The fiber hypothesis captured the public’s nutritional consciousness by virtue of the messianic efforts of a single investigator, a former missionary surgeon named Denis Burkitt, who proposed that this indigestible roughage was a requisite component of a healthy diet. The notion was consistent with Keys’s hypothesis, which was not the case with Cleave’s or Yudkin’s hypothesis, and it resonated also with the era’s countercultural leanings toward diets heavy in vegetables, legumes, and cereal grains.
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.”