Good Calories, Bad Calories

The only evidence that Greenwald and his collaborators considered “indisputable” was that laboratory rats fed “a high-fat, high-calorie diet have a substantially higher incidence of mammary tumors than animals fed a low-fat, calorie-restricted diet.” In this they were right, but they did not rule out the possibility that it was the calories or whatever caused weight gain (what they implied by the adjective “high-calorie”) and not the dietary fat itself that was to blame, which was very likely the case. Even in 1982, when the authors of Diet, Nutrition, and Cancer had reviewed the animal evidence for fat-induced tumor growth, it had been less than indisputable. Adding fat to the diets of lab rats certainly induced tumors or enhanced their growth, but the most effective fats by far at this carcinogenesis process were polyunsaturated fats—saturated fats had little effect unless “supplemented with” polyunsaturated fats. This raised questions about the applicability of these observations to Western diets, which were traditionally low in polyunsaturated fats, at least until the 1960s, when the AHA started advocating polyunsaturated fats as a tool to lower cholesterol. Adding fat to rat chow also caused the rodents to gain weight, which was among the foremost reasons why obesity researchers came to believe that dietary fat caused human obesity. But it was hard to determine in these experiments whether it was the fat or the weight gain itself that led to increased tumor growth.

This laboratory evidence that dietary fat caused breast cancer began to evaporate as soon as Diet, Nutrition, and Cancer was published, and researchers could get funding to study it. By 1984, David Kritchevsky, one of the authors of Diet, Nutrition, and Cancer, had published an article in Cancer Research reporting on experiments that had been explicitly designed to separate out the effects of fat and calories on cancer, at least in rats. As Kritchevsky reported, low-fat, high-calorie diets led to more tumors than high-fat, low-calorie diets, and tumor production was shut down entirely in underfed rats, regardless of how fatty their diet was. Kritchevsky later reported that if rats were given only 75 percent of their typical daily calorie requirements, they could eat five times as much fat as usual and still develop fewer tumors. Mike Pariza of the University of Wisconsin had published similar results in 1986 in the Journal of the National Cancer Institute. “If you restrict calories just a little bit,” Pariza later said, “you completely wipe out this so-called fat enhancement of cancer.” This observation has been confirmed repeatedly. Demetrius Albanes of the National Cancer Institute later described the data as “overwhelmingly striking.” And he added: “Those data have very largely been ignored and strongly downplayed.”

By 1997 when the World Cancer Research Fund and American Institute for Cancer Research released a seven-hundred-page report titled Food, Nutrition and the Prevention of Cancer, the assembled experts could find neither “convincing” nor even “probable” reason to believe that fat-rich diets increased the risk of cancer. A decade later still, Arthur Schatzkin, chief of the nutritional epidemiology branch at the National Cancer Institute, described the accumulated results from those trials designed to test the hypothesis as “largely null.”

Nonetheless, the pervasive belief that eating fat causes breast cancer has persisted, partly because it once seemed undeniable. Purveyors of health advice just can’t seem to let go of the notion. When the American Cancer Society released its nutrition guidelines for cancer prevention in 2002, the document still recommended that we “limit consumption of red meats, especially those high in fat,” because of the same epidemiologic associations that had generated the fat-cancer hypothesis thirty years earlier. By 2006, with the next release of cancer-prevention guidelines by the American Cancer Society, the ACS was acknowledging that “there is little evidence that the total amount of fat consumed increases cancer risk.” But we were still advised to eat less fat and particularly meats (“major contributors of total fat, saturated fat and cholesterol in the American diet”), because “diets high in fat tend to be high in calories and may contribute to obesity, which in turn is associated with increased risks of cancers.” (Saturated fats, in particular, the ACS added, “may have an effect on increasing cancer risk,” a statement that seemed to be based solely on the belief that if saturated fat causes heart disease it probably causes cancer as well.)