If a middle-aged or older
woman with a normal body mass index wants to maintain her weight over an
extended period, she must engage in the equivalent of 60 minutes per day of
physical activity at a moderate intensity, according to new findings by Harvard researchers at Brigham and Women’s Hospital (BWH).

“There is plenty of research on treating overweight and obesity — that is,
looking at strategies for weight loss among overweight or obese persons, but
very little research on preventing weight gain in the first place. Most
overweight and obese persons who lose weight do not successfully maintain their
weight loss over time, and so, from a public health perspective, preventing
that initial weight gain is important,” said I-Min Lee, an associate
professor of epidemiology
at Harvard School of Public Health (HSPH), epidemiologist in the Division of Preventive Medicine at BWH, and associate professor
of medicine at Harvard Medical School.


The findings are published in
the latest issue of the Journal of the American Medical Association.

 Lee and colleagues analyzed data reported from more than 34,000 healthy U.S.
women in the Women’s Health Study over 13 years to examine the relationship
between the level of daily physical activity and weight change over time. Women
in the study reported their leisure-time physical activities every two to three
years. Each time that physical activity was assessed, women were divided into
three groups, according to the amount of time they spent engaged in physical
activity.

The most active group of women spent the equivalent of 420 minutes a week
(60 minutes a day) or more engaged in moderate-intensity physical activity.
 The second group engaged in the equivalent of at least 150 but less than
420 minutes a week of moderate-intensity physical activity, and the least
active group engaged in the equivalent of less than 150 minutes a week of
moderate-intensity physical activity.  An example of a moderate-intensity
physical activity is brisk walking.

These three levels of physical activity were chosen based on the 2008 federal
guidelines for physical activity, which recommended at least 150 minutes a week
of moderate-intensity physical activity for health, and a 2002 Institute of
Medicine report on recommended dietary intakes
, which suggested that 60 minutes
a day of moderate-intensity physical activity was needed to prevent being
overweight, although the scientific basis for this level of activity has been
questioned.

Over the duration of the 13-year study, the average weight of participants
increased by 6 pounds, which is a rate of weight gain similar to that of
comparably aged women in the general population. Compared with the most active
women, both the group physically active for 150 to less than 420 minutes a week,
and the group physically active for less than 150 minutes a week gained
significantly more weight than the most active group. The two less-active
groups also were significantly more likely to gain at least 5 pounds, compared
with the most-active group.

Researchers discovered that the findings differed significantly, according to
women’s body mass index (BMI). Physical activity was associated with less
weight gain only among women with a
normal BMI, which is less than 25. An average U.S. woman who is 5 feet, 5
inches tall must weigh less than 150 pounds to have a normal BMI. Among heavier
women, physical activity — at least, within the levels that study participants
undertook — was not related to less weight gain.

In this study, researchers were able to identify a group of “successful weight
maintainers.” These were women who started with a normal BMI and managed to
maintain their weight, gaining less than 5 pounds at each weight assessment,
throughout the study. These women, 13 percent of participants, consistently
engaged in physical activity that was the equivalent of 60 minutes a day of
moderate-intensity physical activity.

Researchers concluded that:

  • Among middle-aged and older women consuming a
    usual diet with no calorie restriction, moderate-intensity physical
    activity for 60 minutes a day is needed to maintain normal BMI and prevent
    weight gain over time.
  • The 150 minutes a week of moderate-intensity
    physical activity, which can be achieved by 30 minutes a day, five days a week,
    and which is recommended by the federal government, while clearly
    sufficient based on data from many studies to lower the risk of developing
    chronic diseases, is insufficient for weight-gain prevention, without
    restricting caloric intake.

  • Among women who are already overweight or obese,
    physical activity — at least, at levels carried out by participants in
    this study — is not related to weight change, emphasizing the importance
    of controlling caloric intake for weight maintenance in this group.

“These findings shouldn’t obscure the fact that for
health, any physical activity is good, and more is better,” Lee emphasizes. “It
is important to remember that weight is only one aspect of health. Many studies
have shown that being physically active for even 30 minutes a day, five days a
week, significantly reduces the risk of developing many chronic diseases, such
as cardiovascular disease, some cancers, and type 2 diabetes.”

This study was funded by the National Institutes of Health.

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