On Tuesday, a study was published in the online version of the Journal of the American Medical Association that recommends women should work out 60 minutes per day, seven days a week, in order to maintain a normal weight over their lifetime. That figure may come as a shock to many, who have been told they can get away with much less, but this figure is for those women who don't diet
The recommendation is aimed at women who are currently at a normal weight that don't want to diet, but would like to maintain their weight and avoid gaining weight over time. Most Americans tend to gain about 1.5 pounds per year from age 25 to 55.
There is far from a unanimous decision on just how much exercise we need. In fact, other experts say that only 35 minutes per day for seven days per week is sufficient.
Dr. I-Min Lee, an epidemiologist at Brigham and Women's Hospital in Boston and an associate professor at medicine at Harvard University says:
"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. 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 American women who were on average age 54 at the start of it. The participants reported on their physical activity and weight, and health factors including smoking and menopausal status over a period of 13 years. During the study, women gained on average 5.7 pounds.
The study found that only the women who were normal weight at the beginning of the study and who engaged in moderate intensity activity for an average of 60 minutes per day, seven days a week, were able to maintain a normal body weight. A normal body weight is defined as a body mass index of less than 25. This recommended amount of activity is three times higher than what the federal government recommends to lower the risk of chronic aliments including heart disease. The U.S. government recommends a total of 150 minutes per week of exercise.
In the study, moderate intensity activity was defined as walking or hiking, jogging, running, bicycling, aerobic exercise or dance, use of exercise machiens, yoga, tennis, squash, racquetball, and swimming.
Dr. Samuel Klein, a director of the Center for Human Nutrition at Washington University School of Medicine who was not involved in the study, says that the study does not prove that this level of exercise is the only factor in maintaining a normal weight. Klein says that it is possible that other factors contribute. He said:
"Women who decide to be physically active may also decide to eat healthier and do other things to maintain a normal body weight. This study doesn't tell you it's the physical activity."
He says that some people that exercise a lot will eat more as a result, while others will be so exhausted from the exercise that they don't do much activity for the rest of the day.
Dr. Joseph E. Donnelly, director of the Energy Balance Laborator at the University of Kansas, gives yet another perspective on the study. He says that a few randomized, controlled studies have looked at weight management over time, and tend to recommend about 200 to 250 minutes of exercise per week. But he adds:
"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."