Burkitt then spent the next decade lecturing on the dangers of fiber-poor diets. He would condemn modern diets equally for their “catastrophic drop in starch,” for their high fat content—“We eat three times more fat than communities with a minimum prevalence of [Western] diseases,” he would say; “We must reduce our fat!”—and for their lack of fiber, which he considered “the biggest nutritional catastrophe in [the United Kingdom] in the past 100 years.”

Not everyone bought into it. For public-health authorities and health reporters, dietary fat and/or cholesterol continued to be the prime suspects in chronic disease, and dietary fat had already been linked through international comparisons to colon cancer, as well as breast cancer. Burkitt recalled memorable disputes with researchers in the United States who blamed colon cancer on dietary fat, but he insisted that the absence of fiber was responsible. Eventually, they compromised. His opponents, said Burkitt, conceded “that the fact that fat happened to be causative…did not preclude the possibility that fiber might be protective.” Harvard nutritionist Jean Mayer also discounted the significance of fiber, after Burkitt, Walker, and Trowell’s early papers sparked the “furor over fiber” in the United States. But then Mayer, too, saw the wisdom of compromise. The ideal diet, he noted, would minimize the risk of both heart disease and cancer. It would be low in fat, or at least low in saturated fat, and so would be low in meat and dairy products. And it would be high in fiber. “A good diet,” Mayer wrote, “high in fruits and vegetables and with a reasonable amount of undermilled cereals—will give all you need of useful fiber.” The assumption that it would lead to long life and good health, however, was based more on faith and intuition than on science.

Over the last quarter-century, Burkitt’s fiber hypothesis has become yet another example of Francis Bacon’s dictum of “wishful science”—there has been a steady accumulation of evidence refuting the notion that a fiber-deficient diet causes colon cancer, polyps, or diverticulitis, let alone any other disease of civilization. The pattern is precisely what would be expected of a hypothesis that simply isn’t true: the larger and more rigorous the trials set up to test it, the more consistently negative the evidence. Between 1994 and 2000, two observational studies—of forty-seven thousand male health professionals and the eighty-nine thousand women of the Nurses Health Study, both run out of the Harvard School of Public Health—and a half-dozen randomized control trials concluded that fiber consumption is unrelated to the risk of colon cancer, as is, apparently, the consumption of fruits and vegetables. The results of the forty-nine-thousand-women Dietary Modification Trial of the Women’s Health Initiative, published in 2006, confirmed that increasing the fiber in the diet (by eating more whole grains, fruits, and vegetables) had no beneficial effect on colon cancer, nor did it prevent heart disease or breast cancer or induce weight loss.