The old ‘eight glasses a day’ rule isn’t based on science, and experts say some of us may be overdoing it

It’s 10:30 in the morning, and I’m already peeing for the fourth time today.

I peed when I woke up at 6:30 a.m. Then I drank a glass of water, had my habitual three cups of coffee, and peed a couple more times over the course of the next few hours. Now I’m peeing again.

My urine is clear. (It’s almost always clear.) If I drink another glass or two of water between now and lunchtime, and then five more glasses this afternoon and evening, I’ll have hit the eight-glasses-per-day target that everyone seems to think is the key to health and hydration. I’ll probably pee a dozen more times today.

Until recently, I thought I was doing everything right. Water, as the saying goes, is the essence of life. You need it or you die. And if there’s one thing nutrition experts seem to agree on, it’s that dehydration is bad and drinking lots of water is good.

But then I read this 2019 study in the journal Nutrients, which discusses the potential risks of overhydration. Its authors argue that drinking too much water is not only wasteful, but that over time it could lead to bladder distention, kidney dysfunction, or other problems. It cites case reports of otherwise healthy people who drank so much water that they developed swollen kidneys or ruptured urinary tracts.

The research on longevity and mortality is oddly silent on the subject of water consumption, and experts have called the search for a universal daily water requirement “elusive.”

“Urine is a waste product that helps your body balance its levels of sodium and other electrolytes,” says Tamara Hew-Butler, PhD, first author of that study and an associate professor of exercise physiology at Wayne State University in Detroit, Michigan.

If you’re peeing all the time and your pee is clear, she says, that’s an indication that you’re drinking more water than your body can use, and so you’re forcing it to dump fluids in order to maintain homeostasis. While that’s unlikely to be a problem in the short-term, keeping that up for years or decades could lead to some of the urinary tract problems outlined in her study.

The more I dug through the research, the more it seemed like she had a point.

Just how much H2O do you need to drink each day for optimal health? The research on hydration is surprisingly inconsistent.
There’s the long-standing recommendation — sometimes called the eight-by-eight rule — to drink eight, eight-ounce glasses of water each day. But experts have not found solid scientific evidence to support this advice.

“Saying everyone needs eight glasses of water to be healthy is like saying everyone needs to eat a 2,000-calorie diet,” Hew-Butler says. “I weigh 90 pounds and I spend most of my day sitting inside, but I work with football players who weigh 300 pounds and are exercising all day. Our hydration needs are completely different.”

She says the average person needs to replace roughly two liters (or eight cups) of lost fluid each day. But almost anything you drink or eat is going to contain water, which will offset that daily loss. “Coffee, tea, soup, fruits, vegetables — all of that counts,” she says. (Some work has found that people get an average of 20% of their daily fluids from food alone.)

After collecting samples from more than 300 college athletes, she and her colleagues found that up to 55% were dehydrated based on their pee, but none were dehydrated according to their blood samples.

Major health organizations also differ widely in their views on fluid requirements.

The European Food Safety Authority advises women and men to consume two liters and 2.5 liters per day, respectively. Meanwhile, the U.S. National Academy of Medicine recommends 2.7 liters for women and 3.5 for men — or roughly 35–40% more than their European counterparts. (And again, both organizations say that all foods and beverages — not just water — count toward these daily totals.)

It seems like the eight-glasses-a-day maxim has persisted in part because nothing compelling has come along to take its place. The research on longevity and mortality is oddly silent on the subject of water consumption, and experts have called the search for a universal daily water requirement “elusive.”

Part of the difficulty in assessing human hydration needs has to do with how dehydration is measured.

Another of Hew-Butler’s studies found that urine-based hydration analyses — the most common type employed in research — often don’t align with more accurate blood-based measurements. After collecting samples from more than 300 college athletes, she and her colleagues found that up to 55% were dehydrated based on their pee, but none were dehydrated according to their blood samples.

There’s been some well-publicized research showing that even mild dehydration can cause fatigue, headaches, moodiness, or cognitive impairments. But I was surprised to find criticisms of this work. A review in the journal Nutrients argued that these sorts of dehydration studies have produced inconsistent results, and that they often ask people to exercise in hot environments, which could induce fatigue or other symptoms for reasons that have nothing to do with dehydration. (I also found that a lot of dehydration studies have been led by researchers affiliated with Danone or other companies that sell bottled water.)

Meanwhile, research in the journal Nutrition Reviews has concluded that overhydration “may not be as benign as is usually assumed.”

There is no doubt that dehydration is dangerous. Our thirst reflexes tend to decline as we age, and dehydration is a common and serious health problem among the elderly — one that can worsen a number of age-related medical conditions.

There’s also no question that drinking water is generally good for you, and that swigging it in place of sugar-sweetened beverages can reduce your risk for metabolic and cardiovascular diseases.

But if, like me, you assumed there’s no downside to drinking lots of water — that going to the bathroom 15 times a day is a sign of proper hydration — the research to date doesn’t endorse this view. It’s possible that by flooding my digestive plumbing with unneeded H20, I may be asking for a swollen bladder, worn-out kidneys, or other urinary tract issues.

Finally, there’s the environmental cost of excessive water consumption.

Water is a precious resource. Energy is needed to purify it so that it’s potable. Many of us prefer our water bottled, and all that plastic adds up; the U.S. goes through 28 billion plastic water bottles a year. “If you’re worried about climate change, why would you drink more water than you need?” Hew-Butler asks.

She recommends drinking water when you feel thirsty. “Listen to your body,” she says. “It will tell you if you’re drinking too little.” If you notice your urine is dark yellow, that’s also a good indicator that you need a drink.

Water is good for you. But like anything else, you may be able to get too much of a good thing.