince coaches of the importance of keeping their players hydrated? I believe it was a combination.

Many different fluid replacement drinks containing additional substances, such as sodium, potassium and glucose now are available.

Studies have shown that prolonged endurance performance in the heat improves when water intake matches the amount of water lost as sweat. When a fluid replacement drink was ingested instead of water in the same amounts, performance increased even further.

One advantage of a fluid replacement drink is it helps your body hold onto the water, because it contains sodium. Drinking lots of water can trigger a urination response, which can cause modest dehydration before you exercise.

After exercise, even though you are dehydrated, drinking water can trigger urination. Thus, a fluid replacement drink is best before and after exercise, but it’s not as important during exercise, unless you go for more than an hour.

Here are tips on fluid ingestion when exercising in the heat:

• Pre-exercise: Load up on a fluid replacement drink by drinking at least 16 ounces an hour or two before starting.

• During exercise: Consume 6 to 12 ounces of water every 20 minutes when exercising less than one hour. For longer periods of exercise, a fluid replacement drink is superior.

• After exercise: A fluid replacement drink is superior. Load up with 16 to 24 ounces of fluid replacement drink for every pound of weight lost during exercise. But with severe water loss, this can be expensive. An option is to add a dash of salt to drinking water (about a teaspoon per gallon) – at this level you won’t taste it – and consume equal portions of a fluid replacement drink and lightly salted water.

Water intoxication

A rule of thumb when it comes to exercise in hot weather is drink, drink and drink some more. However, if the drink is water, there can be a risk of hyponatremia, or water intoxication, which occurs when excess sodium is lost in sweat and excess water intake dilutes the sodium in the blood. This creates an electrolyte imbalance, which can cause seizures, coma and even death.

The risk is tiny, but it’s still real. To avoid it, add a dash of salt to your water or use a fluid replacement drink.

Also, beware of going to the other extreme and loading up on fluids high in glucose (greater than 12 percent carbs) – fruit juices or soft drinks. This can cause gastrointestinal upset and delay movement of fluid out of the stomach. Fluid replacement drinks are about 7 percent carbs, which is just right.

Bryant Stamford is professor and chairman of the department of exercise science at Hanover College in Hanover, Ind. Address questions or suggestions to “The Body Shop,” The Courier-Journal, P.O. Box 740031, Louisville, Ky. 40201-7431.

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