What dehydration does to your mind after just 2 hours

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Just two hours of vigorous yard work in the summer sun without drinking fluids could be enough to blunt concentration, according to a new study.

After statistically analyzing data from multiple peer-reviewed research papers on dehydration and cognitive ability, researchers found that that cognitive functions often wilt as water departs the body. The data point to functions like attention, coordination, and complex problem solving suffering the most, and activities like reacting quickly when prompted not diminishing much.

“The simplest reaction time tasks were least impacted, even as dehydration got worse, but tasks that require attention were quite impacted,” says Mindy Millard-Stafford, a professor in the School of Biological Sciences at Georgia Tech.

No fluid, no focus

As the bodies of test subjects in various studies lost water, the majority of participants increasingly made errors during attention-related tasks that were mostly repetitive and unexciting, such as punching a button in varying patterns for quite a few minutes. There are situations in life that challenge attentiveness in a similar manner, and when it lapses, snafus can happen.

There’s no hard and fast rule about when exactly such lapses can pop up.

“Maintaining focus in a long meeting, driving a car, a monotonous job in a hot factory that requires you to stay alert are some of them,” says Millard-Stafford, the study’s principal investigator. “Higher-order functions like doing math or applying logic also dropped off.”

The researchers have been concerned that dehydration could raise the risk of an accident, particularly in scenarios that combine heavy sweating and dangerous machinery or military hardware.

Millard-Stafford and first author Matthew Wittbrodt, a former graduate research assistant at Georgia Tech and now a postdoctoral researcher at Emory University, report their work in the journal Medicine & Science in Sports & Exercise.

Sudden onset

There’s no hard and fast rule about when exactly such lapses can pop up, but the researchers examined studies with one to six percent loss of body mass due to dehydration and found more severe impairments started at two percent. That level has been a significant benchmark in related studies.

“If you weigh 200 pounds and you go work out for a few of hours, you drop four pounds, and that’s two percent body mass…”

“There’s already a lot of quantitative documentation that if you lose two percent in water it affects physical abilities like muscle endurance or sports tasks and your ability to regulate your body temperature,” says Millard-Stafford. “We wanted to see if that was similar for cognitive function.”

The researchers looked at 6,591 relevant studies for their comparison, then narrowed them down to 33 papers with scientific criteria and data comparable enough to do metadata analysis. They focused on acute dehydration, which anyone could experience during exertion, heat, and/or not drinking as opposed to chronic dehydration, which can result from a disease or disorder.

How much is too much?

How much fluid loss adds up to two percent body mass loss?

“If you weigh 200 pounds and you go work out for a few of hours, you drop four pounds, and that’s two percent body mass,” Millard-Stafford says. And it can happen fast. “With an hour of moderately intense activity, with a temperature in the mid-80s, and moderate humidity, it’s not uncommon to lose a little over two pounds of water.”

“If you do 12-hour fluid restriction, nothing by mouth, for medical tests, you’ll go down about 1.5 percent,” she says. “Twenty-four hours fluid restriction takes most people about three percent down.”

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And that begins to affect more than cognition or athletic abilities and concentration.

“If you drop four or five percent, you’re going to feel really crummy,” Millard-Stafford says. “Water is the most important nutrient.”

She warns that older people can dry out more easily because they often lose their sensation of thirst and also, their kidneys are less able to concentrate urine, which makes them retain less fluid. People with high body fat content also have lower relative water reserves than do lean folks.

A warning about water

Hydration is important, but so is moderation.

“You can have too much water, something called hyponatremia,” Millard-Stafford says. “Some people overly aggressively, out of a fear of dehydration, drink so much water that they dilute their blood and their brain swells.”

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This leads to death in rare, extreme cases, for example, when long-distance runners constantly drink but don’t sweat much and end up massively overhydrating.

“Water needs to be enough, just right,” Millard-Stafford says.

Also, she warns that while salt avoidance may be good for sedentary people or hypertension patients, whoever sweats needs some salt as well, or they won’t retain the water they drink.

Source: Georgia Tech