BACKGROUND: Dietary factors, particularly insoluble fibers, have been investigated in the pathogenesis of adenomas and colorectal cancer. It has been suggested that dietary fiber confers a protective effect on the risk of recurrent colorectal adenomas. Previous studies have had varying results and have largely been case-control studies, prohibiting any firm clinical recommendations.
POPULATION STUDIED: The investigators enrolled 1429 patients aged 40 to 80 years. All patients had one or more histologically confirmed colorectal adenomas at least 3 mm in diameter removed within the 3 months before recruitment. An adequate nutritional status, normal renal and liver function, and normal functional status were required. Exclusion criteria included: a dietary fiber intake of less than 30 g per day, having invasive cancer within the previous 5 years, a history of colon resection, 2 or more first-degree relatives with colorectal cancer, and severe metabolic disorders or other severe illnesses.
STUDY DESIGN AND VALIDITY: This was a double-blinded randomized prospective trial. All patients underwent a baseline colonoscopy and successfully completed a 6-week run-in period by consuming at least 75% of the amount of a low-fiber supplement (2g/day). They were then randomized to either a high-fiber (13.5g/day, 802 subjects) or a low-fiber (2g/day, 627 subjects) supplement of wheat bran cereal. Since an interim analysis demonstrated that a higher proportion of the high-fiber group discontinued the designated supplement, the original 1:1 randomization schedule was changed to 4:1 in favor of the high-fiber group. Follow-up colonoscopy was planned for 1 year after randomization and again 2 years later. However, national screening recommendations for people with a history of colorectal adenomas changed and led to a decreased rate of colonoscopy at 1 year among subjects enrolled in the latter part of the trial. A strength of the study is the fact that the intervention was realistic and applicable to almost any primary care setting. However, the run-in period did ensure a group that was more compliant than average.
OUTCOMES MEASURED: The primary outcome was the presence or absence of new adenomas at the time of follow-up colonoscopy. Multivariate adjusted odds ratios and relative risks for recurrent adenomas were calculated.
RESULTS: A total of 1303 subjects (91.2%) completed the study. Median follow-up time was 35 months. Half of the subjects in each group had at least one adenoma found during the final follow-up colonoscopy (47% high fiber vs 51.2% low fiber, P=.13). The multivariate adjusted odds ratio for recurrent adenomas in the high-fiber versus low-fiber groups was 0.88 (95% confidence interval [CI], 0.70-1.11; P=.28) and the relative risk of adenoma recurrence was 0.99 (95% CI, 0.71-1.36; P=.93). No statistically significant difference was found between groups when analysis was restricted to the subset of 889 subjects who underwent both a 1-year colonoscopy and a repeat examination 2 years later. There was also no significant difference between groups in regard to adenoma size or histologic appearance. Compliance with the dietary supplement regimen was statistically different between the 2 groups, with a lower rate of consumption of the high-fiber supplement.
The study authors found no statistically significant reduction in the rate of recurrent colorectal adenoma with consumption of a high-dose dietary supplement of wheat bran fiber. Furthermore, a high-fiber diet did not alter the number of adenomas or the histologic features of the recurrent adenomas. The results are consistent with another study1 in the same issue of the journal. Although a high-fiber diet cannot be recommended for the prevention of recurrent colorectal adenomas, there may be other health benefits.