Health in the Media: Fibre Consumption and Breast Cancer
Posted on 1st May 2020 by Ji Yun Stephanie Yeung
A recent Daily Mail headline reads: “Women could be at a higher risk of being diagnosed with breast cancer if they do not eat enough fibre”. Drawing on the same study, The Times reports: “Diet rich in fibre linked to lower risk of breast cancer”.
Headlines are snapshots. Although the suggestion that women are able to lower their risk of breast cancer simply by incorporating more fibre into their diets might sound appealing, these claims risk overlooking other aspects of the systematic review and meta-analysis published in CANCER, a peer-reviewed journal of the American Cancer Society.
Why was this research conducted?
Research has shown that dietary factors contributing to lower levels of circulating insulin, or that modify sex hormone levels, may impact breast cancer incidence. Associations between fibre intake and breast cancer risk have been previously evaluated, but the overall evidence is weak and inconsistent.
The principal investigator previously conducted a large cohort study in 2016 exploring whether a relationship existed between breast cancer risk and dietary fibre intake during adolescence and early adulthood. However, the association between fibre intake from different food sources and the risk of breast cancer had not yet been evaluated through meta-analysis of prospective studies.
Looking at the Evidence
Researchers from the Harvard T.H. Chan School of Public Health investigated the association between the overall risk of breast cancer and both the total intake and types of fibre. For the different types of dietary fibre, studies that met the inclusion criteria assessed the exposure variables of: total fibre, cereal fibre, fruit fibre, vegetable fibre, legume fibre, soluble fibre and insoluble fibre.
The study also assessed these relationships according to menopausal (premenopausal and postmenopausal) and tumour hormone receptor status (oestrogen receptor and progesterone receptor).
Data from 21 publications, including a total of 1,994,910 women, was used for random-effects meta-analysis. By inclusion of only prospective cohort, nested case-control and clinical trial studies, recall and selection biases were minimised.
Findings do show a statistically significant association between high intake of total and soluble fibre and a reduced overall breast cancer incidence. Total fibre consumption was associated with an 8% lower risk of breast cancer (pooled relative risk [RR] comparing the highest versus the lowest category 0.92; 95% confidence interval [CI] 0.88‐0.95). All the different sources of dietary fibre intake (cereal, fruit, vegetable and legume) exhibited similar reduction in breast cancer risk, with only fruit fibre reaching statistical significance.
As for menopausal status, a high intake of total fibre was significantly associated with a decreased risk of breast cancer. While there was an inverse association between intake of total fibre and both oestrogen and progesterone receptor–positive and oestrogen and progesterone receptor–negative breast cancers, this was nonsignificant.
Conclusions & Looking Ahead: What does this change?
The above excerpt from the Daily Mail’s article implies that simply increasing the amount of fibre in one’s diet can protect against future cases of breast cancer. Observing correlations between variables is straightforward. But establishing that one thing causes another is an entirely separate matter. The biological mechanism behind why fibre could prevent breast cancer remains unclear, but its potential role in improving insulin sensitivity and reduction of circulating oestrogen levels has been postulated.
Moreover, authors note that total fibre consumption from different fibre sources was also associated with increased overall dietary quality. Intake of other biologically active ingredients could therefore be contributing to the protective health benefits either alongside or in place of the effects of fibre consumption. On the subject of potential confounding factors, the majority of included studies (17 of the 21 publications) had adjusted for important potential risk factors for breast cancer. An assessment of potential sources of heterogeneity among studies – including region of study and deviations of follow-up, with adjustments for energy intake, alcohol intake, smoking, history of benign breast disease, and family history of breast cancer – found a low to moderate degree of statistical heterogeneity. However, residual confounding is likely to be observed in most observational studies; it is difficult to eliminate the risk for confounders entirely. Given that most of the studies were carried out in North America and Europe regions, the authors also recognise the limitation that the results may not be directly generalisable.
Nevertheless, this meta-analysis found that total fibre intake demonstrated a statistically significant protective association against breast cancer. The study findings are in line with evidence-based dietary guidelines advising the consumption of foods rich in total fibre, highlighting the importance of maintaining a healthy and balanced diet.