Many of the health benefits linked with foods have yet to be fully proven, and following on from last week's report highlighting the conflicting evidence on soya, the latest news on fibre seems equally hazy.
Two studies in this week's issue of The Lancet find that a high-fibre diet contributes to a substantial lowering in the risk of colon cancer. But a report in the International Journal of Epidemiology this month suggests there is no association between increased fibre intake and a reduced risk of bowel cancer. Which only seems to suggest, once again, that further evidence is needed to examine the actions of fibre on health.
In the first study, Ulrike Peters from the National Cancer Institute, Rockville, USA, and colleagues assessed the relation of fibre intake and frequency of colorectal adenoma (non-malignant polyps which are often a precursor of malignant disease). The investigators compared fibre intake of around 34,000 participants (within the Prostate, Lung, Colorectal, and Ovarian (PLCO) Cancer Screening Trial), who did not have polyps, with around 3600 people with at least one adenoma in the distal large bowel (ie, descending colon, sigmoid colon, or rectum).
High intake of dietary fibre was associated with a lower risk of colorectal adenoma, after adjustment for potential dietary and non-dietary risk factors. Participants in the top 20 per cent for dietary fibre intake had around a quarter lower risk of adenoma compared to people in the bottom 20 per cent for fibre intake (representing an increase in fibre intake of 24 grammes per day). This association was strongest for fibre from grains, cereals, and fruits, although there was no association between fibre intake and risk of rectal cancer.
A European study also published in this week's Lancet(p1496) reports similar findings. Sheila Bingham and Nick Day from the UK Medical Research Council, Elio Riboli from the WHO International Agency for Research on Cancer, and colleagues from the European Prospective Investigation into Cancer (EPIC) consortium examined the association between dietary fibre intake and incidence of colorectal cancer in over half a million individuals aged 25-70 years, recruited from ten European countries.
After an average follow-up of 4.5 years, 1065 cases of colorectal cancer were diagnosed. Individuals from the top 20 per cent for fibre intake, who ate 35g fibre per day on average, had their risk of colorectal cancer reduced by a quarter compared with individuals consuming 15g per day on average. This reduction in risk was estimated to be 40 per cent after a more sensitive analysis was done. The protection was strongest for left-sided colon cancer and not significant for rectal cancer.
However a further study, published in the current issue of the International Journal of Epidemiology (IJE), edited at the University of Bristol, found that eating a high fibre diet does not necessarily prevent bowel cancer, although it may be good for preventing other chronic diseases, such as heart disease.
Researchers from the National Cancer Institute in the USA studied a cohort of 45,000 American women between the early 1970s and late 1990s. They looked at intake of sources of fibre (fruits, vegetables, beans and grains) in the women's diets and whether they had bowel cancer some years later.
The authors, led by Professor Volker Mai, found no evidence that the intake of dietary fibre was related to the incidence of bowel cancer. However they say that this finding, and the apparently inconsistent findings of other studies in this area, may in part be explained by the difficulty of collecting accurate information on eating habits in these kinds of study.
The authors also note that while these and other studies suggest that high fibre diets probably do not protect against bowel cancer, a high fibre diet may be good for preventing other chronic diseases, such as heart disease. And there is no evidence that a high fibre diet is bad for health.
In a commentary in The Lancet (p 1487) Lynnette Ferguson and Philip Harris from the University of Auckland, New Zealand, state how the new results on fibre and colon cancer "give fresh impetus to fundamental research to determine the reasons for the protective action of dietary fibre. Whatever the reasons for the results reported by the two studies, eating a diet rich in plant foods, in the form of fruit, vegetables, and whole-grain cereals, probably remains the best option for reducing the risk of colon cancer, and for more general health protection."
Dr Debbie Lawlor of the Department of Social Medicine at Bristol University noted that the fibre issue has been debated since ancient times.
"Hippocrates recognised that wholemeal food passed through the bowel at a much quicker rate and was concerned that in doing so there would be less time for the body to absorb the food's nutritional content. Today rapid transit through the bowel - produced by a high fibre diet - is believed to have some benefit in reducing ingestion of some nutrients, such as cholesterol, that may be bad for health," she said.