Unlike the reaction to Cleave’s hypothesis, which garnered little attention even after Cleave testified to McGovern’s Select Committee, the media pounced on the fiber hypothesis almost immediately. After Trowell published a pair of articles on fiber and heart disease in
Burkitt and Alec Walker followed up Trowell’s articles with an August 1974 review in
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.
“Burkitt’s hypothesis got accepted pretty well worldwide, quite quickly, but it has gradually been disproved,” said Richard Doll, who had endorsed the hypothesis enthusiastically in the mid-1970s. “It still holds up in relation to constipation, but as far as a major factor in the common diseases of the developed world, no, fiber is not the answer. That’s pretty clear.”
As we have seen with other hypotheses, the belief that dietary fiber is an intrinsic part of any healthy diet has been kept alive by factors that have little to do with science: in particular, by Geoffrey Rose’s philosophy of preventive medicine—that if a medical hypothesis has a chance of being true and thus saving lives, it should be treated as if it is—and by the need to give the public some positive advice about how they might prevent or reduce the risk of cancer. This was immediately evident in a
Burkitt’s hypothesis lived on, and it would continue to live, as the fat/ breast-cancer hypothesis continued to live on, in part because the original data that led to it remained unexplained: “Observational studies around the world,” wrote Byers, “continue to find that the risk of colorectal cancer is lower among populations with high intakes of fruits and vegetables and that the risk changes on adoption of a different diet, but we still do not understand why.” It would always be possible to suggest, as Byers had, that the trials could have been done differently—for longer or shorter duration, on younger subjects or older subjects, with more, less, or maybe a different kind of dietary fiber—and that the results would have been more promising. The American Cancer Society and the National Cancer Institute continued to suggest that high-fiber diets, high in fruits and vegetables, might reduce the risk of colon cancer, on the basis that some evidence existed to support the hypothesis and so a prudent diet would still include these ingredients.
The media would also contribute to keeping the fiber hypothesis alive, having first played a significant role in transforming Burkitt’s hypothesis into dogma without benefit of any meaningful long-term clinical trials. “Scientists have known for years that a diet rich in vegetables, fruits and fiber, and low in fat, can greatly reduce—or eliminate—the chances of developing colon cancer,” as a 1998
Although the
