And the regime has to be stuck to rigidly for two years for the benefits to shine through, the scientists added.
Official guidelines currently recommend men and women to do 30 minutes exercise a day to stay trim.
Now it has been shown that this time needs to be almost doubled after a study of more than 200 women who were overweight and obese.
The participants in the study were told to eat between 1,200 and 1,500 calories a day rather than the normal intake of 2,000 and given different exercise regimes.
The women were told to concentrate their exercises on calorie burning or do vigorous or moderate activities.
After six months all of them had lost eight to 10 per cent of their weight and hit an ideal body mass index of between 20 and 25.
But then they faced the problem of keeping their weight down.
After two years, there was an average weight gain of five per cent. But one in four women who maintained their weight loss had one thing in common - the amount of exercise they did.
On average, these women burnt off 1,835 calories per week, doing 275 minutes of moderate exercise - 55 minutes over five days.
This group was also better at sticking to diets.
Dr John Jakicic said that the findings offered women a clear target.
"This clarifies the amount of physical activity that should be targeted for achieving and sustaining this magnitude of weight loss," the Daily Express quoted him, as saying.
"But it also demonstrates the difficulty of sustaining this level of physical activity. Research is needed to improve long-term compliance with this targeted level of physical activity," he added.