The quality of evidence is downgraded by study limitations and by inconsistency (variability in results for different flavonoids in the observational studies).
A Cochrane review [Abstract] 1 included 8 studies with 390,769 participants (384 432 in cohort studies, 4 432 in case-control studies, and 1 905 in one RCT) to assess the effect of dietary flavonoids on the incidence of colorectal adenoma and colorectal cancer (CRC). In some studies the association between total flavonoids and colorectal neoplasms was investigated, while others reported the effects of some flavonoid subclasses. The included studies estimated the flavonoid intake of participants using either a food frequency questionnaire or a self-administered questionnaire. The results were contradictory.
The combined results of total flavonoids associated with prevention of colorectal neoplasms indicated no statically significant difference between the highest flavonoid intake and the lowest (RR 1.03, 95% CI 0.88 to 1.20). The results of three cohort studies did not show an inverse association between total flavonoid intake and CRC risk (RR 1.00, 95% CI 0.80 to 1.25). Further, the results of the RCT showed no association of the total flavonoid intake with risk of adenoma recurrence (RR 1.09, 95% CI 0.93 to 1.28).
The evidence for isoflavones, flavonols, flavones and flavanones was conflicting. One study investigated the association of total isoflavone intake with CRC risk, and two studies with adenoma recurrence. The overall results of the studies showed no effect on risk of colorectal neoplasms with Isoflavone intake (RR 0.98, 95% CI 0.80 to 1.19).
The combined data from three studies investigating the association between flavonol intake and colorectal neoplasms showed an inverse association (RR 0.91, 95% CI 0.84 to 0.98 ). Of the three studies, two investigated the association of flavonol intake with CRC risk; a decreased risk of CRC was observed for the highest intake of flavonols compared to the lowest (RR 0.91, 95% CI 0.83 to 0.99). No inverse association of flavonol intake with adenoma recurrence was observed.
For flavan-3-ols, a reduced risk of CRC was found with high intake of epicatechin (RR 0.85, 95% CI 0.77 to 0.95). There was also some evidence to support that increased intake of procyanidin could reduce the incidence of CRC (RR 0.88, 95% CI 0.79 to 0.98). There was no evidence that suggested that high anthocyanin intake had an inverse association with colorectal adenomas.
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