Until now, what goes on in our brain when we have to make a decision has been a mystery to scientists. But researchers at ETH Zurich, a public research university in Switzerland, have discovered a special brain chemical that mediates this decision.

They found that a messenger substance called orexin and the neurons that produce it are responsible for decisions about whether to exercise or to eat high-calorie snacks.

According to the World Health Organization (WHO), 80% of adolescents and 27% of adults do not get enough exercise. With obesity rates rising worldwide, this study could help inform strategies to tackle this public health crisis, the researchers said.

In their experiments with rats, the researchers showed that orexin is important for this process.

Orexin is one of more than a hundred messenger substances active in the brain. While other chemical messengers such as serotonin and dopamine have long been identified and their roles are well understood, little is known about orexin.

It was discovered relatively recently, about 25 years ago, and scientists are still slowly discovering its functions.

Denis Burdakov, professor of neuroscience at ETH Zurich, is one of the researchers studying orexin.

Burdakov highlights its role in motivation, saying, “Dopamine is often cited as the reason we choose some activities over others.” However, he points out that since dopamine is released during both eating and exercise, this doesn’t fully explain why we might choose one over the other.

To find out, the researchers conducted a simple experiment, asking rats to choose between eight different options during a ten-minute test.

These options included a running wheel and a “milkshake bar” that offered strawberry-flavored milkshakes. “The reason rats like milkshakes is because, like humans, they like sugar and fat,” Burdakov explains.

“Orexin plays a central role in deciding between activity and food,” Burdakov said.

Rats with a functioning orexin system spent more time running and less time on the milkshake bar than rats with a blocked system.

However, when given just one option, both groups behaved similarly. This suggests that orexin primarily helps rats decide between exercise and food when both options are available.

The ETH researchers hope that orexin might also be responsible for this decision in humans, since brain function is practically the same in both species.

They believe that interventions can be developed to help overcome barriers to exercise in both healthy individuals and those who have limited physical activity.

Next, Burdakov wants to figure out how orexin neurons interact with the rest of the brain when deciding between exercise and breakfast.