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Background: Fruit and vegetable intakes have been associated with a reduced risk of colon cancer; however, in more recent studies associations have been less consistent. Statistical power to examine associations by colon site has been limited in previous studies.
Methods: Fruit and vegetable intakes in relation to colon cancer risk were examined in the Pooling Project of Prospective Studies of Diet and Cancer. Relative risks (RRs) and 95% confidence intervals (CIs) were estimated separately in 14 studies using Cox proportional hazards model and then pooled using a random-effects model. Intakes of total fruits and vegetables, total fruits, and total vegetables were categorized according to quintiles and absolute cutpoints. Analyses were conducted for colon cancer overall and for proximal and distal colon cancer separately. All statistical tests were two-sided.
Results: Among 756217 men and women followed for up to 6 to 20 years, depending on the study, 5838 were diagnosed with colon cancer. The pooled multivariable RRs (95% CIs) of colon cancer for the highest versus lowest quintiles of intake were 0.91 (0.82 to 1.01, Ptrend = .19) for total fruits and vegetables, 0.93 (0.85 to 1.02, Ptrend = .28) for total fruits, and 0.94 (0.86 to 1.02, Ptrend = .17) for total vegetables. Similar results were observed when intakes were categorized by identical absolute cut points across studies (pooled multivariable RR = 0.90, 95% CI = 0.77 to 1.05 for 800 or more versus <200 g/day of total fruits and vegetables, Ptrend = .06). The age-standardized incidence rates of colon cancer for these two intake categories were 54 and 61 per 100000 person-years, respectively. When analyzed by colon site, the pooled multivariable RRs (95% CIs) comparing total fruit and vegetable intakes of 800 or more versus less than 200 g/day were 0.74 (0.57 to 0.95, Ptrend = .02) for distal colon cancers and 1.02 (0.82 to 1.27, Ptrend = .57) for proximal colon cancers. Similar site-specific associations were observed for total fruits and total vegetables.
Conclusion: Fruit and vegetable intakes were not strongly associated with colon cancer risk overall but may be associated with a lower risk of distal colon cancer.
The risk of colon cancer in relation to fruit and vegetable consumption has been reported in more than 50 epidemiologic studies. In 1997, an international panel reviewed 21 case-control and four cohort studies and concluded that there was convincing evidence that vegetable consumption reduces the risk of colon and rectal cancers. The panel also stated that the data available for fruit consumption were limited and inconsistent. In a subsequent evaluation of 27 case-control and 13 cohort studies published through early 2003, a different panel concluded that higher vegetable intake probably reduces the risk of colorectal cancer and that higher fruit intake possibly reduces risk. The conclusion for vegetables was more conservative in the latter evaluation because the reported associations for vegetable consumption and colorectal cancer risk were weaker in the cohort studies than in the case-control studies, suggesting that the inverse associations in the case-control studies may have resulted from recall and/or selection biases. Similarly, a meta-analysis published in 2003 reported modest inverse associations between both fruit and vegetable intakes and colorectal cancer in case-control and cohort studies combined; however, the relative risks were essentially null when restricted to the cohort studies. Heterogeneity between the results from the cohort studies was observed for fruit intake, which may have been due to the observed differences in associations between men and women. Also, associations for vegetable intake differed according to cancer site (colon or rectal), although there was no statistically significant heterogeneity between studies for vegetable intakes in this meta-analysis. Inconsistencies in previous studies may also reflect differences in the specific fruits and vegetables consumed or differences in the prevalence of colon cancer risk factors that may modify associations of diet and cancer. Because analyses of specific food items and of modification of associations by other factors are not routinely reported, meta-analyses of the published literature are unable to analyze these potential sources of heterogeneity. Recent evidence also indicates that proximal and distal colon cancers may have distinct etiologies. Previous cohort studies may have been insufficiently powered to examine associations according to colon site.
To better understand fruit and vegetable consumption in relation to colon cancer risk, we analyzed intakes of total and specific fruits and vegetables in a pooled analysis of 14 North American and European prospective cohort studies. All but three of these studies have previously published results on fruit and vegetable intake and colorectal cancer risk. In our analysis, we have included an extended follow-up period for most of the studies. Because colon and rectal cancers may have different etiologies, we restricted our analysis to colon cancer. Using the primary data from each study, we standardized definitions of fruit and vegetable intakes and covariate categories across studies and analyzed the risks of colon cancer overall and of proximal and distal colon cancer separately. We also examined whether associations were modified by colon cancer risk factors.
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