The link between colorectal cancer and dietary fibre was first proposed in 1971 by Denis Burkitt (Cancer Vol. 28, pp. 3-13). The field has been littered with conflicting views ever since.
A new study from the Arizona Cancer Center pools together two studies - the Wheat Bran Fiber Trial and the Polyp Prevention Trial. When considered separately, neither study reported a link between dietary fibre and the risk of colon cancer.
When taken together, however, and in relation to the sex of the participants, a significant benefit of fibre was noted for men, but not women.
"The results of the current analyses indicate that men may experience more benefit from dietary fibre than do women and may help explain some of the discrepant results reported in the literature," wrote lead author Elizabeth Jacobs in the February issue of the American Journal of Clinical Nutrition (Vol. 83, pp. 343-349).
The Wheat Bran Fiber Trial assessed the effect of a high wheat bran fibre supplement (13.5 grams of fibre in two to three cups of cereal per day) against a low wheat bran fibre supplement (two grams of fibre in two to three cups of cereal per day) for 1,429 men and women, randomly assigned to the supplements.
The Polyp Prevention Trial studied the effect of a low-fat, high fibre diet, rich in fruit and vegetables on the recurrence of pre-cancerous polyps in the colon and rectum. Over 2,000 men and women were randomly assigned to either the 'healthy' diet, or a normal diet.
Both studies reported that the high-fibre diets had no effect of the recurrence of colorectal polyps, growths on the inner wall of the colon or rectum. Most colorectal cancers are reported to start in polyps, and the polyps are therefore seen as a marker for the cancer.
When the Arizona researchers re-evaluated the data, they found that the odds ratio (a measure of risk, compared to a reference of 1.00) was 0.91 for all the intervention groups from both studies. This 9 per cent reduction in the risk of colorectal cancer was not statistically significant, thus supporting the view that fibre had no effect on the cancer.
"For men, the intervention was associated with statistically significant reduced odds of recurrence with an odds ration of 0.81; for women, no significant association was observed," reported Jacobs.
The conflicting results from other studies can be explained by the difference in benefits between the sexes, the researchers suggested.
The recently published results of the Women's Health Initiative (WHI) trial also reported no link between a diet low in fat, and high in fruit, vegetables and whole-grain intake. This study was restricted to women and appears to agree with the results from Arizona.
Poor compliance with the dietary intervention in the WHI trial do, however, raise questions about the validity of these results.
There are limitations with the new study. The two studies pooled together - Wheat Bran Fiber Trial and the Polyp Prevention Trial - had very short follow-up periods of only two to four years.
The latency period of the cancer is between 10 and 20 years, suggesting that a follow-up of two to fours years was insufficient to measure any relationship.
Another limitation with this new-pooled analysis is the use of polyps, also called colorectal adenomas, as a marker for actual cancer.
Dr. Arasaradnam from Northern General Hospital in Sheffield, UK, stressed this in an article in the journal Gut in 2004: "The study of dietary fibre using colorectal adenomas as an end point is questionable. This is based on the assumption that adenomas are an adequate surrogate marker for colorectal cancer.
In light of studies showing highly different ratios of adenoma and carcinoma formation… this is a particularly unsafe assumption."
The picture is further blurred by conflicting reports from two big studies in 2005. The Journal of the American Medical Association published the results of a pooled study of 13 cohort studies, which concluded: "High dietary fibre intake was not associated with a reduced risk of colorectal cancer."
On the other hand, the European Prospective Investigation into Cancer and Nutrition (EPIC) study, found a very strong association between a high fibre intake and a reduction in the instance of colorectal cancer. People who ate more than 35 g of fibre a day had a 40 per cent reduced risk than those who consumed only 15 g.
The weight of conflicting evidence on either side of the argument suggests no end is in sight in the near future.
Indeed, Dr Julie Sharp, science information manager at Cancer Research UK, told NutraIngredients.com that the new study did not change the state of play:
"We know that eating a healthy diet, including plenty of fibre, is important in preventing bowel cancer. While it is possible that there are sex differences in response to dietary fibre, other large studies have shown that a high fibre diet is equally important for both men and women."
There are 363,000 new cases of colorectal cancer every year in Europe, with an estimated 945,000 globally. About 492,000 deaths occur from the cancer each year.