This article originally appeared on RealSimple.com.
Despite being told for years that we should all be gulping down eight 8-ounce glasses a day, the actual amount of fluids we need varies from person to person. Now, a new study has revealed what happens in our brains, and our bodies, when we’ve reached our hydration limit—and why it might be dangerous to ignore the feeling that you’ve had enough.
The research, published in the Proceedings of the National Academy of Sciences in the United States of America, found that people had a harder time swallowing after consuming excess amounts of water. This appears to be a protective response to prevent over-drinking, the authors say, which can lead to a potentially fatal condition called hyponatremia, also known as water toxicity.
For the study, the researchers put 20 people in functional MRI (fMRI) scanners and asked them to rate the effort level required to swallow small amounts of water under two different conditions—after exercising, when they were thirsty; and later, after they’d already drank about a liter of water.
The participants rated swallowing in the second scenario three times more difficult than in the first. This was true for both plain water and for water sweetened with sugar.
This showed that they had to “overcome some sort of resistance,” co-author Michael Farrell, Ph.D., associate professor at Monash University in Australia, said in a press release. “This was compatible with our notion that the swallowing reflex becomes inhibited once enough water has been drunk.”
This “swallowing inhibition” seems to help regulate water intake and maintain fluid levels in the human body, the authors wrote, but it’s not foolproof. After all, the participants were still able to swallow, even though it was harder than usual.
In fact, fMRI scans showed that the right prefrontal areas of participants’ brains were much more active when they were trying to swallow after overdrinking—suggesting that the frontal cortex steps in to override the inhibition and allow swallowing on command.
When the body is flooded with water, sodium levels can become abnormally low—a condition called hyponatremia that can lead to lethargy, nausea, seizures, and even death. People who lose a lot of sodium through sweat and don’t replenish their levels (with salt or an electrolyte beverage) are especially at risk.
Thankfully, the study participants hadn’t consumed huge amounts of water, and weren’t in any actual danger. But there are times when people may have more water than is safe, say the authors, and may disregard signals from their brains to stop drinking.
“There have been cases when athletes in marathons were told to load up with water and died, in certain circumstances, because they slavishly followed these recommendations and drank far in excess of need,” Farrell said.
Of course, not drinking enough water is also a common problem in athletes, and in non-athletes as well. Mild dehydration may not lead directly to serious health problems, but it has been linked to low energy levels, headaches, constipation, and even obesity.
Farrell pointed out that elderly people, especially often don’t drink enough water and should watch their intake of fluids.
So how much fluid should you get every day? The Institute of Medicine recommends that men aim for 125 ounces and women aim for 91 ounces a day, but that includes water from foods and other beverages—like juice, tea, and yes, even coffee—as well. That number can also vary based on your age, weight, other health conditions, and how active (and how sweaty) you are.
In short, says Farrell, don’t try to force anything. “Just drink according to thirst rather than an elaborate schedule,” he said. “If we just do what our body demands us to we’ll probably get it right.”