In an analysis combining data from 13 studies, high intake of dietary fiber was not associated with reduced risk of colorectal cancer, according to a study in the December 14 issue of JAMA.
Dietary fiber has been hypothesized to reduce the risk of colorectal cancer, according to background information in the article. However, the results of numerous epidemiological studies have been inconsistent. Ecological correlation studies and many case-control studies have found an inverse association between dietary fiber intake and risk of colorectal cancer. But most prospective cohort studies have found no association between dietary fiber intake and risk of colorectal cancer or adenomas (precursors of colorectal cancer), and randomized clinical trials of dietary fiber supplementation have failed to show reductions in the recurrence of colorectal adenomas.
Yikyung Park, Sc.D., formerly of the Harvard School of Public Health, Boston, and colleagues evaluated the association between dietary fiber intake and risk of colorectal cancer by reanalyzing the primary data from 13 prospective cohort studies (Pooling Project of Prospective Studies of Diet and Cancer). The pooled analysis included 725,628 men and women who were followed-up for 6 to 20 years across studies.
During the follow-up, 8,081 colorectal cancer cases were identified. Among the studies, median (midpoint) energy-adjusted dietary fiber intake ranged from 14 to 28 g/d in men and from 13 to 24 g/d in women. The major source of dietary fiber varied across studies with cereals as a major contributor to dietary fiber intake in the European studies, and fruits and vegetables as the main sources in the North American studies.
Note the factors here that influenced risk. These are the things you want to get more or less of to lower your risk.
In the age-adjusted model, dietary fiber intake was significantly associated with a 16 percent lower risk of colorectal cancer in the highest quintile compared with the lowest. This association was attenuated slightly but still remained statistically significant after adjusting for nondietary risk factors, multivitamin use, and total energy intake. Additional adjustment for dietary folate intake further weakened the association. In the final model, which further adjusted for other dietary factors, such as red meat, total milk, and alcohol intake, only a nonsignificant weak inverse association was found. Fiber intake from cereals, fruits, and vegetables was not associated with risk of colorectal cancer.
The factors that reduced the association between fiber and risk are the factors that probably really do reduce risk. So get less red meat and milk for example. Did alcohol intake reduce or increase the risk of colon cancer? Probably the fruits and vegetables decreased risk due to other compounds (notably vitamins, minerals, and anti-carcinogen compounds) and not due to fiber. The fiber just happened to always be there.
One theory on why fiber should lower colon cancer risk is that it dilutes toxins and speeds the passage of toxins through the intestinal tract. Maybe adjusting for red meat consumption factors out the value of eating the fiber. If a person does eat a lot of red meat then maybe fiber really does protect agianst toxins formed in the intestines from red meat. Though I'm speculating.
But consumption of whole plant food like vegetables that are high in dietary fiber is associated with lower risk of heart disease and diabetes. So you could eat high fiber fruits and vegetables anyway.
"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," the authors write.
"In conclusion, we did not find support for a linear inverse association between dietary fiber intake and risk of colorectal cancer in a pooled analysis of 13 prospective cohort studies. Although high dietary fiber intake may not have a major effect on the risk of colorectal cancer, a diet high in dietary fiber from whole plant foods can be advised because this has been related to lower risks of other chronic conditions such as heart disease and diabetes," the researchers write.
Just eating a high fiber food extract is probably a waste of time. But vegetables continue to be good for you. Most people (and myself) continue to not eat enough veggies.
|Share |||Randall Parker, 2005 December 17 09:27 AM Aging Diet Cancer Studies|