In this cohort, total dietary fiber intake, specifically from vegetables, was inversely associated with colon polyps, with a clear dose-response effect. Fiber intake from fruits and grains did not show a statistically significant effect on colon polyps, though the risk estimates trended in the protective direction. Our results were not altered by mutually adjusting the fiber sources (including vegetables, fruit, and grain fiber in a single model).
Our findings suggest that fiber from legumes, cooked green vegetables, and tomatoes, but not green salad, are needed for the protective effect of “vegetable fiber.” Green salad seems to have no effect on colon polyps risk, perhaps due to its low fiber content, or, since the typical American diet includes green salad in most meals, it is not a discriminating factor. The other vegetable sources may contribute importantly to the protective effect against colon polyp risk.
The protective effects observed might be related not only to fiber content but also because of the presence of chemopreventive phytochemicals.36
The effects of tomatoes and legumes on CRC, specifically, have been investigated in a small number of epidemiologic studies. In AHS-1, Singh and Fraser (1998) identified a complex association between the intake of red meat and legumes for CRC risk. Their findings raised the possibility that a specific factor in legumes neutralizes one or more of the hypothesized carcinogenic mechanisms associated with higher red meat intake.18
These findings support our data that legumes are an important factor for the relationship found between vegetables and colon adenomas.
Other epidemiologic studies, with colon adenomas as the outcome of interest, have shown that frequent consumption of legumes was associated with a reduced incidence14,37
as well as reduced recurrence of colon adenomas.38
Tomatoes have also been reported as a protective against colon adenomas.39–41
Their protection has been associated with fiber and lycopene content that may inhibit cancer proliferation by interfering with the insulin-like-growth factor (IGFs) system, possibly through an effect of IGF-binding proteins (IGFBPs).42
When evaluating the components of the fruit fiber index, excluding winter fruit modified the index toward a statistically significant protective effect, and a major decrease in OR compared to when winter fruit was included in the index. Thus, “winter fruit” may not be a discriminating variable since these popular fruits are usually available year-round. Therefore, adding this variable to the total fruit fiber index may have masked its effect. For grain sources (brown rice, white rice, wheat bread, white bread, or other types of bread), no statistically significant association was found with colon polyp risk. However, due to the limited ability of the food-frequency questions to assess total grain intake we cannot negate an important relation with the outcome.
Our findings support the hypothesis that high intake of dietary fiber protects against first occurrence of colon adenomas. This is supported by only one other prospective study, which found a higher magnitude of protection.43
Similar findings related to the effect of vegetables have been reported in some national11,12,44
case-control studies. For fruits and grains, 2 studies performed in the Prostate, Lung, Colorectal, and Ovarian (PLCO) Cancer Screening Trial found an inverse association between these foods and adenoma risk.45,46
Results from the Nurses' Health Study (NHS) showed an inverse association between frequent consumption of fruits and colorectal adenoma occurrence.14
Results from 2 trials (Polyp Prevention Trial and Wheat Bran Fiber) indicate a protective effect of fiber on recurrent colorectal adenomas, especially among men.47
Using cluster analysis to investigate dietary patterns and the risk for colorectal adenoma, a case-control study also found a protective effect for high fruit intake.13
Nevertheless, other epidemiologic data on incidence,20,23
as well as on recurrence of colon polyps,38,48,49
have not detected an association between dietary fiber and colon adenomas. The possible reasons for these null findings may be the limited dietary fiber information derived from the food-frequency questions, misclassification of true fiber intake due to misreporting, and a limited ability to evaluate long-term dietary exposure.
The major strength of our study is its prospective design with 26 years of follow-up. This eliminates the possibility of recall bias when assessing exposure, including food intake. About 80% of this population (unpublished data) made no major changes in their dietary intake during these years of follow-up, which also reduces the risk of measurement error in the exposure status. However, based on the health recommendations of the Adventist church, it is possible that participants may have overestimated their fruit and vegetable consumption as a result of increased awareness of the potentially beneficial effects on cancer or due to social desirability. But this type of misclassification is most likely nondifferential, biasing the results toward the null.
The unique lifestyle of the Adventist population, with a low percentage of alcohol consumption and tobacco smoking, reduces the possibility of confounding by these nondietary factors. We did not have hospital records of colonoscopy, but only self-reported physician-diagnosed colon polyps. This increases the possibility of measurement error in the outcome assessment. But again, the misclassification is most likely nondifferential. The survivor cohort who volunteered to participate in AHS-2 may have been healthier than the original cohort. This could bias the estimates of colon polyp risk in the target population. Finally, the food-frequency questions were limited in that no data were available on portion sizes and specific food items. Therefore, we used standard portion sizes to estimate consumption. Moreover, we could not adjust for energy intake, an important known factor to consider when evaluating colon adenomas risk.
In conclusion, our findings identify dietary fiber, specifically from vegetables (including legumes), as an important protective factor against colon polyps with a dose-response effect. Fruits (but not winter fruits) may also contribute. Further studies with more complete dietary information throughout the follow-up are needed to verify our findings.