Nonetheless, the view of exercise as a panacea for excess weight soon became conventional wisdom. “Diligent exercisers tend to lose weight,” was how a Washington Post article on the fitness revolution phrased it in 1980. No source for this claim was deemed necessary. All doubts about whether the weight-reducing benefits of exercise actually existed were left behind. In 1983, Jane Brody of the New York Times was counting the numerous ways in which exercise was “the key” to successful weight loss. Exercise, she explained, increases metabolism for hours afterward, which further increases caloric expenditure. It is also “an appetite suppressant, sometimes delaying the return of hunger for hours.” Exercise builds up muscle tissue, Brody said, which in turn burns more calories than fat. And muscle tissue is denser than fat, Brody concluded, “so even if you do not lose any weight, exercise will make you trimmer.” By the end of the decade, as Newsweek observed, exercise was now considered “essential” to any weight-loss program. In 1989 the New York Times counseled readers that, on those infrequent occasions “when exercise isn’t enough” to induce sufficient weight loss, “you must also make sure you don’t overeat.”

The press may have been convinced, but the scientific evidence never supported Mayer’s hypothesis. In October 1973, when the National Institutes of Health hosted its first conference on obesity, Per Björntorp, a Swedish investigator, reported about his own clinical trials on obesity and exercise. After six months of a thrice-weekly exercise program, his seven obese subjects remained both as heavy and as fat as ever. Four years later, when the NIH again hosted a conference on obesity, the conference report concluded that “the importance of exercise in weight control is less than might be believed, 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.” In 1989, when Xavier Pi-Sunyer, director of the Obesity Research Clinic at St. Luke’s–Roosevelt Hospital Center in New York, reviewed the evidence that exercise “without caloric restriction” could lead to weight loss, he still found little reason for optimism, despite what the press was now claiming as gospel. “Decreases, increases, and no changes in body weight and body composition have been observed,” Pi-Sunyer noted. That same year, Danish investigators reported that they had indeed trained previously sedentary individuals to run marathons (26.2 miles). At the end of this eighteen-month training period—a time of almost fanatic exercise—the eighteen men in the study had lost an average of five pounds of body fat. “No change in body composition was observed” among the nine female subjects.