Water being poured into a glass.

During summer’s heat, it’s easy to become dehydrated without realizing it. And dehydration, which occurs when you lose more water via sweat and urine than you’ve taken in, can be especially dangerous for older adults.

“How much water you have affects every body system,” says Jodi Stookey, Ph.D., an epidemiologist in San Francisco.

A lack of sufficient fluid in the body can temporarily cause confusion and put you at risk for falls. When severe, dehydration can lead to a rapid or irregular heart rate, low blood pressure, fainting, and even death.

And dehydration—along with certain chronic illnesses, medications you may be taking, and lack of access to air conditioning—can raise your risk of heatstroke, a condition in which your body becomes very overheated and loses its ability to regulate its temperature. According to an analysis out this week in the New England Journal of Medicine, heatstroke in older adults may be fatal in more than half of cases.

Staying well-hydrated becomes more difficult with age because your sense of thirst tends to diminish with time. Diuretics, often prescribed for high blood pressure and heart failure, can exaggerate water loss.

But you can protect yourself. Here’s how to stay hydrated during the warm months and year-round.

4 Steps for Staying Hydrated

Despite the widespread myth that everyone should consume eight 8-ounce glasses of water each day, there’s really no overarching rule about how much you need to drink to stay hydrated. What’s appropriate can vary a good bit from person to person.

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Generally speaking, the heavier, taller, and more active you are (and the hotter and more humid the weather is), the more fluids you need to take in to cover your losses. To make sure you get enough:

Drink before you feel parched. To make up for that reduced sense of thirst, sip preemptively. By the time you actually feel thirsty, you might be mildly dehydrated.

Sip small amounts throughout the day. If you find it difficult to consume a full glass of water all at once, drink a bit at a time—but do this frequently. Carrying a water bottle with you at all times can help remind you to drink.

Know that other beverages and foods count, too. In fact, all beverages (other than alcoholic drinks) will hydrate you.

That includes caffeinated drinks, even though coffee and tea are mild diuretics and can cause you to urinate more often. But these drinks will add more to your liquid stores than you’ll lose from extra urination, says Janet Mentes, Ph.D., a professor at the UCLA School of Nursing.

Soup, fruits, and vegetables are also good sources of liquid.

Consider your health. Ask your doctor whether medical conditions you have or medications you take affect your hydration needs.

And keep in mind that some health conditions, such as kidney disease and congestive heart failure, may make it dangerous to take in too much fluid. In these cases, your doctor can show you how to stay hydrated safely.

Know the Signs

Dehydration can be challenging to detect as we age because classic signs, such as dry mouth, thirst, fatigue, and skin that fails to spring back quickly when pinched, can also be caused by other factors.

In fact, a 2015 review of research by the independent Cochrane Collaboration found that there was no single reliable test for dehydration.

The color of your urine can sometimes be a clue. In general, healthy urine is the shade of pale straw. The darker your urine, the less hydrated you may be.

But aspirin, multivitamins, and certain fruits and vegetables can also affect the shade of your urine.

If you suspect you might be dehydrated based on your urine’s color and/or the other signs mentioned, try drinking two to three full glasses of water during the course of an hour or two.

If you still have symptoms of dehydration or don’t urinate within 4 hours, it’s wise to contact your doctor.

You should also know the signs of heatstroke, which is a medical emergency. In older adults, according to the NEJM review, early symptoms of heatstroke include confusion, dizziness, weakness, agitation, and other behavioral changes, as well as slurred speech, nausea, and vomiting.

If you suspect it in yourself or someone else, call 911 right away. Then begin trying to cool down the person. Fan the person or get him into air conditioning and apply ice or cold packs.

To prevent heatstroke to begin with, try to keep your body at a normal temperature. Muscle cramps (sometimes called heat cramps) and heat exhaustion, which involves symptoms such as tiredness, weakness, dizziness, heavy sweating, and headache, are signs that you should stop any strenuous activity, cool down, and sip some water.

Stay in air-conditioned spaces, take frequent cool showers or baths, and limit physical activity during heat waves or the hottest parts of the day.

3 Surprising Causes

Taking in too little liquid is an important factor in dehydration, but the following play a role as well:

  • Infections that cause diarrhea, vomiting, excessive sweating, and fever. Tara Cortes, Ph.D., executive director of the Hartford Institute for Geriatric Nursing at New York University, suggests calling a doctor if you vomit repeatedly or have a fever of more than 101° F for more than a day or diarrhea for more than two days.
  • Medication that causes the kidneys to produce more urine, such as diuretics. Some over-the-counter drugs, such as laxatives, may also cause water loss.
  • Health conditions, such as poorly controlled diabetes, that lead to excessive water loss. A 2016 study in the Annals of Family Medicine found that obese people were more likely to be inadequately hydrated as well. Having dementia, Parkinson’s disease, or a stroke can also increase the chance of dehydration.
Editor's Note: A version of this article also appeared in the June 2017 issue of Consumer Reports on Health.