In a Dec. 14 Journal of the American Medical Association (JAMA) report, Yikyung Park, ScD, et al. addresses the relation between dietary fiber intake and risk of colorectal cancer.1 The study examines the commercially accepted hypothesis that dietary fiber reduces the risk of colorectal cancer. The conclusion of the research revealed mixed results, including some instances where fiber supplementation did not decrease the risk of cancer:
The association between dietary fiber intake and risk of colorectal cancer has been inconsistent among observational studies and several factors may explain the disparity: potential biases in each study, the failure to adjust for covariates in the multivariate models, and the range of dietary fiber intake. Inconsistent results also have been reported from randomized clinical trials of dietary fiber supplementation on the recurrence of colorectal adenomas (precursors of colorectal cancer); most trials have found no reduced risk of adenoma recurrence with dietary fiber supplementation compared with placebo.2-5
A statistically significant reduction in risk of colorectal cancer with higher dietary fiber intake has been observed in most case-control studies.6 However, case-control studies are prone to recall bias because dietary assessments are obtained after cancer diagnosis and also are prone to selection bias because control participants who participate are likely to be particularly health-conscious. In addition, publication bias may contribute to the accumulation of literature with significant findings. On the other hand, the Pooling Project is less susceptible to these biases because diet was assessed prior to diagnosis and the studies were not required to have published on the association between dietary fiber intake and risk of colorectal cancer.
In Dr. Fuhrman's book Eat to Live he gives a reason why this connection is inconsistent and provides his recommendation for fiber intake:
It is not the fiber extracted from the plant package that has miraculous health properties. It is the entire plant package considered as a whole, containing nature's anti-cancer nutrients as well as being rich in fiber.
High-fiber foods offer significant protection against both cancer (including colon cancer) and heart disease. I didn't say fiber; I said high-fiber foods. We can't just add a high-fiber candy bar or sprinkle a little Metamucil on our doughnut and french fries and expect to reap the benefits of eating high-fiber foods.
The reality is that healthy, nutritious foods are also very rich in fiber and that those foods associated with disease risk are generally fiber-deficient. Meat and dairy products do not contain any fiber, and foods made from refined grains (such as white bread, white rice, and pasta) have had their fiber removed. Clearly, we must substantially reduce our consumption of these fiber-deficient foods if we expect to lose weight and live a long, healthy life.
Furthermore, in his book Disease-Proof Your Child Dr. Fuhrman tells how the changing Japanese diet demonstrates a connection between higher consumption of animal products and the incidence of colon cancer:
The intake of Total Dietary Fiber (TDF) was evaluated from data from the National Nutrition Survey in Japan for forty-one years beginning in 1947. TDF intake decreased rapidly from 27.4 grams per day in 1947 to 15.8 grams in 1963. Fat intake increased rapidly from 18 grams in 1950 to 56.6 grams in 1987. Of significance in this carefully done study was that the increased occurrence of colon cancer had a twenty-three-to-twenty-four-year lag after the heightened consumption of animal products began. Apparently what the Japanese people did twenty-five years earlier had the strongest effect on inducing cancer, not what they ate ten years or even twenty years earlier.7 Those with the highest consumption of plant fiber in their childhood had the lowest incidence of colon cancer.
Recent studies have also found that eating fruit during childhood had powerful effects to protect against cancer in later life. A sixty-year study of 4,999 participants found that those who consumed more fruit in their childhood (the highest quartile) were 38 percent less likely to develop cancer as adults.8
Diets rich in meat and dairy are powerfully implicated as cancer promoters. Processed, pickled, smoked, or barbequed meats are even more strongly linked to cancer. Separate studies from Europe and the United States found the same results; those who eat meat daily have a three- to four-fold increase incidence of colon, esophageal, and stomach cancers, and the risks are more severe the younger in age people begin these practices.9
1. Yikyung Park, ScD; David J. Hunter, MB, BS; Donna Spiegelman, ScD; Leif Bergkvist, MD; Franco Berrino, MD; Piet A. van den Brandt, PhD; Julie E. Buring, ScD; Graham A. Colditz, MD; Jo L. Freudenheim, PhD; Charles S. Fuchs, MD; Edward Giovannucci, MD; R. Alexandra Goldbohm, PhD; Saxon Graham, PhD; Lisa Harnack, DrPH; Anne M. Hartman, MS; David R. Jacobs, Jr, PhD; Ikuko Kato, MD; Vittorio Krogh, MD; Michael F. Leitzmann, MD; Marjorie L. McCullough, ScD; Anthony B. Miller, MB, BCh; Pirjo Pietinen, ScD; Thomas E. Rohan, MB, BS; Arthur Schatzkin, MD; Walter C. Willett, MD; Alicja Wolk, DMSc; Anne Zeleniuch-Jacquotte, MD; Shumin M. Zhang, ScD; Stephanie A. Smith-Warner, PhD.
2. McKeown-Eyssen GE, Bright-See E, Bruce WR, et al. A randomized trial of a low fat high fibre diet in the recurrence of colorectal polyps: Toronto Polyp Prevention Group. J Clin Epidemiol. 1994;47:525-536.
3. MacLennan R, Macrae F, Bain C, et al. Randomized trial of intake of fat, fiber, and beta carotene to prevent colorectal adenomas: the Australian Polyp Prevention Project. J Natl Cancer Inst. 1995;87:1760-1766.
4. Alberts DS, Martinez ME, Roe DJ, et al. Lack of effect of a high-fiber cereal supplement on the recurrence of colorectal adenomas: Phoenix Colon Cancer Prevention Physicians' Network. N Engl J Med. 2000;342:1156-1162.
5. Schatzkin A, Lanza E, Corle D, et al. Lack of effect of a low-fat, high-fiber diet on the recurrence of colorectal adenomas. N Engl J Med. 2000;342:1149-1155.
6. World Cancer Research Fund; American Institute for Cancer Research Expert Panel, JD Potter, Chair. Food, Nutrition and the Prevention of Cancer: A Global Perspective. Washington, DC: American Institute for Cancer Research; 1997.
7. Tsuji K, Harashima, E, Nakagawa Y, et al. Time-lag effect of dietary fiber and fat intake ratio on Japanese colon cancer mortality. Biomed Environ 1996;9(2-3):223-8.
8. Maynard M, Gunnell D, Emmett P, et al. Fruit, vegetable and antioxidants in childhood and risk of adult cancer: the Boyd Orr cohort. J Epidemiol Community Health 2003;57:218-225.
9. Erickson KL. Dietary pattern analysis: a different approach to analyzing an old problem, cancer of the esophagus and stomach. 2002;75(1):5-7. Satia-Abouta J, Galanko JA, Martin CF, et al. Food groups and colon cancer risk in African-Americans and Caucasians. Int J Cancer 2004;109(5):728-736.