Dehydration can impact more than just your physical health, a new study finds. Researchers found that at a "mild to moderate" level of dehydration—the loss of fluid equal to 2% of their body weight—athletes experienced problems with attention and decision-making, Reuters reports. And those problems got worse as the level of dehydration got worse. In particular, tasks that require motor coordination and executive function suffered; the latter includes tasks like map reading, mental math, and proofreading, among others. "We’ve known that physical performance suffers at a threshold of 2% of body mass ... 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," says study co-author Mindy Millard-Stafford.
Earlier research on how dehydration impacts cognitive functioning utilized small numbers of subjects and yielded mixed results; this study is a meta-analysis, combining data from 33 studies involving a total of 413 adult subjects. It confirms what researchers long believed, according to a professor who was not involved with the study: "The more dehydrated you are the less sharp you are." So how easy is it to lose 2% of your body weight in fluid? Another professor tells NPR hiking at moderate intensity for just one hour during the summer heat could get a person there or close to it; running hard in the heat could get you there in just 30 minutes. "Most people don't realize how high their sweat rate is in the heat," he says. Fatigue, muscle weakness, decreased urine output or dark-gold urine, and confusion are all signs you need to drink more fluid. The current recommendation is that women take in about 91 ounces of water a day and men 125 ounces, though that includes all sources (including food and beverages other than water).