Fibre

In five randomised trials, dietary supplementation with wheat bran or other types of fibre did not affect the rate of recurrence of colorectal adenomas. Two studies tested the effect of dietary counselling to reduce fat consumption and increase fibre intake [25,26]. In one study the subjects in the intervention group were advised to increase fat intake and take wheat-bran supplement [27] and two studies tested purely the effect of fibre supplements [28,29].

The results of the Toronto Polyp Prevention Trial [25] (randomised, not blinded) suggest that there was no significant difference in polyp recurrence between persons assigned to low fat/high fibre diet and those following a normal western diet. The ratio of cumulative incidence rates was 1.2 (95% CI 0.6–2.2) [25]. In a randomised (not blinded) control trial in which the intervention group was assigned to the low fat, high-fibre (18 g per 1000 kcal) and high fruit and vegetable diet, no difference was observed in recurrence rate of colorectal adenomas (and large adenomas) between intervention and control group, who followed their usual diet. The unadjusted risk ratio was 1.00 (95% CI 0.90– 1.12) [26].

In the Australian randomised (not blinded) trial of intake of fat, fibre and ß-carotene to prevent colorectal adenomas there was no significant prevention of new adenomas in any of treatment groups [27].

In a double-blind randomised study, a dietary supplement of wheat-bran fibre had no statistically significant effect against recurrent colorectal adenoma. The multivariate adjusted OR for recurrent adenomas in the high-fibre (13.5 g per day) group compared with the low-fibre (2 g per day) group was 0.88 (95% CI 0.70–1.11; P = 0.28); and the OR of recurrence according to the number of adenomas in high-fibre group compared with the low-fibre group was 0.99 (95% CI 0.71–1.36; P = 0.93) [28].

In a randomised controlled study that tested the protective effects of supplementation with fibre (3.5 g ispaghula husk) and calcium (2 g daily) on colorectal adenomatous polyp recurrence, the adjusted OR for recurrence in the fibre treatment group was 1.67 (95% CI 1.01–2.76; P = 0.42). The OR associated with the fibre treatment was significantly higher in participants with baseline dietary intake above the median than in those with intake below the median (interaction test, P = 0.028) [29].

It appears from the results of these randomised trials that supplementation with fibre does not affect the risk of the recurrence of colorectal polyps. The evidence of a protective effect of fibre against colorectal cancer is purely observational and the use of fibre cannot be recommended for the general population at the present time.