An article appearing this week in the Journal of the American Medical Association is creating quite a buzz, at least among women.
The study examined the exercise and weight histories of over 34,000 women who were enrolled in the Women's Health Study, and concluded that only women who exercised an hour a day (at an intensity equivalent to walking at about a 3 mph pace) maintained their weight. Any less than that, the study said, and the women gained weight.
Consequently, headlines across the land are blaring, "Women Need An Hour of Exercise a Day To Keep From Getting Fat!"
This conclusion is not quite accurate. Plus, in an era when obesity is already being demonized, and especially with new healthcare reforms now in place which (some maintain) will end up collectivizing healthcare resources (such that your obesity will impact my ability to get the healthcare I think I deserve), the notion that women who fail to exercise an hour a day are a menace to society might lead to all kinds of trouble. (One visualizes college-aged members of the proposed Civilian Security Corps showing up each day, and conducting ranks of neighborhood housewives in hour-long forced marches.)
So let's quickly examine the real meaning of this study before the headlines become irreversibly incorporated into the anti-obesity dogma.
1) The study really didn't evaluate the amount of time these women exercised each day. Rather, it evaluated the number of METS (metabolic equivalents) the women expended each week. Roughly speaking, one MET is equivalent to the amount of calories you burn while sitting quietly for 1 hour. So expending 3 METS means you are burning three times the calories you burn while sitting. The investigators found that women whose exercise burned 21 METS per week tended to maintain their weight, while women who burned less than 21 METS gained weight. So, if the exercise you are doing burns 3 METS per hour (such as walking 3 mph), then you would need to exercise 7 hours a week. But if you chose to engage in more vigorous exercise, such as running 12-minute miles (8 METS), then you would only need to exercise around 2 1/2 hours a week to maintain your weight.
2) The study was not a randomized, controlled trial, but was an observational study only, which, furthermore, was based on self-reported exercise levels, and self-reported weights, over a 13-year period. We simply cannot reach any definitive conclusions from this study.
3) It is fundamentally incorrect to equate weight gain (or loss) to exercise levels. One's weight is determined by at least two essential factors: the amount of calories you burn, and the amount of calories you eat. It is entirely possible to lose weight with less than an hour of exercise a day, or to gain weight with more than an hour of exercise a day, depending on how much you eat.
This study threatens to cause the same kind of frustration that was caused a few years ago by an Institute of Medicine report that also recommended that we should all exercise for at least an hour a day. Namely, it might cause those of us who work pretty hard to exercise, but who, at the same time, are mere humans, to throw up our hands in utter frustration, and reach for the Twinkies. The fact is, we can gain a lot of benefit by exercising substantially less than an hour a day.
Lee IM, Djousse L, Sesso HD, et al. Physical Activity and Weight Gain Prevention. JAMA. 2010;303(12):1173-1179.