Our culture of physical exercise began only in the late sixties, coincident with Mayer’s crusade, which explains why our parents might not have been quite so devoted to the idea of spending their leisure time perspiring profusely. In 1977, the New York Times was covering the “exercise explosion” that had come about because the conventional wisdom of the sixties that exercise was “bad for you” had been transformed into the “new conventional wisdom—that strenuous exercise is good for you.” When the Washington Post estimated in 1980 that 100 million Americans were partaking in the “new fitness revolution”—coincident with the start of the current obesity epidemic—it also noted that most of them “would have been derided as ‘health nuts’" only a decade earlier.
Meanwhile, the evidence simply never came around to support Mayer’s hypothesis, even though our beliefs did. My favorite study of the effect of physical activity on weight loss was published in 1989 by a team of Danish researchers. Over the course of eighteen months the Danes trained nonathletes to run a marathon. At the end of this training period, the eighteen men in the study had lost an average of five pounds of body fat. As for the nine women subjects, the Danes reported, “no change in body composition was observed.” That same year, F. Xavier Pi-Sunyer, then director of the St. Luke’s–Roosevelt Hospital Obesity Research Center in New York, reviewed the studies on exercise and weight, and his conclusion was identical to that of the Finnish review’s eleven years later: “Decreases, increases, and no changes in body weight and body composition have been observed,” Pi-Sunyer reported.
Granted, all this still doesn’t explain why we bought into Mayer’s idea that we could exercise more and not compensate by eating more. One simple reason is that the health reporters bought it, and we were reading their articles, not the research literature itself. In 1977, for instance, the National Institutes of Health hosted its second conference on obesity and weight control. “The importance of exercise in weight control is less than might be believed,” the assembled experts concluded, “because increases in energy expenditure due to exercise also tend to increase food consumption, and it is not possible to predict whether the increased caloric output will be outweighed by the greater food intake.” That same year, The New York Times Magazine reported that there was “now strong evidence that regular exercise can and does result in substantial and—so long as the exercise is continued—permanent weight loss.” By 1990, a year after Pi-Sunyer’s pessimistic assessment of the evidence, Newsweek was declaring exercise an “essential” element of any weight-loss program, and the Times had stated that on those infrequent occasions “when exercise isn’t enough” to lose weight, “you must also make sure you don’t overeat.”
As for the authorities themselves, the primary factor fueling their belief in the weight-maintaining benefits of exercise was their natural reluctance to acknowledge otherwise. Although one couldn’t help but be “underwhelmed by” the evidence, as Mayer’s student Judith Stern, a UC Davis nutritionist, wrote in 1986, it would be “shortsighted” to say that exercise was ineffective because it meant ignoring the possible contributions of exercise to the prevention of obesity and to the maintenance of weight loss that might be induced by diet. These, of course, had never been demonstrated either, but they hadn’t been ruled out. This faith-based philosophy came to dominate scientific discussions on exercise and weight, but it couldn’t be reconciled with the simple notion that appetite and calories consumed will increase with an increase in physical activity. Hence, the idea of working up an appetite was jettisoned. Clinicians, researchers, exercise physiologists, even personal trainers at the local gym took to thinking and talking about hunger as though it were a phenomenon exclusive to the brain, a question of willpower (whatever that is), not the natural consequence of a body trying to replenish itself with energy.
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.
For the last 60 years, researchers studying obesity and weight regulation have insisted on treating the human body as a thermodynamic black box: Calories go in one side, they come out the other, and the difference (calories in minus calories out) ends up as either more or less fat. The fat tissue, in this thermodynamic model, has nothing to say in the matter. Thus the official recommendations to eat less and exercise more and assuredly you’ll get thinner. (Or at least not fatter.) And in the strict sense this is true—you can starve a human, or a rat, and he will indeed lose weight—but that misses the point. Humans, rats, and all living organisms are ruled by biology, not thermodynamics. When we deprive ourselves of food, we get hungry. When we push ourselves physically, we get tired.
Our bodies, like all living organisms, have evolved a fantastically complex web of feedback loops. These physiological mechanisms serve fundamentally to work against the inevitable pull of thermodynamics (which is entropy, a.k.a. death) and so make life possible. The necessary condition of life, as the great French physiologist Claude Bernard noted 140 years ago, is to keep the internal environment of an organism stable and conducive to life, regardless of what’s happening on the outside. This is what the Harvard physiologist Walter Cannon, in the thirties, called homeostasis—or the “wisdom of the body,” as he put it. “Somehow the unstable stuff of which we are composed,” Cannon wrote, “had learned the trick of maintaining stability.”