Jan. 19, 2023 – Water is the key to human life. It helps cells survive, it lubricates our joints and helps our metabolism, breathing, waste removal, and temperature regulation.
We can’t survive without it for longer than 3 days. And while scientists, doctors, public health authorities, dietitians, and nutritionists can agree on its importance, one critical question remains: How much water should we be drinking?
For years, we’ve heard that it would be best if humans had at least eight 8-ounce glasses of water daily. Recently, two studies, published only months apart, resulted in a barrage of headlines about daily water intake, only furthering confusion.
- “Why you don’t need to drink 8 cups of water a day” – The Washington Post, Dec. 6
- “8 cups of water a day could be too much” -- NPR, Dec. 7
- “Drink more water: Staying well-hydrated linked to developing fewer chronic conditions, study says.” – USA Today, Jan. 3
Recommendations to drink eight 8-ounce glasses (or 64 ounces, or 1 liter) have been misinterpreted, says Dale Schoeller, PhD, a professor emeritus of nutritional sciences at the University of Wisconsin-Madison, and co-author of one of the studies fueling the latest questions.
“The scientific recommendation was based on all water intake, water and food, you know – an apple contains so much water; soup, so much water per gram, things like that," he says. “For the most part, there’s no accurate and precise method of water intake.”
Schoeller is referring to a 1945 study published by the U.S. Food and Nutrition Board that continues to provide fodder for research and arguments. In the decades since, researchers have continued to grapple over useful ways to calculate water needs, which, according to Schoeller's findings, change regularly, depending on things like age, body size and composition, the amount of energy you put out each day, physical activity, as well as climate and temperature. The total movement of water through the body, or how much water gets into and out of our body daily, is known as water turnover.
“Water turnover is closely related to the concept of water balance,” says Natalia Dmitrieva, PhD, a research scientist in the Laboratory of Cardiovascular Regenerative Medicine at the National Heart, Lung, and Blood Institute in Bethesda, MD, and co-author of the second study that explores hydration levels and their relationship to healthy aging.
“To maintain water balance, our body has to change water output depending on water intake,” she says. “When we don’t drink for a long time (or drink less fluid), the volume of our urine decreases, and it becomes much more concentrated because the kidneys conserve water to match water intake.”
Dmitrieva’s study showed that an important sign of a lack of water balance is an increase in sodium in your blood, which, in turn, appears to promote the risk of chronic conditions and advance the aging process, including dying at a younger age.
Calculate Your Daily Water Needs
Depending who you ask, there is no hard and fast rule for determining how much water to drink every day. Nor do people appear to follow any specific guidance; an unofficial Facebook poll on water intake on my personal page generated answers ranging from ”80 to 96 ounces per day” to “about 6 ounces an hour during the more active part of the day.” Several people said that they had no idea how much they drank.
These responses are hardly surprising. In fact, data demonstrates that fluid intake is often the result of habit or total targets rather than daily need. And as noted, daily needs change on a regular basis, depending on your body’s water turnover rate.
So how should you decide when and how much water to drink every day?
“If we drink low amounts of water, we have an increase in secretion of a hormone called arginine vasopressin from the brain that acts on the kidney to reabsorb as much water as possible; that coincides with an increased perception of thirst. People can use the perception of thirst to help guide them to drink more fluids,” says William Adams, PhD, a hydration expert and an adjunct assistant professor of kinesiology at the University of North Carolina-Greensboro.
Being mindful of what our bodies may be telling us is key, albeit somewhat flawed.
“Our thirst tells us a lot, but by the time we’re thirsty, we’re already 1% to 2% dehydrated, says Melissa Majumdar, a bariatric coordinator at Emory University Hospital Midtown in Atlanta and a national spokesperson for the Academy of Nutrition and Dietetics.
This is a key reason why Jodi Stookey, a nutrition epidemiologist and water and hydration researcher based in San Francisco, suggests that a good rule of thumb is to “err on the side of a little more water than we feel thirsty for, for the day.”
A more reliable sign is urine color. “The darker the urine color, the more concentrated it is,” says Adams. “So, if you wake up in the morning and your urine is super dark (like apple juice color), you probably did a bad job the day before consuming fluids and should make an effort to address it going forward (that day).”
The urine approach is evidence-based and came from research done on athletes in the mid-1990s. The goal? “Aim for a lemonade color,” says Majumdar.
Pacing is also important.
“To drink, you have to pace yourself throughout the day, or you’re inevitably not going to be getting enough,” says Majumdar. “I always tell my patients to have a water bottle with them (or by them on their desk), like it’s a third arm.”
Finally, what should you be drinking?
Stookey says that the best choice is plain clear water if possible.
“When you drink something else – like orange juice or Coca-Cola or milk – it’s more concentrated in the blood, so your own body water has to go into the gut to dilute it in order to absorb those things. Clean water is very different than the other sources,” she says.
Pairing and practice can also help with daily water consumption. So if you are a coffee drinker, perhaps pair a glass of water with it to help build better habits.
And for people who don’t care for plain water, most experts say that carbonated water is a good choice; adding fruit to it adds flavor, helps quench thirst, properly dilutes urine, and points hydration levels in the right direction.