Fifty-five is the magic number when it comes to working out and losing weight, researchers say.

A new study has found that for overweight and obese women seeking to lose weight, fifty-five minutes of exercise a day, five days a week is needed to lose 10 per cent of overall body weight over two years.

Currently, the recommended amount is 30 minutes of moderate physical activity on most days of the week, totaling 150 minutes per week.

The study, conducted by University of Pittsburgh researchers, has been published in the July 28 issue of the Archives of Internal Medicine.

Between Dec. 1, 1999, and Jan. 31, 2003, researchers recruited 201 overweight and obese women with body mass index readings of between 27 to 40. They ranged in age from 21 to 45 years.

The study participants, who were instructed to eat between 1,200 and 1,500 calories a day, were assigned to four groups, each with a different activity level. All groups met to discuss eating and exercise habits and also received supportive phone calls from the research team over a two-year period.

After six months, participants in all of the groups lost an average of eight per cent to 10 per cent of their body weight, though they could not sustain the weight loss over the subsequent 18 months.

Those women who did maintain a weight loss of 10 per cent or more over the two-year period (24.6 per cent of the participants) exercised for 275 minutes — rather than150. They also spoke more frequently with the research team, ate fewer calories and consumed less fat.

"This clarifies the amount of physical activity that should be targeted for achieving and sustaining this magnitude of weight loss, but also demonstrates the difficulty of sustaining this level of physical activity," the authors write.