Physical activity prevents weight gain in middle-aged and older women only if they are already of low or ideal weight, not in those with body mass indexes of 25 or more, according to a large 13-year study.
Even the lighter women must sustain vigorous physical activity—1 hour or more of moderate-intensity activity every day, on average—to prevent weight gain over time. Women who exercise regularly but do so at lower intensity gain weight at the same rate as those who are inactive, said I-Min Lee, Sc.D., of Brigham and Women's Hospital, Boston, and her associates (JAMA 2010;303:1173–9).
These findings imply that following federal and other recommendations advocating 150 minutes of physical activity per week isn't sufficient to prevent weight gain in most middle-aged women, although it may well provide other health benefits, the investigators noted.
Dr. Lee and her colleagues assessed weight changes associated with various levels of physical activity, they said, because different expert groups have recommended widely varying levels of activity. For example, the federal government, the American College of Sports Medicine, and the American Heart Association recommend a minimum of 150 minutes per week, while the Institute of Medicine suggests nearly 3 times that amount, 420 minutes per week, to avoid becoming overweight or obese.
The researchers examined the issue using data from the Women's Health Study, a prospective cohort study of healthy women who had a mean age of 54 years at baseline in 1992. The team assessed 34,079 of these subjects who have been followed up on periodically since the main portion of that study was concluded in 2004.
At baseline, approximately half the women were classified as inactive, performing less than 150 minutes per week of moderate-level physical activity such as brisk walking, hiking, running, bicycling, tennis, swimming, weight training, or aerobic dance or exercise. Another 29% performed an intermediate amount of physical activity, and 22% performed a high level, putting in 21 or more hours of such activity per week.
In an initial data analysis, all three groups showed similar patterns of weight gain over a mean 13 years of follow-up. In particular, the inactive group gained a mean of 0.12 kg and the intermediate group gained 0.11 kg, an insignificant difference.
Further analysis showed, however, that women starting with body mass indexes (BMIs) lower than 25 demonstrated an inverse correlation between physical activity and weight gain, Dr. Lee and her associates said.
Another analysis examined the likelihood of gaining 2.3 kg or more over the course of 3 years. Again, there was no correlation between activity level and weight gain except in the subgroup of women with BMIs lower than 25. Only in those women did a high level of activity prevent weight gain.
Finally, an analysis of the subgroup of women of normal weight who maintained that weight throughout follow-up showed that they spent at least 60 minutes per day performing moderately intense activity, an amount closer to the Institute of Medicine recommendations than to others.
These findings indicate that once women are overweight, “it may be too late” for physical activity to stave off further weight gain, the investigators said.
This study was supported by the National Institutes of Health. Dr. Lee reported serving as a consultant to Virgin HealthMiles and sitting on its scientific advisory board. The authors reported no other financial conflict of interest.
Regular, low-intensity exercise promotes health but may not help overweight middle-aged or older women lose weight.
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