Good Calories, Bad Calories

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 Bjorntorp, 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.

Throughout this period, the research in laboratory animals was equally unsupportive of Mayer’s hypothesis. Male rats might actually limit their food intake after running for hours on a running wheel, as Mayer had suggested was possible, but they ate more on days when they didn’t exercise. They also made up for the exercise by moving less at other times. Moreover, these rats had to be forced to exercise to suppress hunger even temporarily; it did not happen voluntarily. In Mayer’s experiments, the rats were put on a motorized treadmill; they ran because they had no choice. This suggested that any decrease in appetite observed in these less-than- voluntary exercise experiments might have been induced by either stress or exhaustion rather than the exercise itself, and particularly by the use of what are technically known as shock grids to “motivate” the rats. In those experiments that relied on voluntary physical activity, the more the rats ran, the more the rats ate, and weights remained unchanged. When the rats were retired from forced-exercise programs, they ate more than ever and gained weight “more rapidly” than those rats that had been allowed to remain sedentary. With hamsters and gerbils, voluntary running activity produced “permanent increases” in body weight and adiposity— exercising made these rodents fatter, not leaner.

If Mayer’s hypothesis was true, if physical activity played a meaningful role in weight regulation, then researchers’ increasing interest in demonstrating this fact should have led, over the decades, to an unambiguous demonstration that this was the case. On the contrary. “When surveying the scientific literature on the treatment of obesity one cannot help but come away…under whelmed by the minor contribution of exercise to most weight-loss programs,” University of California, Davis, nutritionist Judith Stern, who had obtained her doctorate at Harvard with Mayer, wrote in 1986.

In the past few years, a series of authoritative reports have advocated ever more physical activity for adults— now up to ninety minutes a day of moderate-intensity exercise—but they have done so precisely because the evidence in support of the hypothesis is so unimpressive. No substantial evidence in fact supports this recommendation for weight loss or maintenance.

These reports, from the USDA and others, rely for their conclusions on a handful of systematic reviews of the medical literature that have been published over the past decade. The most comprehensive of these, and the one cited most frequently by these authoritative reports, is a 2000 analysis by two Finnish investigators. The Finnish review reveals that only a dozen or so clinical trials exist that test the benefits of exercise to maintain weight. The great proportion of the studies are observational studies, which survey the amount of physical activity reported by individuals in various populations and then compare this with how much weight these people gain over a certain period of time. These studies—like the famous Framingham Heart Study—are capable only of identifying associations, not cause and effect, and even these associations are inconsistent. Some studies imply that physical activity might inhibit weight gain, the Finnish investigators report; some that it might accelerate weight gain; and some that it has no effect whatsoever. The clinical trials were equally inconsistent. When the Finnish investigators tried to quantify the results of the dozen trials that addressed the effect of an exercise program on weight maintenance, or what the USDA describes as preventing “unhealthy weight gain,” they concluded, depending on the type of trial, that it either led to a decrease of 90 grams (3.2 ounces) per month in weight gained or regained, or to an increase of 50 grams (1.8 ounces). Because “the more rigorous study designs (randomized trials)” yielded the least impressive results, the authors noted, the association between physical activity and weight change, even if it existed, was “more complex” than they might otherwise have assumed. This last point is crucial.

If we consider the last forty years of research as a test of Mayer’s hypothesis that physical activity induces weight loss or even inhibits weight gain, it’s clear the hypothesis leads nowhere meaningful. What Mayer initially insisted had to be true, so much so that he publicly accused the “enemies of exercise” of propagating “pseudo-science,” had devolved over the intervening decades into an analysis of whether the prescription of an exercise program would inhibit weight gain by three ounces each month or accelerate it by two.

The fact that appetite and thus calories consumed will increase to compensate for physical activity, however, was lost along the way. Clinicians, public-health authorities, and even exercise physiologists had taken to thinking and talking about hunger as though it were a phenomenon that was exclusive to the brain, a question of willpower rather than the natural consequence of a physiological drive to replace whatever calories may have been expended. When we are physically active, we work up an appetite. Hunger increases in proportion to the calories we expend, just as restricting the calories in our diet will leave us hungry until we eventually make good the deficit, if not more. The evidence suggests that this is true for both the fat and the lean. It is one of the fundamental observations we have to explain if we’re to understand why we gain weight and how to lose it.

Chapter Sixteen

PARADOXES

The literature on obesity is not only voluminous, it is also full of conflicting and confusing reports and opinions. One might well apply to it the words of Artemus Ward: “The researches of so many eminent scientific men have thrown so much darkness upon the subject that if they continue their researches we shall soon know nothing.”