Replacing Sweat Losses

During hard exercise, your muscles can generate 20 times more heat than when you are at rest. You dissipate that heat by sweating. As the sweat evaporates, it cools the skin. This in turn cools the blood, which cools the inner body. If you did not sweat, you could cook yourself to death. A body temperature higher than 106 degrees Fahrenheit (41 degrees Celsius) damages the cells. At 107.6 degrees Fahrenheit (42 degrees Celsius), cell protein coagulates (as egg whites do when they cook), and the cell dies. This is one serious reason why you shouldn't push yourself beyond your limits in very hot weather.

Some people sweat a lot. For example, James had to put a towel under the exercise bike to mop the sweat that dripped from his body. Although it was a source of embarrassment, I reminded James that sweating is good. It's the body's way of dissipating heat and maintaining a constant internal temperature (98.6 degrees Fahrenheit; 37 degrees Celsius).

James, like many men, produced more sweat than he needed to cool himself. He'd sweat large drops of water, which dripped off his skin rather than evaporate, resulting in a reduced cooling effect. In comparison, women tend to sweat more efficiently than men. But both men and women need to be equally diligent about replacing sweat losses.

James wondered how much he needed to drink to replace his sweat loss. I suggested he learn his sweat rate by weighing himself nude before and after an hour of exercise. For every pound (16 oz, or 0.5 kg) he lost, he needed to drink about 80 to 100 percent of that loss (13 to 16 oz, or 400 to 480 ml) while exercising; this would require training his gut to handle this volume. I also suggested he figure out how many gulps of water equated to 16 ounces (480 ml).

By knowing his sweat rate (4 lb [almost 2 kg], or 64 oz, per hour), he was able to practice "programmed drinking" during exercise in order to minimize sweat losses. James started to drink 16 ounces (16 gulps; half of a 32 oz, or 1 qt, water bottle) every 15 minutes; this matched his thirst and more than doubled what he had previously consumed. His programmed drinking required having the right quantity of enjoyable fluids (chilled, palatable) readily available and even setting an alarm wristwatch to remind him to drink on schedule. He felt so much better after his workout that the extra effort was worthwhile.

Thirst, as defined by a conscious awareness of the desire for water and other fluids, usually controls water intake. The sensation of thirst is triggered when body fluid concentrations are abnormally high. When you sweat, you lose significant amounts of water from your blood. The remaining blood becomes more concentrated and has, for example, an abnormally high sodium level. This triggers the thirst mechanism and increases your desire to drink. To quench your thirst, you need to replace the water losses and bring the blood back to its normal concentration.

Unfortunately for athletes, this thirst mechanism can be an unreliable signal to drink. Thirst can be blunted by exercise or overridden by the mind. Hence, you should plan to drink before you are thirsty. By the time your brain signals thirst, you may have lost 1 percent of your body weight, which is the equivalent of 1.5 pounds (3 cups, or 24 ounces) of sweat for a 150-pound (68 kg) person. This 1 percent loss corresponds with the need for your heart to beat an additional three to five times per minute (Casa et al. 2000). A 2 percent loss fits the definition of dehydrated. A 3 percent loss can significantly impair aerobic performance (Coyle and Montain 1992). Remember, you will voluntarily replace only two-thirds of sweat losses. To be safe, drink enough to quench your thirst, perhaps a little moreā€”but stop drinking if your stomach is sloshing. Enough is enough!

Young children, in particular, have a poorly developed thirst mechanism. At the end of a hot day, children often become very irritable, which may be partially due to dehydration. If you are going to spend the day with children at a place where fluids are not readily available, such as at the beach or a baseball game, bring a cooler stocked with lemonade, juice, and ice water, and schedule frequent fluid breaks to increase everyone's enjoyment of the whole day.

Senior citizens also tend to be less sensitive to thirst sensations than are younger adults. Research with active, healthy men aged 67 to 75 years shows that they were less thirsty and voluntarily drank less water when water deprived for 24 hours compared with similarly deprived younger men aged 20 to 31 years (Phillips et al. 1984). In another study, older hikers became progressively dehydrated during 10 days of strenuous hill walking. The younger hikers remained adequately hydrated (Ainslie et al. 2002). Athletic seniors who participate in any sports should monitor their fluid intake.

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