To the Point: Summer heat and dehydration

July 19, 2006

University Park, Pa. -- As midsummer temperatures spike and sun and humidity make being outdoors uncomfortably warm, people may notice wilting flower leaves and be reminded to water their plants. However, hydrating yourself during hot weather is even more important, even if you don't feel thirsty. Larry Kenney, professor of physiology and kinesiology and an expert on the impact of hot weather and dehydration on people, offers some important facts and reminders about avoiding dehydration.

What exactly is dehydration and how does a person know when he or she is dehydrated?

Kenney: Dehydration is any loss of body water, and the easiest way for people to understand whether they're dehydrated or not is to weigh themselves, particularly before and after an activity. If you weigh yourself before you go out and work in the garden or exercise, then weigh yourself afterward and you've lost a pound, then you need to drink 16 ounces of fluid to make up for that lost pound. The idea is not to gain or lose weight, because both of those, if taken to extremes, could be dangerous. And in humans, the sense of thirst lags behind other responses to dehydration, so certainly by the time you feel severe thirst, you're probably well dehydrated.

Are there other initial signs of dehydration to which we might be advised to pay more attention?

Kenney: I think a lot of people have been dehydrated at one time in their lives, so they have a feel for what those signs or symptoms are. People feel lethargic or get a dull, persistent headache. Other people feel a slight nausea or dizziness. Any time you feel more severe symptoms than that, especially during activity in the heat, like goosebumps, a severe headache, vomiting -- those are signs of more severe heat illness, and those people really need to get out of the heat and be treated.

Are there treatments for different levels of dehydration, and at a certain point beyond self-treatment should medical attention be sought?

Kenney: When people are active in the heat, heat exhaustion is really synonymous with dehydration in many ways, and for mild heat exhaustion, which has some of the symptoms I've just mentioned, getting out of the heat into a cool, air-conditioned area and getting some fluids is usually enough. Any time people get dizzy, pass out, have persistent vomiting or any kind of central nervous system or cognitive symptoms -- if they get confused or can't remember how they got to where they are -- that's a severe medical emergency. Those people are probably bordered on heat stroke and need to seek medical attention, or have someone seek it on their behalf.

The people who are most at risk are the elderly, especially the frail elderly, and infants, because they have a really small body size in relation to their surface area, so they gain heat very quickly, plus they rely on other people for their hydration needs. People taking a variety of drugs and medications also are at risk.

Are there certain medications that cause increased risk of dehydration to patients taking them during hot weather?

Kenney: Almost any drug has the potential to do that, but there are three categories of patients that are the highest risk. First are people who are on antidepressants, because those drugs interfere with sweating responses. Second are people taking diuretics for high blood pressure, because those drugs deplete the body of water. Third are people using any variety of recreational drugs that are stimulants, such as cocaine or ecstasy. There are a lot of documented heat strokes related to raves and other types of places where younger people, for the most part, are using those sorts of drugs -- also speed, amphetamines, anything that revs up the metabolism.

Do you have any recommendations for those who are physically active or regular exercisers?

Kenney: First, they can move the exercise indoors where it's air-conditioned. They can also exercise early in the morning or later in the evening when temperatures are cooler. If they insist on exercising during the hotter times of the day, they need to slow their pace, and the way to do that is to monitor their heart rate. What happens is that the heat and exercise combine to increase the heart rate higher than it would be if exercise were done under cool conditions. So if they know that their typical exercise heart rate is 160 beats per minute, for example, on a cool run, and they go out and still run at 160 beats per minute, by necessity they will have to slow down the pace to do that. So monitoring heart rate is a good safety gauge for exercisers who are so gung-ho that they have to go out at noontime.

Is drinking water the most effective treatment for dehydration, or are there more effective treatments available?

Kenney: If people are dehydrated on a repeated basis -- for instance, people who have outdoor labor jobs, or football players who have "two-a-days" -- they should favor sports drinks over water. That's because the sodium in the sports drink maintains thirst longer, so they end up drinking more of a sports drink, plus it replaces some of the sodium and other electrolytes that are lost in sweat. But if they don't have access to sports drinks, then certainly water is the next best option.

Does the average person need to consume sports drinks during hot weather?

Kenney: People need to drink, but drink sensibly. For the typical person who's not an athlete or a laborer, simple advice would be to drink water and add a bit of salt to their food. For people who are in those other categories, who are going to have more heat stress and be out there on more of a sustained basis, then if they have the ability to do so, drinking sports drinks is beneficial. But also, unless their doctors have told them very specifically not to eat salt, then people should liberally salt their food during these periods of high heat and humidity if they're physically active.

Should people avoid consuming certain things during hot weather?

Kenney: I think there's a lot of advice out there not to drink caffeinated beverages, but I don't think that's really warranted, because when you think about drinking soda or coffee, the small amount of caffeine in it is overbalanced by the amount of water. Alcohol is probably OK in moderation unless you're really out there exercising, but moderation is the key word.

Are there any misconceptions related to dehydration?

Kenney: Some people are prone to cramps in the heat, and while cramps are not life-threatening, they're certainly painful to those people who get them. About 30 years ago we went through the idea that a loss of sodium caused cramping, and then for some reason that theory went out of vogue and people started to talk about potassium. Potassium has absolutely nothing to do with muscle cramps; it really is a sodium and fluid problem. I've heard people tell people that if you have cramps, eat bananas -- you'd have to eat about a seven-foot-long banana to increase any level of potassium in your body. So, really, the idea of adding a little bit of salt to your foods and favoring sports drinks with sodium -- and if you're a cramper, adding even more sodium to the sports drinks -- is a way to prevent cramping.

Another misconception is that back in health class, some people learned that in heat exhaustion you have pale, cold clammy, sweaty skin, but in heat stroke you have hot, dry skin, and that somehow you can use that to differentiate between the two. But if you think about it, that makes very little sense because if you've been out there running five miles on a hot day and you start to experience heat stroke, you've been sweating and your skin is still going to be wet and moist, not hot and dry, especially if the sweat has been evaporating or dripping off. So the real distinction between heat exhaustion, which is serious but treatable without medical intervention, and heat stroke, which always requires medical intervention, are those cognitive symptoms that I talked about before. Anytime people start getting dizzy, acting spacey, not responding when people talk to them -- any sort of central nervous system kind of problem needs immediate medical attention.

The last thing I would say is that people should certainly check on elderly neighbors and people they know without air-conditioning, and parents should be very aware of children in cars. A lot of common sense goes into these recommendations.


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Last Updated July 21, 2011