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Extreme Heat Warning: Beware of Dehydration

Signs and symptoms you may be in trouble

En español | We're a month into summer, and heat records continue to fall. The stifling weather is not only oppressive, but dangerous to your health. Among older people, dehydration is one of the most common hazards of hot, humid weather.

Recent studies have found that even mild dehydration comes with big consequences — from altered mood and impaired memory to headaches and constipation.

How to know when you are dehydrated

Be sure to hydrate before a strenuous workout. — Photo by MM Productions/Corbis

According to the lead author of one study, older people are more susceptible to dehydration. That's because, after age 50 or so, our ability to recognize thirst decreases and our kidneys lose some of their precision in regulating the body's water supply, says Lawrence Armstrong, a researcher with the University of Connecticut Human Performance Laboratory. Some common medications add to the problem, including diuretics (taken for high blood pressure), antihistamines and certain psychiatric drugs. Finally, people who start drinking less because of bladder control problems are at even higher risk.

It's a myth that you only get dehydrated from profuse sweating, says Michael Bergeron, executive director of the National Institute for Athletic Health & Performance at Sanford USD Medical Center in Sioux Falls, S.D. Someone who tries to avoid liquids, he adds, "could get dehydrated even while spending most of the day sitting still in an air-conditioned room."

How to know if you're not drinking enough.
One way is to check the color of your urine. Anything darker than a pale, straw color may mean you need to drink more. (Note: Vitamin B12 darkens urine's hue.) The Institute of Medicine recommends that women take in about 11 cups of fluid a day, and men take in about 15 cups, but that includes liquids found in foods too. Fruits and vegetables add up, since most are at least 80 percent water by weight. A cup of watermelon, for example, has about half a cup of water. Your simplest safeguard, says Armstrong, is drinking eight 8-ounce glasses of water a day.

Still, with the smothering heat, it’s critical to understand the signs and symptoms of dehydration.

Muscle cramps. These don't just happen to young marathon runners. "I see active people in their 70s regularly engaging in activities like playing tennis or mowing the lawn suffering from the same muscle cramping as those doing higher intensity exercise," Bergeron says. The reason: Older adults, especially if they are fit, sweat more than they realize. Your body only has so much fluid to go around and when you sweat it prioritizes, keeping the fluid where it's most vital — the main circulatory system, the brain and other vital organs. "When the nerves that connect to the muscles aren't surrounded by as much water and sodium as they need," they become hypersensitive, causing the muscles to involuntarily contract or spasm, Bergeron says.

Fortunately muscle cramping is often easy to prevent. If you know you're going to have an active day in the heat, be sure to have a sports drink with sodium, either the night before or with breakfast. That's in addition to the recommended eight glasses of water throughout the day. People with high blood pressure don't have to worry about adding this extra salt to their diet. The added sodium in the drink just replenishes the amount they'll be losing, says Bergeron.

Next: Can dehydration impair your memory? »

Dizziness. That unsteady feeling occurs most often during or right after strenuous physical activity, when your body is directing a lot of blood to muscles."Suddenly, your body doesn't have the capacity to get enough blood flow to the brain. At the same time, you're exerting yourself and that increases your body temperature and breathing rate, both of which cause the blood vessels in your brain to dilate," says Bergeron, leading to a dizzy spell.

Headaches. "Any change that alters the body's natural balance, such as dehydration, creates stress and that can trigger a headache. In fact, a headache is your body's alarm system. It's telling you your physiological equilibrium is off," says Juline Bryson, M.D., a neurologist with the Headache Institute at St. Luke's-Roosevelt Hospital Center in Manhattan. Also, when you're liquid-deprived, say headache experts, the blood becomes more concentrated, which causes inflammatory proteins circulating in the blood to irritate nerves surrounding the brain.

Impaired memory and concentration. While the reasons for these symptoms aren't yet clear, researchers at the University of Connecticut Human Performance Laboratory note that dehydration causes changes in electrolyte balances in the blood, which directly affect parts of the mind responsible for reasoning. Changes in electrolyte levels also can alter brain levels of serotonin, which influences mood.

Constipation. When we're dehydrated, the colon redirects fluid into the bloodstream. This leaves us with a harder stool that's harder to pass, says Nadya Swedan, M.D., a Manhattan-based rehabilitation specialist in private practice. And ironically, if you're dehydrated, you only worsen the situation if you try to counter constipation by eating high fiber foods like fruits, vegetables and whole grains. Dietary fiber stimulates the wave-like contractions that move food through the digestive tract, but only in the presence of the right amount of fluid. "If you're not drinking enough, these high fiber foods just sit in your gut," Swedan says.

Holly St. Lifer writes about health and fitness. She also teaches journalism at New York University.

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