According to a US study of 34,000 women, an hour a day of moderate exercise is needed to fight the flab.
Overweight women need to diet as well as exercise, a Harvard team reports in the Journal of the American Medical Association.
The UK government advises adults to exercise for at least half an hour five or more days a week.
The US study followed 34,000 middle-aged women over 13 years.
The women completed regular questionnaires about the amount of time they spent on physical activity.
They were classified into three groups: those who did the equivalent of less than two and a half hours of moderate exercise a week, between two and a half hours and seven hours a week, and more than seven hours a week.
The average age of the women was 54, and the average weight gain was 2.6 kg over the 13 year period.
The high exercise group gained significantly less weight than the rest.
But there was no difference in weight gain between the two lower exercise groups.
The only group who didn't gain weight during the study were normal weight women who exercised for an hour or more a day.
Too late
Among the women who were normal weight at the start, the more they exercised, the less weight they gained.
But among those who were already overweight at the start there was no relation between exercise and weight gain.
The authors say clear guidelines on the amount of physical activity for the prevention of weight gain are essential.
They draw two conclusions: "Firstly once overweight, it may be too late because physical activity - at least, at levels carried out by study participants - was not associated with less weight gain.
"Second, sustaining high levels of physical activity (60 minutes a day) is needed to successfully maintain normal BMI and prevent weight gain."
This level of activity is higher than the minimum that the UK government recommends.
Spare tyre
A Department of Health spokesperson said: "The Chief Medical Officer recommends that adults do 30 minutes of physical activity at least five times a week. Children need 60 minutes of activity every day.
"As part of the Change4Life movement, adults are being encouraged to make simple changes, such as eating more fruit and veg, cutting down on fatty foods and being more active, in their diet and lifestyle which will help them lead longer, healthier lives — and lose that unwanted spare tyre."
But Professor Paul Gately, who runs Carnegie Weight Management Programme at Leeds Metropolitan University, said the government recommendation is the amount of exercise needed to reduce risk of ill health.
He said more exercise was needed to avoid weight gain, and an hour a day was in line with findings of previous research.
He said: "While 30 minutes exercise is recommended for health reasons, people need to exercise for longer to avoid weight gain."
He added that the International Obesity Task Force report in 2004 concluded that people who are overweight have to do more exercise to maintain their weight, up to 90 minutes a day.
http://news.bbc.co.uk/1/hi/health/8586767.stm
