March 23, 2010 -- Weight gain with age is common. But middle-aged women who start out at a healthy weight and get in an hour of moderate activity every day can avoid weight gain, according to a new study.
There is plenty of research on how to lose weight and keep it off, but ''there's very little research on preventing weight gain in the first place," says the study lead author, I-Min Lee, ScD, an associate epidemiologist at Brigham and Women's Hospital and associate professor of medicine at Harvard Medical School in Boston.
So her team addressed that question, trying to pinpoint the amount of physical activity needed to prevent weight gain over time, without calorie restriction, a question that is much debated with little consensus. The study is published in the Journal of the American Medical Association.
Exercise and Weight Control: Study Details
Lee and her colleagues followed more than 34,000 women who had participated in the Women's Health Study. The women's average age at the study start in 1992 was 54.
Women self-reported physical activity and weight at the study start and at years, 3, 6, 8, 10, 12, and 13.
All women ate a normal diet, and there weren't instructions to reduce calories.
The researchers classified the women into three activity groups, depending on the level of activity:
- One group was active less than about 150 minutes a week -- the amount of moderate intensity activity recommended for health benefits (but not necessarily weight control), according to 2008 federal guidelines.
- A second group was active more than 150 minutes a week but less than 420.
- The most active group got in 420 or more minutes a week of moderate activity, or about an hour a day.
The researchers looked at physical activity and weight gain over intervals averaging three years.
Exercise and Weight Control: Study Results
Overall, Lee says, all three groups gained weight over time -- an average of 5.7 pounds.
But the more active the women, the less they gained. ''Compared to women in the most active group, women in the two lesser active groups gained more weight," Lee tells WebMD. ''Compared to the most active women, the two less active groups were more likely to gain 5 pounds over the three-year period. The second most active group was 7% more likely to gain the 5 pounds, and the least active group 11% more likely."
The two lesser active groups were about equal, however, in the amount of weight gained, she says.
Initially, Lee says, the relationship between physical activity and weight control looked like it applied to everyone. But it did not.
Lee and her team also looked at a subgroup of women -- those who started out at a healthy weight -- that is, with a body mass index or BMI of less than 25 -- and maintained a healthy weight throughout -- that is, gained less than 5 pounds at the three-year interval. Thirteen percent of the women, or 4,540, had a BMI lower than 25 at the study start and maintained a healthy weight throughout. ''We found the relationship between physical activity and less weight gain held only for the women with a BMI of less than 25."
For those whose BMI was 25 or more, ''there was no relationship between physical activity and weight gain, but perhaps because they just weren't very active," Lee says. For these women who are already overweight, she says, it seems exercise must be combined with calorie reduction to control weight.
The women who started at a healthy weight and kept their weight healthy consistently got in an hour of moderate intensity physical activity daily, Lee's team found.
Lee isn't sure whether the findings apply to men as they age. "U.S. men [as a whole] are more physically active than women," she says, and less likely to be obese. Lee has served as a consultant for Virgin HealthMiles, a worksite activity program, and is on its scientific advisory board. The study was funded by the National Institutes of Health.
Exercise and Weight Control: Hard Work
The study results are no surprise to Suzanne Phelan, PhD, assistant professor of kinesiology at California Polytechnic State University in San Luis Obispo and adjunct assistant professor of research at Brown Medical School in Providence, R.I., who has researched the topic.
"To keep a normal body weight is hard work," she says. As you age, she says, "you have to exercise a lot whether you are normal weight or maintaining a weight loss."
In her recent research, Phelan found that women who have lost weight and are trying to maintain the loss may even have to put in a few more minutes a day than do normal-weight women who are just trying not to gain as they age.
Exercise can give those with a healthy BMI more bang for their buck, especially if they are lean and have built up muscle mass, says Peter Galier, MD, a staff physician at Santa Monica-UCLA & Orthopaedic Hospital in California and an associate professor of medicine at the University of California Los Angeles David Geffen School of Medicine.
''As you get older, your basal metabolic rate [calories burned at rest] goes down," he says. Those who exercise can maintain muscle mass and perhaps not have as big a decrease as those who don't.
Exercise and Weight Control: What to Do
Although the one hour-a-day finding may be disheartening to the sedentary, Lee says, "I don't want this to be discouraging."
''I would say that any physical activity is good for health," she says. "The government recommendation of 150 minutes a week is clearly enough to reduce your risk of developing many chronic diseases.'' But it appears not to be enough for weight control, she says.
"If you want to do physical activity to control your weight, you have to do a fairly high level of it, and by that we mean 60 minutes a day. Once you are overweight or obese it is hard for physical activity alone to control weight. It has to be balanced with caloric reduction."
Being active doesn't necessarily mean running a marathon. "Most women [in the study] didn't do any vigorous activity," she says, but rather moderate activity.
What counts as moderate?
- Brisk walking, 3 to 4 miles an hour
- Casual bicycling
- Ballroom dancing
- Playing with grandchildren
If an hour or 30 minutes still sounds overwhelming, Phelan says you don't have to do it all at once. “You can do it in 10-minute bouts,” she says.