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The basic problem is that while it's true that exercise burns calories and that you must burn calories to lose weight, exercise has another effect: it can stimulate hunger. That causes us to eat more, which in turn can negate the weight-loss benefits we just accrued. Exercise, in other words, isn't necessarily helping us lose weight. It may even be making it harder.
The Compensation Problem
Earlier this year, the peer-reviewed journal PLoS ONE--PLoS is the nonprofit Public Library of Science--published a remarkable study supervised by a colleague of Ravussin's, Dr. Timothy Church, who holds the rather grand title of chair in health wisdom at LSU. Church's team randomly assigned into four groups 464 overweight women who didn't regularly exercise. Women in three of the groups were asked to work out with a personal trainer for 72 min., 136 min., and 194 min. per week, respectively, for six months. Women in the fourth cluster, the control group, were told to maintain their usual physical-activity routines. All the women were asked not to change their dietary habits and to fill out monthly medical-symptom questionnaires.
The findings were surprising. On average, the women in all the groups, even the control group, lost weight, but the women who exercised--sweating it out with a trainer several days a week for six months--did not lose significantly more weight than the control subjects did. (The control-group women may have lost weight because they were filling out those regular health forms, which may have prompted them to consume fewer doughnuts.) Some of the women in each of the four groups actually gained weight, some more than 10 lb. each.
What's going on here? Church calls it compensation, but you and I might know it as the lip-licking anticipation of perfectly salted, golden-brown French fries after a hard trip to the gym. Whether because exercise made them hungry or because they wanted to reward themselves (or both), most of the women who exercised ate more than they did before they started the experiment. Or they compensated in another way, by moving around a lot less than usual after they got home.
The findings are important because the government and various medical organizations routinely prescribe more and more exercise for those who want to lose weight. In 2007 the American College of Sports Medicine and the American Heart Association issued new guidelines stating that "to lose weight ... 60 to 90 minutes of physical activity may be necessary." That's 60 to 90 minutes on most days of the week, a level that not only is unrealistic for those of us trying to keep or find a job but also could easily produce, on the basis of Church's data, ravenous compensatory eating.
It's true that after six months of working out, most of the exercisers in Church's study were able to trim their waistlines slightly--by about an inch. Even so, they lost no more overall body fat than the control group did. Why not?