Dietary Fiber - Even the Experts are Confused
~ March 1999 No.66 ~
A recent article published in the New England Journal of Medicine has even the experts confused. For some time now it has been accepted by most nutrition and medical researchers that fiber in the diet is good for a couple of reasons. The role of fiber as an agent to lower blood cholesterol has been controversial for some years. However, over time we learned more about the different types of fiber and their different properties. It is now clear that fiber like that found in oat bran can lower blood cholesterol levels and reduce the risk of atherosclerosis. US regulators have even allowed structure / function claims relating fiber and heart disease to appear on food labels.
A second role of dietary fiber concerns its impact on protecting against colon cancer. It was felt that adding sources of fiber to the diet increased fecal bulk, might promote the growth of beneficial bacteria in the lower gut, and generally improved the digestive process. It wasn't clear, but it appeared that the levels of carcinogens in the lower digestive tract were also reduced and this it was believed, reduced the incidence of colon cancer. Some epidemiology studies seemed to support this conclusion.
Now, however, in a study of 88,757 women aged 34-59, US researchers have concluded that there is no statistical evidence that there is a protective effect of dietary fiber against colorectal cancer. Neither total dietary fiber, nor fiber from cereals, fruits or vegetables alone was found to have an impact on the risk of colon cancers.
Is this another French wine paradox or have we been wrong all along to think that dietary fiber could pass down the GI tract and flush out carcinogens? Even the experts are baffled. In an editorial in the same issue as the article reporting no benefits of fiber against colon cancer, Dr. J. Potter poses several explanations as to why the beneficial effects of dietary fiber may not be easy to prove. He cites the example the of experiments to demonstrate the relation between fat and heart disease and calls for more epidemiological and clinical trials. Like fat, fiber is not one compound. It will be necessary to better define the term fiber in the diet. Also lacking at present are good food composition tables that can be used to more accurately calculate the amount of total or individual types of fiber in diets of subjects that have filled out diet summary questionnaires. Epidemiological studies like the one reported in the New England Journal of Medicine base their conclusions on such calculations. Right now they are using the best data available, but it's not good enough.
There is every reason to think that fiber is an important part of a healthy diet. Even the experts agree that, in spite of this apparent set back, grains, fruits and vegetables should be eaten every day.
New England Journal of Medicine