ARCHIVED - Health Canada Scientific Summary on the U.S. Health Claim Regarding Dietary Fibre, Grain Products and Cancer
Bureau of Nutritional Sciences
Food Directorate, Health Products and Food Branch
Since the U. S. health claim pertaining to a diet rich in fibre-containing grain products, fruits, and vegetables was accepted in 1993, a considerable amount of research has been published examining the relationship between cancer and consumption of dietary fibre and cereal foods. Four randomized dietary intervention trials have been conducted in the past few years with patients diagnosed with adenomatous polyps, presumed precursors of colorectal cancer, and several cohort studies have been completed. These studies extended the extensive animal literature that examined the potential link between consumption of dietary fibre and a reduced risk of cancer.
A Medline search was conducted for the period 1993-2000 that included the terms "dietary fiber AND (grains OR cereals OR legumes) AND neoplasms NOT (fruit OR citrus OR vegetables)". A further Medline search was conducted for the period January 1996 to April 2000 using the following key words: cereals, whole grains, neoplasms, and human. The search was expanded to Medline, CAB, Food Science and Technology Abstracts and EMBase for the terms "whole grain OR wheat OR oat OR rye OR barley OR rice OR corn OR cereals" and "cancer OR tumour OR neoplasm OR neoplasms". These articles were screened by title/key words/abstracts for relevance to the subject. All articles were in English or French. Several expert consensus reports were identified including the World Cancer Research Fund/American Institute for Cancer Research 1997 report. The review focussed on this and research conducted since 1996, including 4 controlled trials, 9 cohort studies, 16 case-control studies and 5 ecological studies.
Cancer is a major public health problem affecting approximately 40% of males and 36% of females over their lifetimes. Cancer incidence increases dramatically with age: between the ages of 40-69, the risk of cancer more than doubles for every 10 additional years of age. Risk factors for cancer include non-dietary environmental factors, lifestyle factors, and genetic pre-disposition. However, dietary factors also play a major role.
The fundamental question addressed by this report is whether increasing consumption of diets high in fibre and of cereal foods can have a significant effect on lowering the risk of cancer. The relationship between fruit and vegetable consumption and risk of cancer has been addressed in a previous report (Health Canada, 2000) where it was concluded that sufficient scientific evidence existed to support a Health Claim stating that diets high in fruit and vegetable may lower the risk of some cancers. Thus, the present report focuses on dietary fibre and, specifically, cereal foods.
The evidence presented in the current report does not consistently support a health claim linking dietary fibre, grain products and a lower risk of cancer. Three different expert panels have examined this question and have arrived at different conclusions regarding the strength of the association between grain foods, dietary fibre and cancer. Literature review articles are equally divided about the strength of the association. For colorectal cancer, four randomized intervention trials have failed to demonstrate an effect of diets high in fibre and/or grain supplements on the recurrence of adenomatous polyps in patients with a history of such polyps. In addition, most prospective cohort studies found no effect of grain consumption on cancer risk. The data also fail to support a convincing effect of dietary fibre on cancer.
Part of the problem in determining whether a relationship exists between dietary fibre or grain products and cancer has been methodological. Several different methods have been used to measure the dietary fibre content of foods in nutritional databases. In addition, it is possible that associations between cereal intake and cancer are influenced by methods of processing of the cereals, a factor which has not been assessed consistently or extensively in the literature. An effect of processing is suggested both by plausible biologic mechanisms and by four case-control studies that examined the link between refined grain intake and risk of colon cancer, all of which found a statistically significant higher risk of colon cancer with increasing refined grain intake. These considerations, together with the lack of consensus from expert panels and the literature, indicate that it would be premature at this time, to support a Health Claim that indicates a link exists between cancer risk and consumption of dietary fibre or cereal foods.
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