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.