Getting a bit hungry by fasting and cutting out snacks could actually make you want to exercise – not veg out on the couch as you might think.
And that sudden burst of energy may come from a surprising property of the “hunger hormone” ghrelin, according to new research led by Yuji Tajiri at the Kurume University School of Medicine in Kurume, Japan.
In an era where heeding the call of the stomach just means schlepping from the sofa to the fridge, it might not be obvious why hunger should prompt exercise.
But Tajiri and colleagues remind us that many animals only get a feed if they forage or hunt, so it makes sense that hunger would make them want to move. In fact, studies in mice show that when food is cut back they exercise more.
As for what might be causing that behaviour, the researchers had ghrelin sized up as chief suspect.
The hormone’s main claim to fame is that it makes you hungry. When energy stores dwindle, it is made in the stomach and talks to the brain to get you eating. But ghrelin is working more than one job. Mouse studies show it also helps endurance exercise by turning on adrenalin and the body’s glucose-making machinery.
So, ghrelin looks like it can sustain exercise. But Tajiri’s team wanted to know if it could actually motivate exercise in the first place, and set up some meticulous experiments in mice to investigate.
To start they divided the critters into two groups, one on an all-you-can-eat diet and the other on K rations – feeding just twice a day. As expected, the animals on a limited diet did more voluntary exercise measured through sensors underfoot and turns on a hamster wheel.
But the researchers had introduced a twist.
Some of the mice had been bred to lack ghrelin through a genetic “knockout”. The ones missing ghrelin, the researchers discovered, did much less exercise than their normal cage mates.
Which is something of an Exhibit A that ghrelin could be motivating mice to get on the treadmill.
But the team needed more proof.
So they decided to give those ghrelin-deficient mice a shot in the arm of a ghrelin substitute. It promptly restored their exercise routine. And that looks a lot like an Exhibit B that ghrelin is getting mice moving.
But how, precisely, could a hormone circulating in the bloodstream make you want to exercise?
Tajiri and co had that one covered too. They had done some rather deft brain surgery on the mice, inserting a device that could measure dopamine levels in the reward centre.
Dopamine levels in the brains of those ghrelin knockout mice, it turned out, were way down. Ghrelin was pushing dopamine higher, suggesting the hunger hormone motivates exercise by getting the brain to think it’s fun.
“Our findings suggest that hunger, which promotes ghrelin production, may also be involved in increasing motivation for voluntary exercise, when feeding is limited,” says Tajiri.
Going hungry makes exercise look good – who would have thought?
Tajiri recognises that mice studies don’t prove the same point in people, but he still thinks it has opened the door to a new approach.
“[M]aintaining a healthy eating routine, with regular mealtimes or fasting, could also encourage motivation for exercise in overweight people,” he says.
“If it can be established in clinical practice, it not only opens up new cost-effective diet and exercise strategies but may also indicate a new therapeutic application for ghrelin-mimicking drugs.”
The study appears in the Journal of Endocrinology.