Our hearts don’t mind living in overweight bodies and will keep us alive as long as we work out, Norwegian research now suggests.

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Getting an overweight mouse to work out was key to the discovery.

“When a mouse trains, its heart starts to function normally,” said researcher Anne Hafstad of the University of Tromsø.

“I doubt any medicine can have as good a result on the heart as exercise does,” said Hafstad, writing in the Norwegian research journal, Forskning.

For ten weeks she fattened up mice on carbohydrate-rich junk food, and they responded by being lethargic and showing weaker heart rhythm. Another ten weeks of working out while on junk food followed.

Five times a week, Hafstad upped their sets of repetitious treadmill work. She said their daily lot was the equivalent for humans of walking uphill for two hours and then doing an hour of sprint intervals.

The mice’s fat count plummeted during both moderate and intense training, despite the trashy diet.

“The effect on heart function was surprising,” said Hafstad, adding: “The heart became more elastic, its chambers worked together better and oxygen use normalised.”

The research was unusual for having brought the mouse’s body into the first stages of so-called Type 2 diabetes: insulin resistance and stiffened walls in the heart. Stiffened hearts play a large part in the up to 15,000 acute heart attacks experienced in Norway every year.

The mice’s diabetic heart made it all the more surprising that exercise changed the condition of all 20 animals tested.

“We've never had such good results before, even with anti-diabetes treatment,” she said.

“We have never before seen a normalisation of heart function.”

She said the mice went from being stressed and lethargic to being happy after exercising.