The 60-minute-a-day recommendation, released online in 2010 in the Journal of the American Medical Assn., is aimed at women of normal weight who don’t want to diet but do want to avoid gaining weight over time. Most Americans gain about 1.5 pounds a year between age 25 and 55.
“We wanted to see in regular folks — people not on any particular diet — what level of physical activity do you need to prevent weight gain over time,” said the lead author of the study, Dr. I-Min Lee, an epidemiologist at Brigham and Women’s Hospital in Boston and an associate professor of medicine at Harvard University. “It’s a large amount of activity. If you’re not willing to do a high amount of activity, you need to curtail your calories a lot.”
The study was based on surveys of more than 34,000 U.S. women who were, on average, age 54 at the start of it. They reported their physical activity and weight, as well as health factors such as smoking and menopausal status, over 13 years. On average, the women gained 5.7 pounds during the study.
Only those women who were normal weight at the start of the study and engaged in moderate-intensity activity an average of 60 minutes per day, seven days a week, maintained a normal body weight, defined as a body mass index of less than 25. That amount of exercise is three times higher than the amount recommended by the federal government — 150 minutes per week — to lower the risk of chronic ailments such as heart disease.
Moderate-intensity activity was defined in the study as walking or hiking, jogging, running, bicycling, aerobic exercise or dance, use of exercise machines, yoga, tennis, squash, racquetball and swimming. Housework and gardening were not included in the analysis.
“Women who decide to be physically active may also decide to eat healthier and do other things to maintain a normal body weight,” he said. “This study doesn’t tell you it’s the physical activity.”
“It’s complex,” Klein said. “It’s all about energy balance. It takes a very small imbalance to gain a significant amount of weight over many years.”
Although few randomized, controlled studies — the gold standard in research — have looked at the issue of weight maintenance over time, they tend to point to about 200 to 250 minutes of exercise a week, said Dr. Joseph E. Donnelly, director of the Energy Balance Laboratory at the University of Kansas.
“But there is huge individual variation,” he said. “It’s very difficult to filter out what it takes for the average person not to gain weight when you don’t do randomized studies.”
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