The Fiber Myth

The fiber hypothesis is based on the study of a single investigator named Denis Burkitt, former missionary surgeon in Uganda. He proposed that a diet high in fiber, as he observed among native populations in Africa, is the key to prevent chronic diseases especially colon and rectal cancer.

On the other side of the debate were Cleave and Yudkin, who proposed that the addition of sugar and refined carbohydrates to a traditional diet, is to blame for the development of digestive cancers and other chronic diseases.

By the late 1970s we had 2 opposing theories. Is it:
the absence of fiber
OR
the presence of sugar and refined carbohydrates in the diet
is the cause of the appearance of chronic diseases?

Between 1994 and 2000, there were two observational studies and a few randomized control trials to test Burkitt's fiber hypothesis. A few examples:

The Nurses Health Study, from the Harvard School of Public Health, where they examined diet and disease relationship in 89,000 nurses and 47,000 male health professionals around the United States. The "Study found no protective effect of dietary fiber on the development of either colorectal adenomas or carcinomas." Consumption of a fiber rich diet is unrelated to the risk of colon cancer.

The EPIC study involved more than 500,000 individuals from 10 countries and found that total dietary fibre consumption was inversely proportional to colorectal cancer risk. This association was stronger for colon cancer, particularly left sided, rather than for rectal cancer.

In 2006, the Women's Health Initiative trial of 49,000 women confirmed that whole grains, fruits and vegetables had no beneficial effect on colon cancer, nor did it prevent heart disease or breast cancer.

Despite all these conclusions the fiber hypothesis is still very much alive, even though it has been disproved many times since the 1970s.

Burkitt's theory failed to explain other populations relative lack of chronic diseases in "carnivorous" isolated populations:

"How to explain the absence of these chronic diseases in cultures whose traditional diets contained predominantly fat and protein and little or no plant foods and thus little or no fiber - the Masai and the Samburu, the Native Americans of the Great Plains, the Inuit? And why did chronic diseases begin appearing in these populations only with the availability of Western diets, if they weren't eating copious fiber prior to this nutrition transition?"
                                                                              - Gary Taubes

I believe Cleave and Yudkin. A high sugar/refined carbohydrate diet - rather that the absence of fiber - is to blame for the appearance of chronic diseases in these native populations. (Great article here.) You can also have more information about why we are more adapted to a high fat diet, rather that a high fiber/high fruits and vegetables diet, if you go to Barry Groves blog.