New research suggests that fibre supplementation offers no protection against
colorectal cancer and may even have a detrimental effect. Calcium supplementation,
however, may be beneficial.
The efficacy of dietary supplementation with soluble fibre or calcium in the
prevention of colorectal adenoma was investigated by Dr Claire Bonithon-Kopp
(faculty of medicine, Dijon, France) and colleagues for the European cancer
prevention organisation study group. A total of 552 patients with a history
of colorectal adenoma were randomly assigned to three groups and received either
2g elemental calcium daily (as two sachets in water twice a day), 3.5g ispaghula
husk as one sachet in water each day, or placebo. All patients were assessed
for adenomas by colonoscopy at entry to the trial and after three years. The
researchers say that adenomas are thought to be precursors of most colorectal
cancers.
They found that at least one adenoma developed in 16 per cent of the calcium
group, 29 per cent of the ispaghula group and 20 per cent of the placebo group.
Adjusted odds ratios for risk of adenoma recurrence were 0.64 for calcium treatment
and 1.57 for fibre treatment. The researchers say that fibre supplementation
resulted in a significant increase in adenoma recurrence rate and that calcium
had a slight, but non-significant, beneficial effect.
The researchers say that their findings “do not accord with the hypothesis
of a protective effect of fibre on the risk of colon cancer”. There was
no evidence that differences in dietary intake between the groups could explain
the result. They suggest that detrimental effects of soluble fibre could occur
because of an increased production of short-chain fatty acids by bacterial fermentation
and an excessive acidification which may stimulate proliferation of epithelela
cells.
The researchers suggest that poor compliance may partly explain the weakness
of the association between calcium treatment and adenoma recurrence. However,
calcium treatment seemed to have beneficial effects whatever the degree of compliance,
they say. The researchers suggest that calcium may have a protective effect
against colon carcinogenesis by binding secondary bile acids and fatty acids
to form insoluble soaps in the bowel lumen, thus reducing their proliferative
effect.
They conclude that dietary supplementation with wheat bran fibre or ispaghula
husk may not be effective for preventing colorectal adenoma recurrence. However,
they add: “Our findings should not prevent recommendations for high consumption
of vegetables, fruits and cereals because this approach has potentially beneficial
effects on other chronic disease, especially coronary heart disease.” (Lancet
2000;356:1300.)