Why too much exercise can lead to making bad decisions

Too much exercise doesn't just take a toll on your body, a new study found. It also taxes your brain.

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The researchers tested male triathletes, all of whom volunteered for the study. "They were strongly motivated to be part of this program, at least at the beginning," said Bastien Blain, an author of the study and a postdoctoral fellow at University College London.

“Overtraining syndrome is a form of burnout, defined in endurance athletes by unexplained performance drop associated with intense fatigue sensation,” researchers wrote in their study, which was published Thursday in the journal Current Biology.

For the study, half the athletes increased their training by 40% while the other half trained as they normally would.

After three weeks, all athletes were put in a brain scanner. They were asked questions designed to determine a preference for immediate gratification or long-term reward.

"For example, we ask, 'Do you prefer $10 now or $60 in six months,' " Blain told NPR.

The athletes who overtrained “were, in fact, choosing more immediate gratification than the other group of athletes," Blain said.

Not only that, MRIs of the athletes’ brains showed more fatigue in the part of the brain responsible for cognitive control.

"Cognitive control in this situation is the capacity to maintain exercise despite things like muscle pain," Blain told CNN. "And what we found is there is an intellectual component involved in exercising and it has a finite capacity. You cannot use it forever."

Essentially, your brain will burn out and affect your ability to exercise. When there is a lot of activity in the brain’s cognitive control area, Blain told NPR, “athletes are able to ignore signals from screaming muscles and focus on winning.” But overtraining can fatigue that part of the brain, he said, and a person is less able to push their body.

Because this study focused only on elite athletes, the "brain fatigue" conclusions can't be applied to the average person without more research, the group said.

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