Exercise limits weight gain in normal-weight but not heavier women

As most of us know, about two-thirds of adults in the United States are overweight or obese, and excess weight is a major health problem. Weight gain occurs when energy intake (calories consumed) exceeds energy output (calories burned).

Many studies have shown that physical activity can promote weight loss among people who are overweight or obese, but far fewer have investigated whether it can prevent unhealthy weight gain in the first place. That's the focus of a study by Harvard Medical School researchers published in the March 24/31, 2010, issue of The Journal of the American Medical Association. The research team was led by Prof. I-Min Lee of Harvard Medical School and Harvard School of Public Health, who is on the advisory board of Harvard Women's Health Watch.

The researchers analyzed data provided by 34,079 healthy women, average age 54, who were participating in the long-term Women's Health Study. Between 1992 and 2007, the women reported their body weight and physical activities every three years. They also provided information on matters that could affect the link between physical activity and weight change, such as smoking, postmenopausal hormone use, alcohol intake, and diet.

Participants were divided into three groups based on their level of physical activity, with energy expended in each group expressed in metabolic equivalent (MET) hours per week. A MET is a unit used to estimate the energy expended during physical activity, relative to the energy expended while sitting quietly.

The results

Not surprisingly, moderate-intensity activity, such as brisk walking, consumes more METs than lower-intensity activities such as yoga or stretching. In the study, women at the lowest activity level got less than 7.5 MET hours per week, the minimum recommended in federal guidelines (it's the amount expended in walking briskly for 30 minutes, five days a week). The middle group got 7.5 to less than 21 MET hours per week; and the most active women got 21 MET hours or more per week, which requires at least 60 minutes per day of moderately intense physical activity — or 30 minutes per day of vigorous activity.

Over the course of the study, the women gained 5.7 pounds, on average. Those who exercised less tended to gain more: over any three-year period, women in the low and middle activity levels were more likely to gain 5 or more pounds than the most active women. When the researchers looked only at the 4,540 women who were normal weight (a body mass index of less than 25) at the study's outset and who had managed to hold their weight in the normal range, they found that these weight-maintainers averaged 60 minutes of moderately intense activity per day.

What it means for women looking to control weight

Among normal-weight women, the likelihood of putting on weight decreased as physical activity level increased. Among women who were overweight or obese, there was no relationship between physical activity and weight gain.

Thus, for women who aren't overweight or obese, exercise can keep off excess pounds, but it must add up to about seven hours per week of moderately intense activity such as brisk walking or casual bicycling — or 3.5 hours per week of vigorous activity such as jogging or aerobic dancing.

For women who are already overweight or obese, increased physical activity alone is not enough to prevent further weight gain. These women also need to reduce their calorie intake. But they should still get at least 30 minutes of moderately intense physical activity most days for the sake of the many health benefits, which include a reduced risk of chronic conditions such as heart disease and stroke.