The October 1, 2007 issue of New York magazine informs us that there is practically no scientific evidence for the proposition that increased exercise is an effective long-term strategy for fighting obesity. In “The Scientist and the Stairmaster,” Gary Taubes describes how completely the conventional wisdom on obesity has shifted from what it was in the 1940s:
More-strenuous exercise, these physicians further argued, doesn’t help matters—because it works up an appetite. “Vigorous muscle exercise usually results in immediate demand for a large meal,” noted Hugo Rony of Northwestern University in his 1940 textbook, Obesity and Leanness. “Consistently high or low energy expenditures result in consistently high or low levels of appetite. Thus men doing heavy physical work spontaneously eat more than men engaged in sedentary occupations. Statistics show that the average daily caloric intake of lumberjacks is more than 5,000 calories, while that of tailors is only about 2,500 calories. Persons who change their occupation from light to heavy work or vice versa soon develop corresponding changes in their appetite.” If a tailor becomes a lumberjack and, by doing so, takes to eating like one, why assume that the same won’t happen, albeit on a lesser scale, to an overweight tailor who decides to work out like a lumberjack for an hour a day?
Since perhaps the 1980s, conventional wisdom has been all on the other side, yet empirical testing has failed to bear it out. The gist of the article is that while obesity does tend to be highly correlated with sedentary lifestyles, there is no telling which one causes which — or indeed, whether they might both be the twin effects of some other cause we do not yet understand very well.
Ultimately, the relationship between physical activity and fatness comes down to the question of cause and effect. Is Lance Armstrong excessively lean because he burns off a few thousand calories a day cycling, or is he driven to expend that energy because his body is constitutionally set against storing calories as fat? If his fat tissue is resistant to accumulating calories, his body has little choice but to burn them as quickly as possible: what Rony and his contemporaries called the “activity impulse”—a physiological drive, not a conscious one. His body is telling him to get on his bike and ride, not his mind. Those of us who run to fat would have the opposite problem. Our fat tissue wants to store calories, leaving our muscles with a relative dearth of energy to burn. It’s not willpower we lack, but fuel.
There are, of course, many good reasons to exercise, including sheer enjoyment. Tim Peach, I believe exercises mainly in the hope of beating me up some day. But if Taubes is right, Tim will have to bring his A game, because he will find me rather heavier than he will ever be.