Evidently a re-analysis of the Harvard Women's Study.
"So the researchers assessed data from the 34,079 women in the Women's
Health Study from 1992 to 2007."

...."adding that for heavier women, it's a "case of 'too late,' if you
will -- for a woman who has a body mass index of 25 or higher, there was
no relation between physical activity and weight change."

===========================================================================
http://jama.ama-assn.org/cgi/content/short/303/12/1173

By Kristina Fiore, Staff Writer, MedPage Today

To Stay Trim, Women Need an Hour of Exercise Daily-

Published: March 23, 2010
Reviewed by Robert Jasmer, MD; Associate Clinical Professor of Medicine,
University of California, San Francisco.    
Video source: JAMA

Normal-weight women need 60 minutes a day of moderate exercise in order
to maintain a healthy weight, researchers said.

Those who exercised fewer than 420 minutes a week gained significantly
more weight than those who met this target, I-Min Lee, MBBS, ScD, of
Brigham & Women's Hospital in Boston, and colleagues reported in the
March 24/31 issue of the Journal of the American Medical Association.

For the heaviest women, however, there was no relationship between
exercise and weight gain.

"If you want to prevent your weight gain over time, you need to be
physically active at the level of 60 minutes of moderate-intensity
physical activity, so the equivalent of one hour a day of brisk walking
or 30 minutes a day of jogging or running," Lee said, adding that for
heavier women, it's a "case of 'too late,' if you will -- for a woman
who has a body mass index of 25 or higher, there was no relation between
physical activity and weight change."

Federal guidelines have recommended at least 150 minutes a week of
moderate-intensity exercise in order to gain substantial health
benefits, but it remains unclear whether that amount can also prevent
weight gain.

So the researchers assessed data from the 34,079 women in the Women's
Health Study from 1992 to 2007. Their mean age was 54 and they were all
healthy at baseline.

Over a mean follow-up of 13 years, the women gained a mean of 2.6 kg
(5.7 lbs).

The researchers found that those who exercised the most -- 420 minutes
or more a week -- gained significantly less weight than those who
exercised less than 420 minutes per week (P=0.003).

Compared with the most active group, those who exercised the least
gained an additional 0.12 kg over a three-year period, and those who
exercised between 150 and 420 minutes per week gained an additional 0.11
kg over that time, Lee said.

There was no significant difference between the two latter groups.

They also found that women getting more than 420 minutes of exercise per
week were significantly less likely to gain 2.3 kg (5.1 lbs) over a
three-year period.

In adjusted analyses, those in the group that got the least exercise
were 11% more likely to gain that amount of weight, and those expending
between 150 and 420 minutes per week were 7% more likely to do so.

"These two lesser-activity groups of women were significantly more
likely to gain weight compared with the most active group of women," Lee
said.

The researchers also found that physical activity was inversely related
to weight gain, but only among women with a body mass index (BMI) under
25. For those with a BMI greater than that, exercise had no effect on
weight control.

"[For] the women who were overweight or obese, physical activity -- with
the range done in the study -- was not sufficient to control their
weight," Lee said.

Finally, the researchers investigated how much physical activity was
done by women who successfully maintained normal weight.

A total of 13.3% of the cohort had done so by gaining less than 2.3 kg
(5.07 lbs) over the 13-year study period.

The mean activity level among this group was 21.5 "metabolic equivalent"
hours per week, or just over 60 minutes a day of moderate-intensity
exercise.

The researchers said the results "highlight two important points for
weight gain prevention."

First, once patients are overweight, they said, it may be too late for
physical activity to have any effects. Second, sustaining high levels of
exercise is needed to maintain a normal BMI.

Still, Lee cautioned not to "lose sight of the fact that any amount of
physical activity, even 150 minutes a week, is sufficient to lower the
risk of developing many chronic diseases."

They noted that the study was limited by self-report of recreational
physical activity and body weight.

However, their findings suggest that the "2008 federal recommendation
for 150 minutes per week, while clearly sufficient to lower the risks of
chronic diseases, is insufficient for weight gain prevention absent
caloric restriction."

The study was supported by grants from the National Institutes of Health.

Lee reported serving as a consultant to Virgin HealthMiles and sits on
its scientific advisory board.

The other authors reported no disclosures.

Primary source: Journal of the American Medical Association
Source reference:
Lee IM, et al "Physical activity and weight gain prevention" JAMA 2010;
303(12): 1173-79.

http://jama.ama-assn.org/cgi/content/short/303/12/1173