By this time, though, exercise had a profoundly influential proponent: the Harvard nutritionist Jean Mayer, who would almost single-handedly overturn a century of clinical evidence and anecdotal experience. In the 1950s, when Mayer established himself as the leading authority on obesity in the United States, he did so based more on the romance of his background than his expertise as a clinical scientist: he was the son of the famous French physiologist André Mayer, and he had fought in the French resistance during World War II.

Mayer represented a new breed of obesity authority, of a kind that would now come to dominate the field. His predecessors—among them Louis Newburgh, Hugo Rony, Hilde Bruch, Frank Evans, Julius Bauer, and Russell Wilder—had all been physicians who worked closely with obese patients. Collectively, they had treated thousands of them. Their views on the cause of obesity differed, often radically, but their firsthand experience was unquestionable. Mayer was not a clinician. His training was in physiological chemistry; he had obtained a doctorate at Yale 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 obesity, but he never treated obese patients, so his hypotheses were less fettered by any anecdotal or real-life experience.

As early as 1953, after just two years of research on genetically obese mice, Mayer was extolling the virtues of exercise for weight control. By the end of the decade, he was getting credit from the New York Times for having “debunked” the “popular theories,” argued by clinicians and their obese patients, that exercise had little influence on weight. Mayer knew that the obese often eat no more than the lean, and often even less. This seemed to exclude overeating, which meant the obese had to be less physically active. Otherwise, how could they have achieved positive energy balance and become obese? Mayer himself first reported this phenomenon in a strain of laboratory mice that were prone to both obesity and diabetes. They ate little more than their lean littermates, he noted, but their activity was “almost nil” this sedentary behavior could explain their propensity to grow fat.

Through the 1960s, Mayer documented this relationship between energy intake, inactivity, and obesity in a series of human studies. He noted that high-school girls who were overweight ate “several hundred calories less” than those who weren’t. “The laws of thermodynamics, however, were not flouted by this finding,” he said, because the obese girls spent only a third as much time in physical activity as the lean girls; they spent four times as many hours watching television. Mayer studied adolescent girls at summer camp and reported that the obese girls expended “far less energy,” even during scheduled exercise periods, than their non-obese counterparts. He also studied infants. “The striking phenomenon is that the fatter babies were quiet, placid babies that had moderate intake,” Mayer reported, “whereas the babies who had the highest intake tended to be very thin babies, cried a lot, moved a lot, and became very tense.” Thus, Mayer concluded, “some individuals are born very quiet, inactive, and placid and with moderate intake get fat, and some individuals from the very beginning are very active and do not get particularly fat even with high intake.”