Was it hard to concentrate during that long meeting? Does the crossword seem a little tougher? You could be mildly dehydrated.
A growing body of evidence finds that being just a little dehydrated is tied to a range of subtle effects — from mood changes to 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 this month, based on 33 studies.
How long does it take to become mildly dehydrated in the summer heat? Not long at all, studies show, especially when you exercise 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. If you’re going hard during a run, you can reach that level of dehydration in about 30 minutes.