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How to Prevent Dehydration in the Heat during Summertime Exercise

Jun 19, 2010


by Sheri Colberg, PhD



It may be even harder to convince your patients about the benefits of regular physical activity during the hotter summer months. Adequate fluid intake is essential to living well at any age, but is especially critical in the heat when the risk of dehydration peaks. As people grow older, they begin to lose some of their normal thirst sensations, thereby increasing the risk for dehydration unless they make a conscious effort to drink more.


If exercising with any elevation in blood sugar, take care to drink enough water, as it will be easier than normal to dehydrate. Elevated sugars can increase water losses through excessive urination, so the risk for losing extra fluids is greater with poorly controlled blood glucose levels. Exercising itself compounds the risk by increasing sweating (thus loss of water), which can rapidly add to a dehydrated state. Since exercising in the heat can be especially dangerous for older individuals — who may not release heat as effectively as younger adults — adequate fluid replacement and frequent rest need to be high priorities for them.

Hydration Tips for Exercise

     Drink cool, plain water during and following exercise, especially during warmer conditions, and take frequent breaks to have a chance to cool down, preferably out of the heat and direct sunlight.

     Drink only when you feel thirsty and don’t to force yourself to drink more than the amount of fluid that satisfies your thirst or water intoxication may result.

     To know how much fluid to replace after exercise, weigh yourself before and after a prolonged activity and only replace up to the weight you have lost (1 liter of water weighs 1 kilogram, or 2.2 pounds).

     If you prefer fluids with some flavor, try flavored waters, sports drinks that have no added carbohydrates or calories (such as Champion Lyte), or Crystal Light (with a pinch of salt if you want it to taste and act more like a sports drink).

     Only drink regular sports drinks (containing glucose) when you need some carbohydrate to prevent or treat hypoglycemia during activities.

     Exercise during the cooler times of day — first thing in the morning or at dusk –whenever possible, and avoid the peak solar times when you gain more heat directly from the sun’s rays (usually 11:00 AM to about 2:00 PM).

Despite all the emphasis on drinking enough, drinking too much can also be a problem. It’s actually as easy to harm yourself with excessive fluid intake during exercise. If you drink too much water and other fluids during exercise, you’ll increase your risk of diluting the sodium content of your blood, potentially causing a medical condition known as hyponatremia, or water intoxication, and putting you at risk for seizures, coma, and even death. While the American Diabetes Association recommends adequate hydration prior to exercise (e.g., 17 ounces of fluid taken two hours before exercise) with fluids consumed early and frequently to cover sweat losses during exercise, or the maximal amount of fluid you can tolerate, updated guidelines are definitely needed. To avoid overhydrating, you should start drinking when you actually feel thirsty. The only exception would be for people with poorly controlled diabetes since they may have an elevated thirst threshold (meaning that they don’t feel thirsty as soon, even when dehydrated). In that case, start drinking small amounts of water as soon as you start sweating.

Preventing dehydration without overloading on fluids is an individual balancing act. You should not be gaining weight during a physical activity. Really, you will be sweating and losing water in other ways, so your weight should go down (albeit temporarily until you rehydrate). Replace only the weight that you have lost.  Later on, after exercise, continue to use thirst as your guide, rehydrating after the fact with water or other non-caloric fluids, but if you have consumed a lot of fluid during an activity, wait until you start to urinate before drinking any more. 

As for whether you should drink water, sports drinks, or other fluids, it depends on your blood glucose needs. For shorter activities (lasting an hour or less), plain water is fine, unless you need some extra carbohydrate to prevent your blood sugar from dropping too much, in which case you can drink a sports drink like Gatorade or PowerAde. You don’t need to worry about replacing electrolytes, like sodium, potassium, and chloride, unless you are exercising outdoors in hot weather for more than two hours at a time, and even then, you can usually wait to replace electrolytes naturally with your food the next time you eat. 

Luckily for the coffee drinkers out there, it’s only a myth that caffeinated drinks will hydrate you less well than caffeine-free ones, but keep in mind that too much caffeine can cause the bones to lose calcium, so the decaf options may be better ones for that reason alone. Some other tips are to make certain to increase your fluid intake when you have a fever. If you get diarrhea, try to consume fluids containing calories (rather than something like low or no calorie sodas). Also, remember that adequate fluid intake is by far the best constipation cure out there.

To sign up for five free healthy living reports via e-mail or a 52-week fitness program delivered via e-mail, log on to www.lifelongexercise.com. For more information, also check out my web site at www.shericolberg.com. If you need tips for getting started on an exercise program, consult The 7 Step Diabetes Fitness Plan. People with any type of diabetes who are already more active may benefit from reading the Diabetic Athlete’s Handbook.

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