WEDNESDAY July 30, 2008 (foodconsumer.org) -- Exercise is known to help weight loss, but there has been no definite answer to how long an overweight person needs to exercise to sustain weight loss.   A new study finds that to have such an effect, an overweight woman need at least to exercise 55 minutes each day for five days a week.

The study, which appears in the July 28 Archives of Internal Medicine, showed this magnitude of exercising intensity could sustain a weight loss of 10 percent over two years.

In the United States, more than 65 percent people are considered overweight, according to the background information in the study report.

"Among obese adults, long-term weight loss and prevention of weight regain have been less than desired," researchers write.   "Therefore, there is a need for more effective interventions."

Currently, 30 minutes of moderate physical activity on most days of the week, for a total of 150 minutes per week are recommended. The study suggests that more exercise may be needed to enhance a long-term weight loss effect.

For the study, John M. Jakicic, Ph.D., of the University of Pittsburgh, and colleagues calculated the amount of exercise needed for 201 overweight and obese women in a weight loss intervention between 1999 and 2003.

All the women were advised to eat between 1,200 and 1,500 calories per day. Participants were divided into four groups. Women in each group were assigned to do exercise for a physical activity amount (1,000 calories vs. 2,000 calories per week) and intensity (moderate vs. vigorous).   Instruction was given for modifying eating and exercise habits during the two-year period.

The researchers found after six months, women in all four groups experienced an average loss of 8 percent to 10 percent of their initial body weight.

However, most of them were not able to sustain this weight loss. After two years, the women's weight was on average 5 percent lower than their initial weight, with no difference between groups.

Those who did maintain a loss of 10 percent or more over two years (24.6 percent) reported performing more physical activity resulting in burning of an average of 1,835 calories per week, or 275 minutes per week over the baseline level of activity than those who lost less weight.

But even among this group who sustain 10 percent weight loss, exercise may not be the only thing they did to have the effect.   These people also completed more phone calls with the research staff and engaged in more eating behaviors recommended for weight loss and had a lower intake of dietary 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 researchers write.

"Research is needed to improve long-term compliance with this targeted level of physical activity. Moreover, continued contact with the intervention staff and the ability to sustain recommended eating behaviors also may be important contributing factors to maintaining a significant weight loss that exceeds 10 percent of initial body weight, which suggests that physical activity does not function independently of these other behaviors."