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 DIETARY Fiber and CANCER  

Abstract:  Fiber has been shown to reduce cholesterol and cardiovascular diseases.  Research now also shows that it can reduce risk of colorectal, breast, and other female cancer.  A reduction in risk of these cancers of about 30% appears associated with a dietary increase of 15 grams per day of fiber.  But this benefit probably will develop only slowly over a 15 year period of time.

The Research Results:  Three separate meta-analyses of 13 to 16 studies each published before 1994 showed that highest Fiber diets reduced colorectal cancer nearly in half, each with high joint statistical significance.  Results of these plus those for 13 other studies are provided in Table FC following. Twenty of the twenty-one individual risk ratios measured show an advantage for Fiber.  The balance of other data suggests that a risk ratio of 0.7 will be within or close to the error margins of the listed individual studies for colorectal cancer.  The research risk ratios for colorectal adenomas – precursors of cancer – vary but generally are consistent with a risk ratio of 0.70.

The research also shows that Fiber reduces risk of other causes of cancer.  The average risk ratio obtained on 5 studies of breast cancer is 0.52, and an average from risks of two other causes of cancer is a risk of 0.52.  There has been much speculation that Fiber protects by lowering transit time thru the digestive system. The fact that Fiber reduces cancer of the breast and at other sites at least as much or more suggests that other mechanisms are probably primary.

The result from the Nurses study although within the 0.7 risk factor range did show a lower benefit than most other studies. This is similar to lower benefits for antioxidants on cancer for this population.  A possible reason for this is that this group included mostly pre-menopausal women at start.  It is possible the Fiber will not benefit this group appreciably.  However, as will follow, Life Ahead assumes benefits from Fiber for most people should accrue mostly over the long term that usually will include the post-menopausal years.

About Study Duration:  Because cancer develops slowly over the years, the effect of differences in dietary Fiber as for most other modifying factors should depend on their duration. It is extremely unlikely that a change in diet Fiber will accomplish an immediate change in overall risk of cancer.  Little information on the effect of duration of Fiber differences was found.  But some of the studies were for time periods up to 25 years, and for measured durations of up to 20 years.  All studies in Table FC were either case control or prospective.  Assuming a usual 10 years of diet duration of before study baselines, total average durations of dietary Fiber differences were probably at least 15 years. The short term study #9 did note a smaller benefit of only 0.88 for 1.5 years average duration of the 3 year study, but margin of error here is too high to draw a general conclusion from this one study.  Life Ahead assumes as discussed following that benefits from Fiber will develop only slowly as a compounding risk benefit each year that a diet difference is maintained.  This basis is confirmed further by the results from some clinical studies that follow.

The Clinical Studies:  It seems unlikely that any effect on cancer of a diet change of either antioxidants or Fiber could be measured in a clinical trial of less than 10 years because this still would provide an average exposure duration of only 5 years.  For example it takes 10-20  years minimum for cancer to develop significantly from cigarettes.  Results of one study  (Jacobs, J Natl Cancer Inst 2002,94:1620) found as expected little effect for a short term increase in dietary Fiber of about 11 gm/day over a baseline of 17.5 gm per day.  Another attempt, (Pierce, Control Clin Trials 2002,2:728) found no effect in 4 years (2 years avg duration) of substantially changing diets of breast cancer survivors. (Shike, M, J Clin Oncol 2002, 20:3592) found no effect on PSA in a 4 year (2 yrs avg duration) trial on Prostate cancer survivors.  The Life Ahead method can provide a forecast of the probable benefit from a randomized clinical trial of various durations. This expected benefit will be very small.  For example, a Life Ahead forecast of risk for a Fiber difference of 15 grams per day from a 10 year long clinical study is a risk ratio value of only 0.89 that might not be measurable in a practical study.

The Life Ahead Basis for Fiber And Cancer:  Based on the available research, Life Ahead presently includes a risk factor benefit for dietary Fiber on colorectal, breast, and female genital cancers of 0.70 for a difference of 15 grams/day, and applies this to risk to an average duration of diet difference of 15 years.  This factor is applied to differences in Fiber from an assumed average population value difference of about 18 grams intake per day, from 13 to 25 average grams per day.   The maximum accepted amount of Fiber for Life Ahead risk is 25 grams per day. This 0.70 risk probably is a conservative estimate of a likely benefit, and is a risk benefit of about 0.976 for each year.   A maximum duration for risk computation is taken as 15 years. A maximum risk risk benefit computed for any number of years or amount of Fiber is 0.70.  The presently computed cancer risk benefit for dietary Fiber may be conservative because this factor might reduce risk of other sites of cancer that yet have been adequately researched.  It also is conservative because higher values than 25 grams per day may provide further benefits.

Increasing Intake of Dietary Fiber:  A present target for Fiber in Life Ahead is 25 grams per day (but adjusted further for user gender and body weight).  This desired value may be difficult to achieve via most usual diets.  See the diet library values in Life Ahead for Fiber sorted in order of descending amounts of Fiber, and compare these values with those now in a usual diet. Cereals head  the list for Fiber, but many other foods can contribute.  Several Fiber enhancing supplements are available and are widely advertised.  Psyllium can increase Fiber appreciably, and has been shown to reduce serum cholesterol.  Thus it probably will help in reducing risk of cardiovascular diseases.  But no actual direct research data were found confirming that a Fiber supplement actually reduces risk of either cardiovascular disease or cancer. Adequate Fiber can help avoid constipation, and is thought to help prevent diverticular disease.            

Unfortunately, useful data confirming a benefit for Fiber supplements may not become available soon.  Assuming Fiber protects increasingly over 15 years of time as now is computed in Life Ahead, no practical and timely clinical type study may be able to show an effect. Until substantial groups of individuals have taken these supplements over ten or more years it may not be possible to ascertain directly the benefits of Fiber supplements in reducing disease.  Life Ahead cannot include a factor in its quantification until it is well supported by actual research data.  But fiber supplements may provide benefits not now acknowledged by the program.