Major Review: More Fiber Lowers Risk of Breast Cancer
Research shows that diets high in fiber reduce the risk of colorectal cancer. Now, a major analysis of the literature that included almost one million women suggests that dietary fiber may also protect against breast cancer.
The research was funded by the World Cancer Research Fund as part of AICR/WCRF's Continuous Update Project (CUP), an ongoing review of cancer prevention research. It builds on a 2007 and 2009 AICR/WCRF review of the literature, which concluded the evidence linking dietary fiber and breast cancer prevention was too limited or inconsistent to draw a conclusion.
Here, lead author Dagfinn Aune, a nutritional epidemiologist at Imperial College London who is part of the CUP team, talks about the findings.
Q: Your analysis looked at 16 studies on fiber and breast cancer risk. What were the main findings?
A: We found that there was a 5 percent reduction in risk of breast cancer for each 10 grams per day of dietary fiber women eat. Our analyses suggest that increasing dietary fiber intake may be a promising approach to reduce breast cancer risk. Although the association is weak, breast cancer is such a common cancer and everybody eats, so increasing fiber intake could still result in many breast cancer cases being prevented.
"We found that there was a 5 percent reduction in risk of breast cancer for each 10 grams per day of dietary fiber women eat."
Q: And you looked at prospective studies, which follow a population over time?
A: Yes. We only included prospective studies to avoid potential biases that can affect retrospective case-control studies, such as recall and selection biases.
Q: Did the results surprise you, given that the WCRF/AICR 2007 report and its updates found the evidence was inconclusive.
A: We were not very surprised by the findings. Updating the evidence is the role of the CUP and with more published studies, we are able to detect associations now that could not be detected in 2007, when the number of published studies was lower.
Because we included all the available prospective studies we had more statistical power to detect a significant association than each individual study had on its own and that is probably one of the reasons we found an association in contrast to the previously inconsistent results from individual studies. Also, the hypothesis has been supported by results from experimental studies.
Q: Can you talk about how the CUP systematic process works in general?
A: There is a protocol that we follow which specifies how the searches are done, how the analyses are conducted etc. We search in the PubMed database and all the searches are then screened for relevant studies. We then extract the relevant data from the studies into a database. Much of the work we do goes into conducting the searches, screening studies, extracting data and cleaning up datasets before analyses can be conducted and reports be written. We have double-checking procedures of article selection and data extraction.
Q: You looked at many types of fibers independently, from vegetables to grains; can you talk about what you found?
A: Yes, we looked at fruit fiber, vegetable fiber and cereal fiber in relation to breast cancer risk. We did not find a significant association for any of these, but this may have been due to limited statistical power because of the limited number of studies that reported on fiber types. There was some suggestion of an inverse association for fruit and cereal fiber, but not vegetable fiber, but neither of these results was significant. However, we did find a significant association for soluble fibers, but not insoluble fibers.
Q: What are some possible mechanisms why dietary fiber may protect against breast cancer?
A: Experimental studies both in humans and animals have shown that higher intake of dietary fiber can reduce estrogen levels in blood. Fiber may bind estrogens in the colon and increase the fecal excretion of estrogens. In addition, high fiber intake can reduce hyperinsulinemia and may also reduce the risk of overweight and obesity, which are established risk factors for postmenopausal breast cancer. In our analysis the association appeared to be independent of body fatness, thus, it appears that the latter explanation is not the main mechanism.
A: I have a passion for preventive medicine. The many indefinite answers in the field of nutritional epidemiology make it even more exciting to work in this area by trying to come up with clearer answers through systematic reviews and meta-analyses of the overall evidence. We have shown in several meta-analyses now that even though individual studies by themselves may not provide clear answers with regard to specific diet-cancer relationships, meta-analyses of the evidence can bring more definitive answers. Being part of the Continuous Update Project is great because the work we do will result in recommendations that can help reduce the world cancer burden.