The one thing that might be said about exercise with certainty is that it tends to makes us hungry. Maybe not immediately, but eventually. Burn more calories and the odds are very good that we’ll consume more as well. And this simple fact alone might explain both the scientific evidence and a nation’s worth of sorely disappointing anecdotal experience.
It’s difficult to get health authorities to talk about the disconnect between their official recommendations and the scientific evidence that underlies it because they want to encourage us to exercise, even if their primary reason for doing so is highly debatable. Steve Blair, for instance, a University of South Carolina exercise scientist and a co-author of the AHA-ACSM guidelines, says he was “short, fat, and bald” when he started running in his thirties and he is short, fatter, and balder now, at age 68. In the intervening years, he estimates, he has run close to 80,000 miles and gained about 30 pounds.
When I asked Blair whether he thought he might be leaner had he run even more, he had to think about it. “I don’t see how I could have been more active,” he said. “Thirty years ago, I was running 50 miles a week. I had no time to do more. But if I could have gone out over the last couple of decades for two to three hours a day, maybe I would not have gained this weight.” And maybe he would have anyway. If we trust the AHA-ACSM report he co-authored, there is little reason to believe that the amount he runs makes any difference. Nonetheless, Blair personally believes he would be fatter still if he hadn’t been running. Why?
There was a time when virtually no one believed exercise would help a person lose weight. Until the sixties, clinicians who treated obese and overweight patients dismissed the notion as naïve. When Russell Wilder, an obesity and diabetes specialist at the Mayo Clinic, lectured on obesity in 1932, he said his fat patients tended to lose more weight with bed rest, “while unusually strenuous physical exercise slows the rate of loss.”
The problem, as he and his contemporaries saw it, is that light exercise burns an insignificant number of calories, amounts that are undone by comparatively effortless changes in diet. In 1942, Louis Newburgh of the University of Michigan calculated that a 250-pound man expends only three calories climbing a flight of stairs—the equivalent of depriving himself of a quarter-teaspoon of sugar or a hundredth of an ounce of butter. “He will have to climb twenty flights of stairs to rid himself of the energy contained in one slice of bread!” Newburgh observed. So why not skip the stairs, skip the bread, and call it a day?
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?
Credit for why we came to believe otherwise goes to one man, Jean Mayer, who began his career at Harvard in the early fifties, went on to become the most influential nutritionist in the country, and then, for sixteen years, served as president of Tufts University (where there is now a Jean Mayer USDA Human Nutrition Research Center on Aging). As an authority on human weight regulation, Mayer was among the very first of a new breed, a type that has since come to dominate the field. His predecessors—Wilder, Rony, Newburgh, and others—had all been physicians who worked closely with obese and overweight patients. Mayer was not. His training was in physiological chemistry; he had obtained a doctorate at Yale with a dissertation on the interrelationship of vitamins A and C in rats. In the ensuing decades, he would publish hundreds of papers on different aspects of nutrition, including why we get fat, but he never had to reduce obese patients as part of his clinical obligation, and so his hypotheses were less fettered by anecdotal or real-life experience.
As early as 1953, after just a few years of research on laboratory mice, Mayer began extolling the virtues of exercise for weight control. By 1959, the New York Times was crediting him with having “debunked [the] popular theories” that exercise played little role in weight control. Mayer knew that the obese often eat no more than the lean and occasionally even less. This seemed to exclude gluttony as a cause of their weight gain, which meant that these fat people had to be less physically active. Otherwise, how could they take in more calories than they expend and so become fat?