“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 New England Journal of Medicine editorial that accompanied back-to-back April 2000 reports on two major trials—one on fourteen hundred subjects of the Phoenix [Arizona] Colon Cancer Prevention Physicians’ Network, and one $30 million trial from the National Cancer Institute—both of which confirmed that fiber had no effect on colon cancer. The editorial was written by Tim Byers, a professor of preventive medicine at the University of Colorado, who said that the two trials had been short-term and focused only on the early stages of cancer. For this reason, they should not be interpreted as “evidence that a high-fiber cereal supplement or a low-fat high-fiber diet is not effective in protecting against the later stages of development of colorectal cancer.” Byers was wrong, in that the results certainly were evidence that a high-fiber diet would not protect against the later stages of colorectal cancer; they simply weren’t sufficient evidence for us to accept the conclusion wholeheartedly as true.

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.