Let us begin with a short quiz: a few questions to ponder during the 30 (or 60 or 90) minutes a day you spend burning off excess calories at the gym, or perhaps while feeling guilty because you’re not so engaged. If lean people are more physically active than fat people�one fact in the often-murky science of weight control that’s been established beyond reasonable doubt� does that mean that working out will make a fat person lean? Does it mean that sitting around will make a lean person fat? How about a mathematical variation on these questions: Let’s say we go to the gym and burn off 3,500 calories every week�that’s 700 calories a session, five times a week. Since a pound of fat is equivalent to 3,500 calories, does that mean we’ll be a pound slimmer for every week we exercise? And will we continue to slim down at this pace for as long as we continue to exercise?
For most of us, fear of flab is the reason we exercise, the motivation that drives us to the gym. It’s also why public-health authorities have taken to encouraging ever more exercise as part of a healthy lifestyle. If we’re fat or fatter than ideal, we work out. Burn calories. Expend energy. Still fat? Burn more. The dietary guidelines of the U.S. Department of Agriculture, for instance, now recommend that we engage in up to 60 minutes daily of �moderate to vigorous intensity� physical activity just to maintain weight�that is, keep us from fattening further. Considering the ubiquity of the message, the hold it has on our lives, and the elegant simplicity of the notion�burn calories, lose weight�wouldn’t it be nice to believe it were true? The catch is that science suggests it’s not, and so the answer to all of the above quiz questions is �no.�
Just last month, the American Heart Association and the American College of Sports Medicine published joint guidelines for physical activity and health. They suggested that 30 minutes of moderate physical activity five days a week is necessary to �promote and maintain health.� What they didn’t say, though, was that more physical activity will lead us to lose weight. Indeed, the best they could say about the relationship between fat and exercise was this: �It is reasonable to assume that persons with relatively high daily energy expenditures would be less likely to gain weight over time, compared with those who have low energy expenditures. So far, data to support this hypothesis are not particularly compelling.� In other words, despite half a century of efforts to prove otherwise, scientists still can’t say that exercise will help keep off the pounds.
The 30 minutes recommended by the AHA-ACSM report is a departure from the recent guidelines of other authoritative organizations�the Institute of Medicine of the National Academies and the International Association for the Study of Obesity�both of which, like the USDA, have recommended that we exercise for up to 60 minutes a day to avoid what the USDA calls �unhealthy weight gain.� But the reason for this 60-minute recommendation is precisely that so little evidence exists to support the notion that exercising less has any effect.
The report that these experts cite most often as grounds for their assessments was published in 2000 by two Finnish researchers who surveyed all the relevant research on exercise and weight of the previous twenty years. Yet the Finnish report, the most scientifically rigorous review of the evidence to date, can hardly be said to have cleared up the matter. When the Finnish investigators looked at the results of the dozen best-constructed experimental trials that addressed weight maintenance�that is, successful dieters who were trying to keep off the pounds they had shed�they found that everyone regains weight. And depending on the type of trial, exercise would either decrease the rate of that gain (by 3.2 ounces per month) or increase its rate (by 1.8 ounces). As the Finns themselves concluded, with characteristic understatement, the relationship between exercise and weight is �more complex� than they might otherwise have imagined.
This is not to say that there aren’t excellent reasons to be physically active, as these reports invariably point out. We might just enjoy exercise. We may increase our overall fitness; we may live longer, perhaps by reducing our risk of heart disease or diabetes; we’ll probably feel better about ourselves. (Of course, this may be purely a cultural phenomenon. It’s hard to imagine that the French, for instance, would improve their self-esteem by spending more time at the gym.) But there’s no reason to think that we will lose any significant amount of weight, and little reason to think we will prevent ourselves from gaining it.