The tests, conducted at the University of Connecticut's Human Performance Laboratory, showed that it didn't matter if a person had just walked for 40 minutes on a treadmill or was sitting at rest, the adverse effects from mild dehydration were the same. Mild dehydration is defined as an approximately 1.5 percent loss in normal water volume in the body.
Lawrence E. Armstrong, one of the studies' lead scientists, said that the test results affirm the importance of staying properly hydrated at all times and not just during exercise, extreme heat, or exertion.
"Our thirst sensation doesn't really appear until we are 1 percent or 2 percent dehydrated. By then dehydration is already setting in and starting to impact how our mind and body perform," Armstrong said.
"Dehydration affects all people, and staying properly hydrated is just as important for those who work all day at a computer as it is for marathon runners, who can lose up to 8 percent of their body weight as water when they compete," he said.
Separate groups of young women and men were tested. Twenty-five women took part in one study. Their average age was 23. The men's group consisted of 26 men with an average age of 20. All of the participants were healthy, active individuals, who were neither high-performance athletes nor sedentary - typically exercising for 30 to 60 minutes per day.
Each participant took part in three evaluations that were separated by 28 days. All of the participants walked on a treadmill to induce dehydration, and all of the subjects were hydrated the evening before the evaluations commenced.
As part of the evaluation, the subjects were put through a battery of cognitive tests that measured vigilance, concentration, reaction time, learning, memory, and reasoning. The results were compared against a separate series of tests when the individuals were not dehydrated.
According to one of the studies, in the tests involving the young women, mild dehydration caused headaches, fatigue, and difficulty concentrating. The female subjects also perceived tasks as more difficult when slightly dehydrated, although there was no substantive reduction in their cognitive abilities.
In the tests involving the young men, mild dehydration caused some difficulty with mental tasks, particularly in the areas of vigilance and working memory, according to the results of the second UConn study.
While the young men also experienced fatigue, tension, and anxiety when mildly dehydrated, adverse changes in mood and symptoms were "substantially greater in females than in males, both at rest and during exercise," according to the study.
"Even mild dehydration that can occur during the course of our ordinary daily activities can degrade how we are feeling - especially for women, who appear to be more susceptible to the adverse effects of low levels of dehydration than men," Harris Lieberman, one of the studies' co-authors, said.
"In both sexes these adverse mood changes may limit the motivation required to engage in even moderate aerobic exercise. Mild dehydration may also interfere with other daily activities, even when there is no physical demand component present," Lieberman said.
Why women and men are so adversely affected by mild dehydration is unclear, and more research is necessary. But other research has shown that neurons in the brain detect dehydration and may signal other parts of the brain, regulating mood when dehydration occurs. This process could be part of an ancient warning system protecting humans from more dire consequences, and alerting them to the need for water to survive.
In order to stay properly hydrated, experts like Armstrong recommend that individuals drink eight, 8-ounce glasses of water a day, which is approximately equivalent to about 2 liters of water.
People can check their hydration status by monitoring the colour of their urine. Urine should be a very pale yellow in individuals who are properly hydrated. Urine that is dark yellow or tan in colour indicates greater dehydration. Proper hydration is particularly important for high-risk groups, such as the elderly, people with diabetes, and children.
The study conducted on women has been published in The Journal of Nutrition, and the study on men has been published in the British Journal of Nutrition.