Severe dehydration can result in confusion and delirium, but little is known about the effect of mild dehydration on cognitive function.

To find out, researchers conducted this small study to investigate the effects of dehydration on cognitive performance of 25 young women.

The women took part in three daylong sessions. In one, three 40-minute bouts of treadmill walking were used to induce dehydration, in the second, a diuretic was given in addition to the treadmill walking, while in the third, the women were adequately hydrated when they did the three treadmill tests.

During and after each bout of exercise the women took a series of cognitive tests.

Fluids lost during the two dehydration sessions were not replaced, whereas they were replaced in the adequate hydration session.

Interestingly, the women could not distinguish between being dehydrated and adequately hydrated.

The researchers combined the results from the exercise only and exercise plus diuretic sessions and compared those results to results from the adequate hydration session.

The average degree of dehydration was a 1.36% decrease in body mass. When the women were dehydrated they reported less vigor, more fatigue, more mood disturbances, lower concentration, increased perception of task difficulty, and greater headache severity both when they were at rest and while exercising. But many other aspects of cognitive function were not affected.