American researchers said this week that “couch potatoes” build up dangerous pockets of fat more quickly than originally thought.

But vigorous exercise can take if off pretty quickly, the team at Duke University in North Carolina reported.

The team of researchers looked at visceral fat – the hidden flab tucked in among the organs. It is often invisible but unlike an obvious paunch or heavy thighs, it is linked with insulin resistance – pre-diabetes – and heart disease.

Speaking to a meeting of the American College of Sports Medicine in San Francisco, Cris Slentz said he was surprised at how rapidly fat accumulated deep in the abdomens of patients who did not exercise.

Volunteers who did no exercise had an 8.6 per cent increase in visceral fat after eight months, while those who exercised the most lost 8.1 per cent of their visceral fat, Slentz said.

“The results of our investigation show that in sedentary overweight adults, who continue to choose a sedentary lifestyle, the detrimental effects are worse and more rapid than we previously thought,” Slentz said in a statement.

Exercise takes the fat away quickly, but it has to be pretty vigorous, Slentz and colleagues found.

“Participants who exercised at a level equivalent to 17 miles (28 km) of jogging each week saw significant declines in visceral fat, subcutaneous abdominal fat and total abdominal fat,” Slentz said.

“While this may seem like a lot of exercise, our previously sedentary and overweight subjects were quite capable of doing this amount.”