In the summer of 1969, Burkitt began studying stool characteristics in available subjects. “Finished bowel transit tests on family,” he recorded in his diary on July 4. The following month, he visited Alec Walker, who ran the human biology department at the South African Institute of Medical Research. Walker had been studying the rising tide of chronic diseases in urban Bantus in South Africa since the late 1940s, and he was the rare investigator who shared with Burkitt an interest in human feces and constipation. Walker had done extensive studies linking the relative lack of constipation among black convicts in the local prison, as well as the lack of appendicitis in the Bantus at large, to their traditional high-fiber diets. (Walker publicly dismissed the hypothesis that sugar or refined carbohydrates caused heart disease, but he also reported that the Bantus developed chronic disease only after they moved into the city and began consuming “more white bread, sugar, soft drinks and European liquor.”) Walker had also just submitted an article to the British Medical Journal linking the very low mortality rates from colon cancer among the Bantus to their bowel motility, a characteristic, he wrote, that was “largely lost” among Western societies. Walker’s research gave Burkitt the confidence to devote his efforts to the study of stool characteristics and bowel behavior, hoping to associate in a scientific manner fiber deficiency, constipation, and the presence of chronic diseases.

It was precisely this work that led to the fiber hypothesis and its present place in our nutritional consciousness. In 1972, Burkitt and Walker published an article in The Lancet supporting their theory and discussing their measurements of transit time and stool characteristics in twelve hundred human subjects. In rural areas, unaffected by industrialization, they reported, “diets containing the natural amount of fiber are eaten and result in large, soft stools that traverse the intestine rapidly. By contrast, the refined low-fiber foods of the economically-developed countries produce small firm stools which pass through the gut very slowly.” Thus, the relative constipation endemic in the developed world, they suggested, appeared to play a causative role in bowel-related disorders: appendicitis, diverticulitis, and both benign and malignant tumors of the colon and rectum, all of which showed the classic distribution of diseases of civilizations. “All these diseases are very closely associated epidemiologically,” Burkitt and Walker explained. “These diseases are still rare in developing countries and in rural Japan, where eating habits have changed but little, but they are all seen increasingly in Japanese who live in Hawaii and California and are increasing in Japan in those who have changed to a Western diet. In no country or region is one of these diseases common and the others rare save that appendicitis, which afflicts the young, appears about a generation before the other conditions.”