Recent studies suggest that being even the slightest bit dehydrated manifests in certain physiological effects. These effects can range from subtle to extreme and can include anything from muddled thinking, confusion, to mood changes.
According to the Georgia Institute of Technology director of Exercise Physiology Laboratory, Mindy Millard-Stafford, “We find that when people are mildly dehydrated they really don’t do as well on tasks that require complex processing or on tasks that require a lot of their attention.”
Analyzing the results of 33 studies, she warns that it really does not take very much to become just mildly dehydrated, particularly in the high temperatures of the mid-summer season.
And this is especially true if you are exercising outdoors, advises University of Connecticut professor of kinesiology, Doug Casa. Also the CEO of the Korey Stringer Institute, Casa notes, “If I were hiking at moderate intensity for one hour, I could reach about 1.5 percent to 2 percent dehydration.”
Furthermore, he attests, sweating out just one liter of water equates to 2 percent of dehydration for an average-size person. Casa goes on to say, “Most people don’t realize how high their sweat rate is in the heat,” adding that if you are planning a hard run, you can reach this level of mild dehydration after just 30 minutes.
More importantly, at this level of dehydration—remember it is only 2 percent—most of us are only feeling slightly thirsty (or not thirsty at all). Casa warns that “Most people can’t perceive that they’re 1.5 percent dehydrated.”
But the unfortunate reality about this is that while you may not know you are dehydrated (because you don’t feel any different), the effects on your body and mental performance could begin to show.
For example, one recent study had young, healthy, active women take several cognitive tests after restricting fluid intake to only 6 ounces (or less) per day. Lead researcher Nina Stachenfeld—of the Yale School of Medicine and the John B. Pierce Laboratory—comments, “We did manage to dehydrate them by [about] 1 percent just by telling them not to drink for the day.”
One cognitive test was a card game that requires a lot of attention because the rules continue to change the longer you play. Stachenfeld asserts: “When the women were dehydrated they had about 12 percent more total errors [in their games].”