Fibre and prevention of chronic diseasesBMJ 2011; 343 doi: http://dx.doi.org/10.1136/bmj.d6938 (Published 10 November 2011) Cite this as: BMJ 2011;343:d6938
- Anne Tjønneland, professor and head of unit,
- Anja Olsen, project office manager
In the linked systematic review and meta-analysis (doi:10.1136/bmj.d6617), Aune and colleagues assess the association between the intake of dietary fibre and whole grains and the risk of colorectal cancer.1
It was first hypothesised in 1988 that food items rich in dietary fibre may prevent colorectal cancer,2 but randomised trials studying dietary fibre have not supported the association.3 This is a classic situation within nutritional epidemiology; a food item is related to decreased incidence of disease and the biological effect is attributed to a single component, but when this component is tested in randomised trials the results are not what was expected. This ought to have taught researchers to study the dietary sources and not only one specific component, which is exactly what Aune and colleagues have done.1 Their systematic review and meta-analysis of prospective observational studies clearly shows that a high intake of fibre from cereals and high consumption of wholegrain foods is significantly associated with a reduced risk of colorectal cancer, whereas no preventive effects can be seen with other sources of dietary fibre.
Fruits, vegetables, legumes, and grains are our main sources of dietary fibre. The first three food groups have received major interest in cancer epidemiology. Fruits and vegetables have been considered especially important in preventing cancer,4 although more recent research has questioned this.5 Grains have received considerably less interest.
Whole grains, by definition, contain all the anatomical fractions of the cereal product, either intact or reconstituted in the components,6 but since the industrial revolution people have favoured white flour. In 2001, the first analysis of consumption of whole grains in the United Kingdom found that it was extremely low—about a third of British adults failed to consume any wholegrain foods and 90% consumed less than three servings a day.7 Equally low intakes of whole grain are seen in the United States and most of Europe, with wholegrain foods forming a substantial role in the diet only in some northern and eastern European countries.
When refining the grain, most of the germ and the bran—and therefore many of the bioactive compounds—are removed. Depending of the type of grain, about 80% of the fibre, and a substantial number of essential minerals, vitamins, and bioactive compounds—such as polyphenols, sulphur amino acids, and lignans—are lost.8 When studying wholegrain foods it is therefore important to assess much more than dietary fibre. Many of the specific mechanisms involved in the health benefits of whole grains still need to be explored and explained in detail, but factors such as increased satiety, stabilised glucose homoeostasis and insulin response, and the fermentation of fibre and resistant starch in the colon to produce short chain fatty acids are probably important.
Aune and colleagues’ meta-analysis adds to the current evidence of the many health effects of whole grains.1 Observational studies have shown that wholegrain foods probably protect against the development of obesity, type 2 diabetes, and cardiovascular disease,6 but data in relation to cancer have so far been limited.
From a public health point of view, it is difficult to disagree with the recommendation for people to eat more wholegrain foods because evidence in their favour is rapidly accumulating. To increase the intake of these foods in Western countries, the health benefits must be actively communicated and the accessibility of wholegrain products greatly improved, preferably with a simple labelling system that helps consumers to choose products with high wholegrain contents.
Aune and colleagues’ meta-analysis indicates that it is the whole grain and not the fibre component alone that has beneficial effects. Types of fibre differ between different food groups and it is unclear why fibre from grains is associated with decreased risk when fibre from fruit, for example, is not. Increasing the fibre content of refined grain products is therefore not likely to be sufficient.
Although a high intake of whole grain can be recommended, research is still needed to explain the biological mechanisms responsible for the beneficial effects of these foods in detail, including the effects of different types of grain. Some research indicates that wholegrain rye may be even more beneficial than other types of whole grain,6 but this needs to be studied further. Barriers to increasing the intake of wholegrain products also need to be studied; many people might think that wholegrain foods are less tasty than refined alternatives. However, at least for children, limited availability of wholegrain foods in the household, not preference, has been shown to be the major obstacle to intake.9 With time, people may even find that they prefer whole grains.
Cite this as: BMJ 2011;343:d6938
Competing interests: All authors have completed the ICMJE uniform disclosure form at www.icmje.org/coi_disclosure.pdf (available on request from the corresponding author) and declare: no support from any organisation for the submitted work; no financial relationships with any organisations that might have an interest in the submitted work in the previous three years; no other relationships or activities that could appear to have influenced the submitted work.
Provenance and peer review: Commissioned; not externally peer reviewed.