It’s easy to know when you’re really dehydrated—your mouth is parched, you’re likely overheated and all you can think about is chugging a giant glass of ice-cold water.

But knowing when you’re mildly dehydrated is much harder, because the signs aren’t always as apparent.

And, it’s far more common for people to be mildly dehydrated than to be in dire straits.

The problem is, even running just a little bit low on fluids can mess with our brains, according to two new studies—one on men and one on women.

Both studies included healthy college-aged men and women who took part in cognitive evaluations over the course of a few weeks in order to compare scores from their well-hydrated and dehydrated states. And it’s amazing what the researchers found…


Researchers wanted to make sure that all subjects were similarly hydrated at the start of the experiments. So they had each participant drink eight extra ounces of water (beyond whatever their individual normal intake was) both the night before and the morning of testing.

During the “dehydrated” experiment, participants walked on treadmills in a warm room to produce water loss, which was measured by weighing them and recording urine output. During rest periods, the subjects were not allowed to drink any fluids (except for about two ounces of water in the middle of the experiment to wet their palates). After this routine, participants had lost about 1.5% of their starting water volume—the definition of mild dehydration.

During the “hydrated” experiment, participants walked on treadmills for the same amount of time, except that during rest periods researchers gave them mineral water—the exact amount that each individual had lost through urine and sweat during the exercise.

In both their dehydrated and well-hydrated states, before and after each treadmill experiment, participants took the same series of tests measuring cognitive factors such as concentration, vigilance (detecting stimuli), visual reaction time, memory, learning, logical reasoning and perceived difficulty of the task. They also reported their moods and any physical problems, such as fatigue and headaches.

Results: In women, compared to when they were hydrated, the mild dehydration caused them to have trouble concentrating and to perceive the mental tasks as more difficult, though they didn’t score any worse on the mental tasks. The mild dehydration also caused the women fatigue and headaches. In men, compared to when they were hydrated, the mild dehydration caused them to not score as well on mental tasks, especially tasks involving vigilance and working memory. The mild dehydration did not cause the men fatigue or headaches. Researchers reported that differences in hormones may be the reason that men and women experienced different effects.

To learn more about the results, I called Lawrence Armstrong, PhD, a professor of kinesiology at the University of Connecticut in Storrs, who led both studies.


Why would mild dehydration produce such pronounced effects? Dr. Armstrong offered an intriguing theory. It’s possible that the billions of neurons in our brains detect mild dehydration early and negatively impact those bodily functions listed above as a warning to us: We need to get water before more dire consequences occur!

It’s really important to pay attention not just to thirst, but also to some of those other more subtle signs mentioned above, said Dr. Armstrong, such as fatigue, headache, and difficulty concentrating, remembering or learning. “If you feel any of those things, think about how much you’ve had to drink over the last 24 to 48 hours,” he said.

As a rule of thumb, Dr. Armstrong advises drinking eight cups of water each day to stay properly hydrated. If you exercise for under an hour, drink an extra 1.5 to 2.5 cups of water, advises the Mayo Clinic—and if you exercise for over an hour or if you sweat intensely, drink even more water than that. For every glass of alcohol that you drink, have an extra cup of water. Of course, keep an eye on your urine color—dark yellow or tan means that you need more to drink. Pale yellow is ideal.

Remember, it certainly doesn’t require running a marathon to run low on fluids. Mild dehydration can occur even when we’re doing mundane tasks —such as eating or watching TV—because we constantly lose water through our skin and during the simple act of breathing, Dr. Armstrong said. What about soft drinks, juice drinks and the many other beverages available? Water is the simplest and most natural fluid—so always keep some nearby, and sip away!