Soaked: The Myths and Facts of Hydration
Popular culture is awash in misconceptions about our daily fluid requirements
This article is part of a Wise & Well Special Report: Extreme Heat and Human Health.
When I was an adolescent I completed a 10-hour, 15-mile hike up and down Yosemite’s Half Dome more than once. Each time, I drank some water when I set out, and again when I returned. And that was all. I am not endorsing that practice, but I would like to point out that I am still here, kidneys and all. Today my kids cannot (read: will not) leave home for more than a few minutes without their water bottles.
My offspring sometimes ask me how much water they should drink. I tell them about my advanced methodology, which I call my “Total Hydration Internal Regulation Sensing Technique,” or THIRST. That’s right, I tell them. Drink if you are thirsty.
Like much of my priceless wisdom, this is met by eye rolls despite the fact that I am a board-certified physician (as I like to remind them).
“No seriously, Dad. How much?”
Origin of a myth
Conventional wisdom has long held that we need to drink 64 ounces of water a day (that’s half a gallon, or nearly 2 liters). This is often expressed in terms of 8-ounce glasses, giving rise to the “8 x 8” rule.
Where did this number come from?
It all began innocently enough in the 1945 edition of Recommended Daily Allowances put out by the US Food and Nutrition Board (USFNB), which estimated that an adult human requires 1 milliliter (imagine a cube measuring one centimeter, or about one-half of an inch, on each side) of total water for every calorie of energy expended.
Men burn roughly 2,500 calories per day on average, and women burn roughly 2000 calories on average. That would translate to about 2 liters (half a gallon) of total water for women, and about 2.5 liters of total water for men. And so, the logic goes, an average person (whatever that is) should have a total water intake of 2 to 2.5 liters per day.
A lot of water has flowed under the bridge since 1945, as has a lot of scientific research. How has this estimate held up, and what does “total water” mean, anyway?
Water in, water out
Let’s talk science for a moment. A good measure of whether or not you are dehydrated is something called “osmolality.” Osmolality is a measure of the total concentration of electrolytes in your blood. If you lose too much water, your blood gets more concentrated, and your osmolality goes up. If you take in (way) too much water, your blood gets diluted, and your osmolality goes down.
Shifts in osmolality can be very dangerous, particularly for your brain. Luckily (and by “luckily” I mean, “as a result of hundreds of millions of years of fine-tuning through the evolutionary process of natural selection”), your body has an extraordinary ability to maintain a constant osmolality under a wide range of hydration circumstances.
Your body achieves this feat through two mechanisms: thirst, and regulation of urine volume.
The ability of our brains to drive our behavior is as fascinating as it is powerful. Sex, hunger, pain avoidance. And thirst. I remember as a kid — growing up in the arid central valley of California — the feeling of getting a drink after an afternoon of yard work out in the sun. I would turn on the hose, wait for what seemed like an eternity for the stale, 100-degree water to get flushed out, and then gulp down the cool fresh water.
I could not imagine any other beverage tasting so good.
A shift of just 2% in our blood’s osmolality is detectable by the hypothalamus — a region deep in the inner reaches of the brain. The hypothalamus then sends signals to the cortex — the outer wrapper of the brain where decisions are made. Under the influence of the hypothalamus, we become increasingly focused on finding something to drink.
Until we get that drink, the kidneys do a great job of slowing down urine production so our osmolality doesn’t get further out of whack. And if we drink too much, our kidneys turn up the urine production so our osmolality does not over-correct.
The kidneys can create anywhere from almost zero to nearly 1 liter of urine every hour to keep things in balance. (A healthy adult should be making around a liter of urine per day — around 40 milliliters, or just over an ounce, per hour).
For example: The Third National Health and Nutrition Examination Survey (NHANES III) conducted by the US government between 1988 and 1994 collected, among many other things, data on total water consumption as well as blood samples from which blood electrolyte levels could be measured. The bottom 10% of water consumers in the study consumed only 1 liter of total water a day, while the top 10% consumed almost 8 liters of total water per day.
Despite this eight-fold range in water consumption, osmolality varied by only 3% across the 12,000 participants in the survey.
What counts as water?
Note the phrase total water. This includes water in the food we eat (comprising on average between 20% and 50% of our daily intake of water), and water in beverages other than pure water.
We are often told that caffeinated and alcoholic beverages do not count toward the total because they lead to increased urine production, or “diuresis.” But this is not the full story.
“It’s a myth that caffeinated beverages or those containing alcohol are dehydrating because they make you urinate,” says Howard E. LeWine, MD, Internist at Brigham and Women’s Hospital in Boston. “They do, but over the course of the day, the water from these beverages still leads to a net positive contribution to total fluid consumption”
Of course, caffeine and alcohol have health consequences of their own. As in so much of life, moderation is the key.
