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WILL INCREASING THE INTAKE OF DIETARY FIBER REDUCE THE RISK OF BOWEL CANCER?

(February 2001)

Thirty years ago, epidemiologic data suggested that greater amounts of insoluble fiber, particularly from grains, decreased the risk of bowel cancer. Despite a paucity of persuasive data, that dictum (more fiber, less bowel cancer) became virtually public health and nutrition gospel. Three studies now strongly challenge that belief. The first and most pertinent of the three was published in The New England Journal of Medicine in 1999. It is another report from the Nurses’ Health Study that started in 1976 and has enrolled more than 120,000 nurses who periodically answer questionnaires. The present analysis is based on 88,757 women ages 34 to 59 years followed for sixteen years. During that period, 787 women developed bowel cancer and another 1,012 women were found to have intestinal polyps (benign growths) that could become cancerous. They were divided into five groups of fiber intake according to a dietary questionnaire administered in 1980. The fiber intake ranged from a daily average of 10 grams in the lowest group (less than in the average American diet) to a high of 25 grams (about double the average American intake). There was no relationship between total fiber intake and likelihood of bowel cancer or polyps. When individual fiber types were considered - from cereals, vegetables, and fruits - there was no evidence cereal fiber had an influence on bowel cancer risk. Fruit fiber may have had a small protective effect, but vegetable fiber appeared to possibly increase the risk.

The authors conclude "our data do not support the existence of an important protective effect of dietary fiber against bowel cancer."

The other two studies were published together in the April 6, 2000 issue of The New England Journal of Medicine. The two studies together included more than 3,000 men and women who had a history of intestinal polyps; one-half were placed on a high fiber diet and all were followed for three to four years looking for new polyps. There was no evidence that a high fiber diet prevented the development of new polyps.

Commentary: These studies have been greeted with dismay because they go against the standard recommendations to include in the range of 25 grams of fiber, particularly insoluble fiber from wheat-based cereals and breads, fruits and vegetables, in the diet. Although the validity of the studies has been questioned, the problem is not with the studies, but with the pronouncements over the last thirty years of nutrition policy makers, nutritionists, cancer specialists, and others who told the public that including fiber in the diet would definitely provide significant protection against bowel cancer. The evidence was never persuasive. Healthful Life has maintained consistently for the last fifteen years that the protective effect of fiber in bowel cancer was unproven.

The argument that vegetable fiber increases the risk of bowel cancer looks weak. This was not found with polyps, and there are some inconsistencies in the data in the report. For example, the alleged risk for bowel cancer increased as vegetable fiber intake increased from the lowest to the middle intake categories, but then progressively decreased in the two highest fiber intake groups. One would expect, if the findings were really significant, that the greater the vegetable fiber intake, the greater the risk. This aspect of the report should not be taken seriously.



for the lack of protection against potentially dangerous polyps or bowel cancer by dietary fiber . These are important studies. They should be a lesson for policy makers and others - do not make claims for health promotion and disease prevention that are based on insufficient evidence. If you have to reverse or withdraw your pronouncements, the public will lose faith in anything you say, and that would be terrible for the cause of responsible health promotion.

What are the bottom lines for public policy:

  1. claims about fiber and bowel cancer are not supported by the evidence.

    2.

  2. still, getting enough soluble and insoluble fiber in your diet from cereals, breads, fruits, and vegetables (20 to 25 grams a day) is a good idea because:

    3.

    1. soluble fiber (at least 3 grams a day) in oat-based cereals, oat breads, and muffins can reduce blood cholesterol levels to a modest extent in a significant percentage (but not all) people and, thereby, modestly reduce the risk of coronary heart disease and heart attacks.

      2.

    2. insoluble fiber (wheat, fruit, vegetables) does reduce the risk of certain inflammatory diseases of the bowel (diverticulitis definitely, appendicitis probably).

      3.

    3. foods high in fiber may reduce the risk of coronary heart disease and heart attacks, although this, too, is not yet proven.
      (SEE TABLES FOR FIBER CONTENT)

      4.

    4. fruits and vegetables (that are often high in fiber) as a group (but not specifically any individual fruit or vegetable) have been shown convincingly to offer modest protection against some cancers. The specific substances in fruits and vegetables that provide the protection are not known.

      5.

    5. fruits and vegetables contain important vitamins and minerals and many other substances that can contribute to your health.

      6.

    6. cereals not only provide fiber, but are also fortified with important vitamins (especially folic acid).

      7.

So, there are good reasons for endorsing diets that include adequate amounts of both soluble and insoluble fiber, but reduction in the risk of bowel cancer is not one of them. However, we should note that our nutrition expert believes that cereal fiber may turn out to protect against bowel cancer; he argues that, in the Nurses’ Health Study, the amount of cereal consumed was not adequate to produce protection.

Fuchs, C.S., et al. Dietary fiber and risk of colorectal cancer and adenoma in women. The New England Journal of Medicine. Vol 340. (January 21) Pgs 169-176. 1999.

Schatzkin, A., et al. Lack of effect of a low fat, high fiber diet on the recurrence of colorectal adenomas. The New England Journal of Medicine. Vol 342. (April 20). Pgs 1149-1155. 2000.

Alberts, D.S., et al. Lack of effect of a high fiber cereal supplement on the recurrence of colorectal adenoma. The New England Journal of Medicine. Vol 342. (April 20). Pgs 1156-1162. 2000.

 


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