Researchers found that athletes who lost fluid equal to 2 percent their weight took a hit to their cognition. Even this mild to moderate level of dehydration- the loss of 2 pounds for someone who weighs 100 pounds and four pounds for someone weighing 200 - led to attention problems and impaired decision making, according to the report in Medicine & Science in Sports & Exercise.

In particular, dehydration led to impairment in tasks requiring attention, motor coordination, and so-called executive function, which includes things like map recognition, grammatical reasoning, mental math, and proofreading, for example.

“We’ve known that physical performance suffers at a threshold of 2 percent of body mass, particularly when it’s from exercise in a warm environment,” said study coauthor Mindy Millard-Stafford, a professor in the school of biological sciences and director of the physiology lab at the Georgia Institute of Technology.

“So the question was, what happens in the brain with the same amount of loss, which is pretty common with people who are active or work outside in the heat. Just like a muscle cell needs water, so do the cells in our brain.”

While the effects weren’t huge at 2 percent, they increased with increasing dehydration, Millard-Stafford said.

The new study isn’t the first to look at the impact of dehydration on cognition. But earlier research yielded mixed findings, possibly because studies were based on small numbers of subjects. Millard-Stafford decided to get around that pitfall by performing a meta-analysis, an approach that combines data from many smaller studies.

After scouring the medical literature, the researchers found 33 studies involving a total of 413 adults. Participants lost fluids amounting to 1 to 6 percent of their body mass either through exercise alone, exercise coupled with heat, heat alone or fluid restriction.

The study may be particularly timely as much of the country suffers through a heat wave. And it may remind weekend warriors as well as more dedicated athletes of the importance of staying hydrated.

“I think this reinforces something we thought was true,” said Dr. Ronald Roth, a professor of emergency medicine at the University of Pittsburgh School of Medicine, medical director of the Dick’s Sporting Goods-Pittsburgh Marathon and one of the team physicians for the Pittsburgh Steelers. “The big picture here is that the more dehydrated you are the less sharp you are. And your decision-making abilities get lost sooner than later.”

Before this new research came along, a person might have happened upon one of the earlier studies that found no cognitive effects from dehydration and thought that dehydration wouldn’t harm their thinking, Roth said.