Keep reminding clients to drink plenty of fluids. New research shows that cognitive abilities—attention, coordination, complex problem solving and reaction time—begin to decline with as little as 1% loss of body mass from dehydration, with more severe impairments showing up at 2%. Dehydration affects attention first and with more severity than other cognitive abilities.

Principal investigator Mindy L. Millard-Stafford, PhD, director of the exercise physiology laboratory at Georgia Institute of Technology, Atlanta, said that cognitive abilities affected in the study included “maintaining focus in a long meeting, driving a car [and doing] a monotonous job in a hot factory that requires you to stay alert.” She added that “higher-order functions like doing math or applying logic also dropped off.”

Researchers based their findings on a review and meta-analysis of 33 studies with a total of 413 participants. “There’s already a lot of quantitative documentation that if you lose 2% in water, it affects physical abilities like muscle endurance or sports tasks and your ability to regulate body temperature,” said Millard-Stafford. “We wanted to see if that was similar for cognitive function.”