Study: Mild Dehydration Can Negatively Affect Mental Performance

A new study has confirmed that even the mildest of dehydration can send your mental performance off balance. The effects could be as minor as mood changes and more major, like muddled thinking.

“We find that when people are mildly dehydrated they really don’t do as well on tasks that require complex processing or on tasks that require a lot of their attention,” says Mindy Millard-Stafford, director of the Exercise Physiology Laboratory at Georgia Institute of Technology. She published an analysis of the evidence linking dehydration to a decreased mental performance this month. Her findings were based on 33 studies, according to a report by NPR. 

Scientists say it doesn’t take long at all to become dehydrated either, especially if you’re moving around in the heat outdoors. “If I were hiking at moderate intensity for one hour, I could reach about 1.5 percent to 2 percent dehydration,” says Doug Casa, a professor of kinesiology at the University of Connecticut, and CEO of the Korey Stringer Institute. For an average-size person, 2 percent dehydration equates to sweating out about a liter of water. “Most people don’t realize how high their sweat rate is in the heat,” Casa says.

“Most people can’t perceive that they’re 1.5 percent dehydrated,” Casa says. At that rate, the effects of the dehydration are far more subtle.

Though the study was small and funded by PepsiCo, (which also sells bottled water) Nina Stachenfeld of the Yale School of Medicine and the John B. Pierce Laboratory, who led the research, designed the methods and completed the analysis independently. And other scientists say her findings fit with a growing body of independent evidence that points to similar conclusions.

Stachenfeld asked women to complete simple cognitive tasks after limiting their water intake to 6 ounces for the entire day.  This put them in the “mild” state of dehydration. The women were 12% more likely to make mistakes on the tests.  But reaction time did not appear to suffer.

“I absolutely think there could be big implications of having a mild cognitive deficiency with small amounts of dehydration,” Casa says. If you’re a student, for example, a 12 percent increase in errors on a test could matter quite a bit. And whether you’re a pilot, a soldier, a surgeon or a scholar, many daily tasks depend on the ability to be precise and pay attention.

Although water intake varies across the board, typically, people get about 20 percent of the water they need daily from fruits, vegetable, and other food. Women need about 91 ounces of water per day (in total, so about 72.8 ounces less the water taken into the body via ingestion of foods) and men require about 125 ounces of water per day (about 100 ounces after accounting for food water.)

Water needs vary from person to person too. For example, body weight, muscle mass, physical activity, and heat exposure can increase the amount of fluid a person needs.