(CNN) -- It turns out that overworking your brain with either physical or mental exercise may lower your ability to delay self-gratification. And that may set you up for poor choices in your eating habits, self-care and finances.
A new study published Thursday in the journal Current Biology asked elite endurance athletes to overtrain for three out of nine weeks, and compared them to a group who did a normal 9-week training program.
Not only did those overworked athletes perform worse on a cycling test administered at the end of the overtraining, MRIs of their brains during behavioral tasks showed more fatigue in the cognitive control part of the brain system.
"Cognitive control in this situation is the capacity to maintain exercise despite things like muscle pain," said study author Bastien Blain, a research associate at University College London. "And what we found is there is an intellectual component involved in exercising and it has a finite capacity. You cannot use it forever."
In other words, your brain will burn out and affect your body's ability to exercise. But that's not all. Overworking that part of the brain also reduced the athletes' abilities to resist temptation of an immediate reward.
"For example, they were asked whether they preferred $10 now or $50 in six months," Blain said. "And those who overtrained were more likely to choose the immediate reward, which is interesting. It could provide a mechanism to explain why some athletes are using drugs to improve their performance."
Blain had done a similar study in 2016 on mental burnout. A group of 58 adults performed hard executive tasks over a 6-hour period, then underwent MRI imaging and were asked if they would choose $5 now or $50 later. Just as with the physical burnout, researchers found overworked brains were much more likely to choose immediate self-gratification.
Perhaps less is more?
One caution about the exercise study is that it only looked at endurance athletes, said Dr. Marc-Andre Cornier, who is associate director of Colorado University's Anschutz Health and Wellness Center.
"This is potentially very important for the higher end athlete who is overdoing it," Cornier said. "But does this have anything to do with the average Joe going to the gym? You can't conclude that from this study."
In fact, a preliminary study done by Cornier found that regular moderate exercise -- an hour on the treadmill four to five times a week -- was associated with reducing the brain activity in regions associated with impulse control and the desire to eat.
"What we found is that 6 months of exercise in certain people who were otherwise sedentary altered the brain response to food," Cornier said. "Those people ate less and they lost weight. And the more we saw that change in brain activity due to exercise, the more weight they lost."
But not everyone's brain responded to exercise, Cornier found. Some people just continued to exercise and overeat. Why?
"We don't know yet," Cornier said, adding that a study is underway to ferret that out. "Is it genetically how they've been wired?"
The good news is the brain can be rewired, so there's hope that these newer imaging studies can try to pinpoint the parts of the brain that might be targeted, with interventions or even drugs.
"The brain is plastic and you do form new connections," Cornier said. "I think that's exciting because it makes you think that we can do something about some of these chronic behavioral issues like food intake or depression. But why one individual would be different than another is the million dollar question."
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