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Do You Really Need 8 Glasses of Water a Day?

Accumulating research shows that staying hydrated is important for older adults

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Ask any adult how many 8-ounce glasses of water they’re supposed to drink per day and they’ll probably deliver the one-word answer before you’ve had a chance to get the question out: eight.

Not seven. Not nine. Eight.

That’s the number that took hold in 1945 when the U.S. Food and Nutrition Board of the National Research Council made its official recommendation for water intake. Though hidden within a more complex equation, the recommendation translated to roughly 64 ounces a day. And it referred to a person’s total daily water intake — not just pure water but also the amount taken in from foods and other beverages — though it was widely misinterpreted to mean that everybody, everywhere should drink eight 8-ounce glasses of plain water every day.

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Up for debate in the decades since: Do you really need that much?

The answer isn’t so simple.

“There are many factors that affect water needs and hydration status, such as body size, climate, activity and metabolic rate, etc., so water needs may differ from one individual to another and even for the same person under different circumstances,” says Paul Jacques, a senior scientist at the U.S. Department of Agriculture Human Nutrition Research Center for Aging at Tufts University.

Although the eight-glasses rule is an easy recommendation to remember, depending on how it’s interpreted, it might not meet the current recommendations for total water intake (meaning water contained in other beverages counts), which is 13 cups per day for men and nine cups a day for women, Jacques says. If you include water content from food, that number is even higher.

The health-hydration connection  

Though experts may not agree on the precise amount of water we should drink to meet the total daily DRI (or dietary reference intake) for overall water consumption, this much is clear: Staying hydrated is a big deal.

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“You might think that since your body has so much water — more than 50 percent — it shouldn’t make a difference, even if you lose some,” says Stavros Kavouras, a professor of nutrition and director of the Hydration Science Lab at Arizona State University. “However, the data suggest that even a 1 to 2 percent water deficit is enough to lower exercise performance, impair cognitive function and worsen cardiovascular function.”

That’s not all.

new study from the National Institutes of Health (NIH) suggests that adults who aren’t sufficiently hydrated may age faster, face a higher risk of chronic diseases — such as lung disease, diabetes, heart failure and stroke — and be more likely to die younger than those who stay well-hydrated.

The results are based on 30 years’ worth of data collected from more than 11,000 adults in the U.S. who were 45 to 66 years old when the study began. The researchers looked at levels of sodium in the participants’ blood to determine hydration (higher concentrations are a sign that they most likely weren’t consuming enough fluids) and found that those with blood-sodium levels at the higher end of the normal range had aged faster than those at the lower end of the range.

“While more research is needed, the main theory regarding the mechanism by which hydration status might affect health and aging is through elevated levels of antidiuretic hormone,” Jacques says. That’s the hormone “that helps regulate total body water by [controlling] water excretion in the kidneys and by stimulating thirst.” If you’re underhydrated, that hormone is elevated, he says.

Exacerbating the problem: Research shows we tend to drink less as we age. One study published in 2019 in the journal SAGE Open Nursing suggests that up to 40 percent of older adults may be chronically underhydrated. The obvious culprits: Certain medications, such as diuretics, may negatively affect water balance, plus some older adults may intentionally reduce their water intake because of incontinence or other concerns about bladder control. What’s more, the sensation of thirst tends to decline with age.

“When you’re younger, you get thirsty and you get something to drink, but older people in the same situation don’t feel thirst,” says Rosanne Leipzig, M.D., professor of medicine in the Department of Geriatrics and Palliative Medicine at the Icahn School of Medicine at Mount Sinai in New York City and author of Honest Aging. “It’s unclear why, but it has amazing repercussions.”

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Staying hydrated

Generally speaking, eight 8-ounce glasses of water per day, “even though it was not originally developed based on scientific data, is very close to what we need,” Kavouras says.

Of course, lifestyle, climate and overall health play a role, he adds. “You will not have the same water needs if you train five hours a day in the middle of the summer, versus having a fairly sedentary lifestyle in the winter, but eight glasses a day is a good goal for most people.”

How do you know if you’re not properly hydrated? Look for warning signs such as dry mouth, sluggishness and dizziness. Another indication: if you're going to the bathroom two to three times a day (and not every two to three hours) and producing urine that has a strong odor and looks more like apple juice than lemonade.

To help remedy the situation, aim to eat more foods with high water content. For instance, cantaloupe, strawberries, watermelon, lettuce, celery, spinach and cooked squash are all at least 90 percent water; yogurt, apples, grapes, oranges, carrots, pears, pineapples and cooked broccoli are 80 to 89 percent water.

“Hydration is a big deal — especially for older adults,” says Roopa Anmolsingh, M.D., with the Cleveland Clinic Center for Geriatric Medicine. “As a geriatrician, I would advise older people to discuss with their primary care providers what their individualized recommended daily water intake should be.” 

Video: 5 Signs of Dehydration

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