Taking the stairs, taking a hike, taking a yoga class, or any other moderate physical activity recently helped thousands of healthy women maintain their weight for 13 years without cutting calories, a new study reports. The only catch is that it only worked for women with a normal body mass index (BMI) who exercised for an hour daily.

The federal government, along with the American Heart Association and others, recommends getting 150 minutes of moderate physical activity a week (about 20 minutes a day), whereas the Institute of Medicine (IOM) has advocated for 450 minutes (about 60 minutes a day). According to the National Institutes of Health, the ranges of BMIs are:

•    Less than 18.5 is underweight;

•    18.5 to 24.9 is normal;

•    25 to 29.9 is overweight and

•    more than 30 is obese.

Between 1992 and 2007, researchers behind the new study checked in with 34,079 relatively healthy middle age and older women seven times to gather information on weight and physical activity levels. For women under the age of 65 and with BMIs below 25, exercise made a big difference. Those in that group who got less than 420 minutes of moderate activity each week "gained significantly more weight" than those who were able to squeeze in more than 60 minutes a day, reported the team, which was led by I-Min Lee, of the Division of Preventive Medicine at Brigham and Women's Hospital at Harvard Medical School.

Unfortunately, for women who started the study either overweight or obese and who maintained average U.S. eating habits (didn't trim their caloric intake), the amount of moderate activity did not seem to affect their statistical likelihood of losing weight.

The researchers concluded that this might indicate that, "once overweight, it may be too late" to rely on moderate activity alone to achieve a healthier BMI. The results were published online March 23 in JAMA, Journal of the American Medical Association.

Even though the federal recommendation for 150 minutes of physical activity each week is "clearly sufficient to lower the risks of chronic diseases," the researcher noted, it "is insufficient for weight gain prevention absent caloric restriction."

In the U.S., where one in three adults is considered obese, the sheer numbers "present a tremendous health care challenge in treatment and cost relating to the many adverse health conditions associated with excess body weight," the researchers noted. Even small weight gains, such as the average 2.6 kilograms that study participants put on over the course of the study, can make a cumulative impact on health—for both individuals and the overall disease burden—especially as "the majority of persons losing weight do not maintain their weight loss," the authors noted. So keeping the pounds off in the first place is, indeed, "preferable," as Lee and colleagues wrote.

Image courtesy of iStockphoto/diego_cervo