Narita, et al., describe the findings of their study in an article titled “Dietary fiber intake and risk of breast cancer defined by estrogen and progesterone receptor status: the Japan Public Health Center-based Prospective Study” published in Cancer Causes & Control. The researchers explored possible associations between dietary fiber intake and risk of breast cancer and whether the association is influenced by reproductive factors and hormone receptor status in 44,444 women participating in the Japan Public Health Center-based Prospective Study (JPHC Study).
JPHC Study participants were recruited from municipalities supervised by 11 public health centers across Japan in 1990 (Cohort I) and 1993 (Cohort II). Five years after recruitment, participants completed a food frequency questionnaire (FFQ) that assessed average consumption frequency and portion size of 138 food and beverage items consumed in the past year. Intake of seasonal fruits and vegetables was estimated by asking the frequency of intake and length of each season. Breast cancer was identified by patient notification and record linkage with a population-based cancer registry. In this study, 97 percent of cancer was determined microscopically with estrogen receptor (ER) and progesterone receptor (PR) status determined by immunohistochemistry or enzyme-linked immunosorbent assay (ELISA).
For data analysis, the participants were divided by quartiles according to their energy-adjusted intakes of total fiber, soluble fiber, insoluble fiber, natto (fermented soybean), and rice. Multivariate statistical models were adjusted for a number of potential confounding factors. Among other findings, the researchers report no association between the consumption of fiber and breast cancer risk was identified in this cohort after adjusting for all potential confounding factors. However, the researchers report the highest intake of dietary fiber, with a median intake of 18.1 g/day, was associated with decreased risk of breast cancer.
In interpreting the data, the researchers note that “The overall null associations in the present study might be explained by small ranges of fiber consumption between quartiles.” Furthermore they state “In conclusion, while previous studies have found protective effects of high dietary fiber consumption against breast cancer, our findings were unable to draw clear comparison due to lower intake of fiber among Japanese population. Future research is needed to further investigate the association in the Japanese or Asian population between high levels of dietary fiber intake and the potential effect on breast cancer survival and recurrence.” However, the authors note the following limitations:
- The study is limited by self-reported data that may introduce bias, FFQ may have been less accurate than a 24 hour diet recall as shown by low correlation of FFQ and 28-day (or 14-day) dietary records, confounding factors introduced through health-conscious behavior changes, and ER/PR breast cancer subtype data availability being limited to 50 percent of the breast cancer cases in the study.
- Diet, environment, and lifestyle behaviors of participants in this study may not reflect other populations.
- Possible mechanisms of dietary risk factors influencing the incidence of breast cancer are not understood and several potential mechanisms of action for dietary fibers have been proposed.