To discuss cancer, we need to first return to the subject of cancer in isolated populations eating traditional diets. The modern incarnation of these observations begins with John Higginson, who was the founding director of the World Health Organization’s International Agency for Research on Cancer (IARC), a position he would hold for two decades. In the 1950s, Higginson studied cancer incidence in native African populations and compared them with incidence in the United States and Denmark, the two nations for which equivalent data existed. With a few exceptions, Higginson reported, cancer in African natives was remarkably uncommon. This led Higginson to conclude that most human cancers were caused by environmental factors, and that diet and lifestyle factors were the primary suspects. “It would seem, therefore, that the majority of human cancer is potentially preventable,” as the World Health Organization concluded in 1964, a view that evolved into the new orthodoxy.

Cancer epidemiologists then tried to establish what proportion of cancers these might be. Higginson suggested 70 to 80 percent of all cancers could be prevented; others said as many as 90 percent. In 1981, the Oxford epidemiologists Richard Doll and Richard Peto published the seminal work on this subject: a 120-page analysis in the Journal of the National Cancer Institute that reviewed the existing evidence on changes in cancer incidence over time, changes upon migration from one region of the world to another, and differences in cancer rates between communities and nations. (Colon cancer, for example, was ten times more common in rural Connecticut than in Nigeria; breast cancer was diagnosed eight times more often in British Columbia than in the non-Jewish population of Israel.) Based on this evidence, Doll and Peto concluded that at least 75 to 80 percent of cancers in the United States might be avoidable with appropriate changes in diet and lifestyle.

In the quarter-century since Doll and Peto published their analysis, it has been cited in nearly two thousand journal articles, and yet the fundamental implications have been largely lost. The two most important conclusions in their analysis were that man-made chemicals—in pollution, food additives, and occupational exposure—play a minimal role in human cancers, and that diet played the largest role—causing 35 percent of all cancers, though the uncertainties were considered so vast that the number could be as low as 10 percent or as high as 70 percent.