More evidence that fibre cuts bowel cancer risk

“Eating more cereals and whole grains could reduce the risk of developing colorectal cancer,” BBC News has today reported. The broadcaster says a major examination of fibre research has shown that people can cut their risk of bowel cancers by 20% by eating three portions of foods such as wholegrain breads, cereals and porridge each day.

It has long been thought that a high intake of fibre might cut the risk of bowel and rectal cancers, so British and Dutch researchers decided to review all relevant studies on the topic. After a thorough search, they combined the results of 21 studies that had examined how dietary fibre intake related to later cancer development. The overall results suggest that for each 10g of fibre eaten per day risk of colorectal cancer fell by 10%.

However, the researchers do stress that the associations they observed are uncertain, as there is a possibility that unmeasured dietary or lifestyle factors could be affecting the relationship. For example, people who eat more fibre may also avoid other behaviours thought to increase bowel cancer risk, such as eating red meat, smoking or drinking excessively. Also, all dietary studies are difficult to perform given the complexity involved in capturing people’s long-term eating habits accurately.

Despite these minor reservations, these results lend further support to the long-standing theory that getting enough fibre can help lower the risk of developing bowel cancer, which is currently England’s third most common cancer.

Where did the story come from?

The study was carried out by researchers from Imperial College London, the University of Leeds and Wageningen University in the Netherlands. Funding was provided by the World Cancer Research Fund.

The study was published in the peer-reviewed British Medical Journal.

The newspapers have reflected the findings of this review accurately.

What kind of research was this?

This was a systematic review and meta-analysis that aimed to investigate the link between dietary fibre, particularly wholegrain intake, and the development of cancer. To do so it combined and analysed the results of all available prospective observational studies on the subject.

Colorectal cancer (CRC) is the third most common cancer worldwide, and dietary factors have often been associated with the risk of developing it. Red meat and processed meat are suspected to increase risk, while fibre, fruit and vegetables have long been thought to decrease the risk.

What did the research involve?

To gather studies the reviewers searched research databases up to December 2010, also looking for relevant research in the reference lists of the studies that they identified. To be eligible for inclusion the studies had to be either:

• prospective cohorts that had examined dietary intake and then followed participants to see who developed CRC, or
• case-controls that had looked at people with and without CRC and then looked back at their diet prior to cancer development

Studies also needed to have quantitatively measured dietary intake, reported on the number of years of follow-up and to have estimated risk figures for cancer (either as hazard ratios or risk ratios).

In combining results of the studies, the researchers used statistical methods that took into account differences between the individual studies’ results (heterogeneity) and calculated overall risk of cancer for the highest fibre intake versus the lowest intake. These analyses looked at total dietary fibre intake, intake from specific dietary sources and wholegrain intake. Where more detailed intake on fibre was available, they also looked for evidence of a “dose-response” trend (where results showed an association between increasing fibre intake and decreasing cancer risk, a finding that would support the theory that fibre actively cuts cancer risk).

What were the basic results?

Twenty-one studies provided information for the analysis of highest-versus-lowest dietary intake and cancer risk, and 18 of these studies had sufficient information to allow for dose-response analysis. Twelve of the studies were from the United States, five from Europe and four from Asia.

Significant findings calculated by the researchers were as follows:

• High vs. low intake of total dietary fibre: a 12% decrease in risk of CRC with high intake (relative risk 0.88, 95% confidence interval 0.82 to 0.94; 19 studies)
• Dose-response analysis for total dietary fibre: a 10% decrease in CRC risk with 10g a day intake of total fibre (relative risk 0.90, 95% confidence interval 0.86 to 0.94; 16 studies)
• High vs. low intake of cereal fibre: a 10% decrease in CRC risk with high intake (relative risk 0.90, 95% confidence interval 0.83 to 0.96; eight studies)
• Dose-response analysis for cereal fibre: a 10% decrease in CRC risk with 10g a day intake of cereal fibre (relative risk 0.90, 95% confidence interval 0.83 to 0.97; eight studies)
• High vs. low intake of whole grains: a 21% decrease in CRC risk with high intake (relative risk 0.79, 95% confidence interval 0.72 to 0.86; four studies)
• Dose-response analysis for whole grains: a 10% decrease in CRC risk with 90g a day intake of whole grains (equivalent to three servings) (relative risk 0.83, 95% confidence interval 0.78 to 0.89; six studies)

The researchers found no significant association between CRC and:

• intake of fruit fibre (high vs. low, or dose response)
• intake of vegetable fibre (high vs. low, or dose response)
• intake of legume fibre (high vs. low, or dose response)

There was no significant heterogeneity between the included studies, in other words, no significant differences in the studies’ designs that would prevent them from being combined in a meaningful way.

How did the researchers interpret the results?

The researchers conclude that a high intake of dietary fibre, in particular fibres from cereal and wholegrains, is associated with a reduced risk of colorectal cancer. They say that further studies are needed that give more detailed results, especially by subtype of fibre, and that take into account potential confounders.

Conclusion

A higher intake of fibre has long been thought to decrease risk of colorectal cancer, and this large and valuable review has helped to assess and analyse the existing body of evidence on the matter. It has found that higher intakes of total fibre, cereal fibre and wholegrains are all linked to a decrease in the risk of colorectal cancer, a disease that currently kills around 16,000 people in England each year.

The review has several strengths, including that it has searched for all available literature on the subject and that it looked only at prospective studies that analysed intake prior to cancer development. Also, as the researchers say, they found no evidence of publication bias (where only studies finding a positive association had been published).

There are some inherent limitations to a systematic review such as this:

• Dietary study and assessment is notoriously hard to perform due to factors such as how accurately people may describe or recall their diet and the way a person’s eating habits may change over time. In this case, the individual studies used different ways to categorise how much fibre a person ate, but it is not clear how people were questioned about their intake. As the researchers say, further studies need to consider ways to reduce this potential for inaccuracy.
• The studies differed in design, included population, methods of dietary assessment, follow-up duration, and the confounding factors that they adjusted for. Also, it is not clear from the review paper how the individual studies confirmed cancer in the cases. However, it is worth noting that heterogeneity (differences) between the individual studies was low, indicating that it was appropriate to combine their results.
• The researchers particularly highlighted that the associations that they have found are only weak, possibly due to the influence of other unmeasured factors (those that could be associated with both dietary fibre and CRC risk). Such factors could include other aspects of the diet (for example, people who eat more fibre may also choose to eat less red meat) or other lifestyle factors (people who eat more fibre may also be less likely to smoke or drink excessively, and more likely to take regular exercise).

Despite these limitations, the review is well-conducted and a valuable source of evidence that suggests that eating more fibre can be protective against colorectal cancer. The results suggest a 10% reduction in risk of colorectal cancer for each 10g-a-day intake of total dietary fibre and cereal fibre, and a roughly 20% reduction for each three servings of wholegrains daily (considered to be 90g a day). This “dose response” relationship also adds weight to the idea that fibre is genuinely protective and that it plays an important role in a balanced diet.