But Mayer’s hypothesis always had shortcomings. First, the association between reduced physical activity and obesity doesn’t tell us what is cause and what is effect. “It is a common observation,” noted Hugo Rony, “that many obese persons are lazy, i.e., show decreased impulse to muscle activity. This may be, in part, an effect that excess weight would have on the activity impulse of any normal person.” It’s also possible that both obesity and physical inactivity are the symptoms of the same underlying cause. This was a likely explanation for the inactivity and obesity that Mayer had observed in his laboratory mice. The same genetic mutation that rendered these mice sedentary could also have induced obesity (and perhaps diabetes).

Another problem, as we discussed in the last chapter, is that obesity is also associated with poverty, and even extreme poverty, and that should be a compelling argument against physical inactivity as a cause of the disease. Those who earn their living through manual labor tend to be the less advantaged members of societies in developed nations, and yet they will have the greatest obesity rates.

A third problem was the observation that exercise accomplishes little in the way of tilting the caloric balance when compared with a very modest restriction of intake—walking a few miles as opposed to eating one less slice of bread—and that increasing activity will increase appetite. Mayer ignored the comparison of intake and expenditure by focusing on expenditure alone. “For a long period the role of exercise in weight control was disregarded, if not actually ridiculed,” he wrote in a 1965 New York Times Magazine article. “One reason often advanced for this neglect is that ‘exercise consumes very little energy.’…Somehow the impression was given that any such exercise had to be accomplished in a single uninterrupted session. Actually, exercise does correspond to a caloric expenditure that can be considerable, and this expenditure will take place in a day or a decade.” And so the expenditure of calories by exercise, no matter how small, according to Mayer, would accumulate, leading to long-term weight reduction. This, of course, would be true only if the excess expenditure went unaccompanied by a compensatory increase in appetite and intake.

Mayer acknowledged that exercise could increase food intake, but said it wasn’t “necessarily” the case. This was the heart of Mayer’s hypothesis—a purported loophole in the relationship between appetite and physical activity. “If exercise is decreased below a certain point,” Mayer told the New York Times in 1961, “food intake no longer decreases. In other words, walking one-half hour a day may be equivalent to only four slices of bread, but if you don’t walk the half hour, you still want to eat the four slices….” Mayer based this conclusion on two of his own studies from the mid-1950s.*78 The first was with laboratory rats and purported to demonstrate that rats that are exercised for one to two hours every day will actually eat less than rats that don’t exercise at all. The second was a study of mill workers in West Bengal, India, and stands as a reminder that dreadful science can pass for seminal research in the field of obesity.