Water seems to come with the rule of eight: Drink eight 8-ounce glasses per day for good health, according to popular and much-repeated advice.
But is that really the case? Here’s what to know about how much water you really need:
What are the health benefits of water?
“There’s a pretty long list of reasons why maintaining proper hydration status is important for maintaining someone’s health,” Brenda Davy, a professor in the department of human nutrition, foods and exercise at Virginia Tech, tells TODAY.com.
Davy, who is also a registered dietitian nutritionist, studies how water and beverage intake affects health. Proper hydration has been linked to good cognitive function, optimal energy levels, weight control, and a lower risk urinary tract infections and kidney stones, she says.
Water also helps your body keep a normal temperature, lubricate joints, and get rid of wastes through urine, perspiration and bowel movements, according to the Centers for Disease Control and Prevention.
Adults who aren’t getting enough fluids — as measured by higher sodium levels in the blood — may be more likely to die younger, a National Institutes of Health study published in 2023 found.
“The results suggest that proper hydration may slow down aging and prolong a disease-free life,” said Natalia Dmitrieva, a study author and researcher at the National Heart, Lung and Blood Institute, in a statement.
But the study doesn’t prove that drinking more water will prevent chronic disease, experts told NBC News. The relationship between drinking fluids and age-related chronic diseases remains “highly speculative,” said Dr. Lawrence Appel, director of the Welch Center for Prevention, Epidemiology and Clinical Research at Johns Hopkins University.
What is the recommended water intake?
There is no recommendation from the CDC for how much plain water everyone should drink every day. But when it comes to total daily fluid intake from a variety of beverages and foods, the agency points to recommendations from the National Academies of Medicine.
It suggests women drink 9 cups of beverages, including water, per day, while men should drink 13 cups of beverages, including water.
Those recommendations take into account that we get about 20% to 30% of our daily fluid needs from foods such as fruits and vegetables.
There is some research from the 1930s and 1940s to support the eight 8-ounce glasses per day rule, though it is only a “rough guideline,” Davy says.
But a 2022 study published in Science casts doubt on whether people need to drink that much water.
“Most people will find that they don’t need eight glasses per day,” Herman Pontzer, co-author of the paper and a professor of evolutionary anthropology at Duke University, tells TODAY.com.
“Our study shows that ‘one size fits all’ doesn’t work for hydration. People should listen to their bodies and drink when they’re thirsty. Bigger people generally need more water. Athletes or those who work in physically demanding jobs need more water, as do people in hot environments.”
You may also need to drink more if you are running a fever, or suffering from diarrhea or vomiting.
There’s no harm to drinking eight glasses of water per day if you want to do it, but no clear benefit either, Pontzer notes. But Davy considers that amount a good general guideline: “I don’t see a big problem with encouraging individuals to drink 8 cups of water a day,” she says.
Does it need to be water?
Fluid intake doesn’t have to mean just water, but “many of us would recommend water as the ideal beverage for hydration because it does not contain calories,” Davy says.
Beverages like unsweetened coffee or tea are also good ways to maintain adequate fluid intake. Research shows moderate daily coffee drinking — three to six cups per day — isn’t dehydrating.
Can you rely on thirst to know if you’re getting enough fluids?
Our natural thirst sensation mechanisms are probably adequate to help us avoid more severe dehydration, but perhaps not sensitive enough to avoid milder degrees of it, Davy says.
Research shows it’s common for adults to be 1% to 2% below optimal hydration levels, which could potentially impact cognitive function and lead to brain fog, she adds.
It may be a particular problem later in life. Our thirst mechanisms decline as we age, so older adults are more prone to dehydration than younger people, Davy says.
Besides thirst, how else does the body signal dehydration?
Check your urine color. The usual shade is usually straw yellow or a light lemonade color. A darker hue indicates you need to drink more. It’s a pretty simple practical method, but be aware some vitamins and supplements may impact urine color, Davy says.
Also try the pinch test. Simply squeeze the skin on one of your finger knuckles for about 3 seconds, then let go.
If you’re well hydrated, the skin will return to its original position in a couple of seconds. But if you’re dehydrated, the skin loses its elasticity and will stay in a pinched position for a moment.
It is better to drink tap water or bottled water?
“I like to promote tap water as the ideal source of water because it is accessible and affordable and safe for most individuals,” Davy says.
“Tap water also has the advantage of being fluoridated in most communities, which is important for good dental health.”
How does water intake impact weight?
Davy and her colleagues have done studies showing that drinking about 2 cups of water before eating a meal may be a helpful weight management strategy for middle-aged people and older adults.
It might help promote feelings of fullness so people eat less and drink fewer calorie-containing beverages.