Maybe Fiber Doesn't Prevent Colon Cancer . . .

Summarized by Robert W. Griffith, MD
January 16, 2006

Introduction

Some say that fiber in the diet prevents colon cancer, others say it doesn't. Most of these opinions are based on so-called 'observational' or 'cross-sectional' studies, where the diets of a group of people with colon cancer are compared with those of a group of similar people who don't have cancer. In cross-sectional studies an apparent link between a specified factor (diet) and a disease (colorectal cancer) may be estimated, but it only represents a snapshot in time, and doesn't indicate whether the particular diet preceded or followed the diagnosis of cancer.

To get a better handle on a possible link between fiber and colon cancer, Harvard scientists have gathered the information from published prospective studies, in which groups of volunteers (the groups are called 'cohorts') are studied from a baseline time point for a number of years. The findings are reported in the Journal of the American Medical Association, and we summarize them here.

What was done

Thirteen prospective cohort studies were identified for analysis. They included more than 725,000 men and women who were followed for 6 to 20 years. During the follow-up period over 8,000 cases of colorectal cancer were diagnosed among the participants.

All them were allocated to one of five equally-sized groups, called quintiles, according to the amount of fiber in their diet - separate quintiles were created for each sex. The frequency of occurrence of colorectal cancer was then calculated for each quintile.

What the results showed

The average dietary fiber intake was 14 - 28 grams a day in men and 13 - 24 grams a day in women. The total fiber intake at the highest level (the top quintile) was 2 to 3 times as great as that in the lowest quintile.

In studies reported from Europe cereals were the main sources of fiber, compared with fruits and vegetables in the North American studies.

The first analysis compared the frequencies of colorectal cancer in the highest fiber- intake quintiles with those of the lowest. It showed a statistically significant difference in favor of the highest fiber intake: the 'risk ratio' was 0.84, or, in other words, for every 100 cases of cancer in the lowest fiber quintile there were only 84 cases in highest fiber quintile. However, this significant difference disappeared after the numbers were adjusted for other risk factors that could have influenced the result - the intake of folic acid, alcohol use, and red meat consumption. (Thus those who had the highest fiber intake ate less red meat, drank less alcohol, or took in more folate.)

What these results tell us

Unfortunately, the study doesn't provide a clear-cut answer to the question "Does dietary fiber reduce the risk of colon cancer?" After careful analysis, the answer appears to be "No", but it wasn't possible to isolate the intake of fiber from the intake of factors that are known to influence the frequency of colorectal cancer, such as alcohol, red meat, smoking, and folic acid. Moreover, fiber isn't always just fiber. Sources are not always simply roughage, and most are not even 'fibrous'. And many dietary fibers are digested by bacteria in the colon.

One might expect fiber could have a protective effect by one or more mechanisms: dilution of potential carcinogens in the feces, reducing transit times of feces through the colon, and binding of carcinogenic bile acids. So the results of this analysis were somewhat of a surprise.

Even if the protective action of dietary fiber isn't obvious for colorectal cancer, there's no reason for healthy people to slow down on their fiber intake. Fiber has been shown to protect against hemorrhoids, constipation, high blood pressure, high cholesterol levels, cardiovascular disease and diabetes. So even if colorectal cancer must be removed from the list (maybe temporarily), there's still good reason to eat your cereal, fruit and veggies.

Source

• Dietary fiber intake and risk of colon cancer, A pooled analysis of prospective cohort studies. PY. Park , HDJ. Hunter , SD. Spiegelman ,  al., , 2005, vol. JAMA , pp. 2849--2857

Related Links
How to Try and Avoid Colon Cancer
Does Fiber Affect Colon Cancer Risk?
Calcium May Reduce Colon Cancer Risk in Men
Lycopene May Reduce the Risk of Colorectal Cancer

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