Will the average person please stand up
My Mom calls. “How do I know if I am getting enough water?” she asks.
It’s hard not to suspect my children put her up to this. I explain to her my THIRST system, then add, “You’ve survived nearly nine decades, Mom. Just keep doing what you are doing.”
On the other hand, she raises a worthwhile point. Age, body size, body fat, altitude, humidity, temperature, and activity level all contribute to how much water our body loses each day. These factors make it all the more difficult to make generalizations about how much to drink.
A major international study published in November of 2022 precisely quantified the water turnover in over 5,000 people from 23 countries. The study, led by Yosuke Yamada, PhD, a researcher with Japan’s National Institute of Biomedical Innovation, largely confirmed prior estimates of average water requirements.
However, the researchers documented that water turnover was highly variable, ranging from 1 liter per day to up to 10 liters per day, largely depending on the factors listed above.
“The current study clearly indicates that one size does not fit all for drinking water guidelines, and the common suggestion that we should drink 8 × 8oz glasses of water per day is not backed by objective evidence,” the study concluded.
What about the container?
Humans have transported water presumably since the dawn of the species. In addition to drinking, water was moved from here to there for irrigation, cooking, and washing. Bottling of water finds its roots at least as far back as the 16th Century. But these efforts were generally not related to maintaining hydration. Rather, it was felt that water from certain locations had medicinal qualities making particular water desirable for export. Indeed, it was the fascination with mineral waters that led to the industrial production of carbonated water in the early 19th Century.
As understanding of germ theory emerged, bottled water also emerged as a strategy for avoiding contaminated water sources and their associated diseases. However, this need for bottled water evaporated in the developed world with widespread application of water chlorination of municipal water supplies — starting first in the UK in 1905 and promptly spreading from there.
It is worth noting that while the chlorination of water saves countless lives through the prevention of waterborne disease, chlorine and its byproducts have health risks of their own. These risks, while worthy of ongoing study and mitigation, are nonetheless orders of magnitude smaller than the risks posed by water contaminated with infectious organisms.
Modern municipal water supplies have also been associated with public health disasters of their own — including the recent water crisis in Flint, Michigan, which culminated in widespread lead poisoning and other health concerns.
While often touted for health benefits, water bottling is itself not a benign process. It is estimated that in the United States alone, 17 million barrels of oil are consumed in the manufacture of plastic water bottles each year. While this is equivalent to only one day of automobile traffic, it is still far greater than the resource consumption required for tap water.
In addition, of course, many if not most of those bottles then end up in landfills, along trails in otherwise pristine forests, or the ocean.
With the above caveats in mind, for most of us the healthiest way to consume water — for both our bodies and the environment — is drinking tap water from a reusable container, possibly filtered to remove chlorine and its byproducts.
But seriously, how much should I drink?
Due to the body’s spectacular ability to regulate itself, it is impossible to define how much we should drink. Thus, the US Food and Nutrition Board (USFNB) does not define a “Recommended Dietary Allowance” for water (as it does for, say, calcium or carbohydrates). Rather, it simply defines an “adequate intake” for water, which is nothing more than the median of what people report drinking in surveys like NHANES.
According to the USFNB’s latest report on water and electrolyte intake, the adequate intake for total water intake for young men and women (ages 19 to 30 years) is 3.7 liters and 2.7 liters per day, respectively. Adequate intake is slightly higher (and thirst drive lower) at older ages.
Again, “adequate intake” is a misnomer. These are not thresholds for “adequate” intake, which would imply drinking less water would be “inadequate.” They are just the averages of what people consume.
“As with adequate intakes for other nutrients, for a healthy person, daily consumption below the AI may not confer additional risk because a wide range of intakes is compatible with normal hydration,” says the USFNB report.
I should emphasize the phrase “healthy person” above. Certain health conditions may affect how much water you should drink. And despite my cajoling of my mother, it is true that older individuals can have decreased thirst drive and often need to be more attentive to water intake. If you are not sure whether or not you have specific requirements, always check with your doctor before changing your health habits.
In the face of all this variability, the logical conclusion would be that rather than aim at some fixed (and arbitrary) water consumption goal, healthy individuals below 65 years of age should just follow our thirst. That approach has served our species well for hundreds of thousands of years.
Now if you will excuse me, I need to grab a huge mug of coffee and go tell my kids, “I told you so.”
Correction: The original story was edited to add information about decreased thirst drive in older individuals, in response to some helpful reader comments.
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