Dietary fiber has been found to be associated with lower risk of breast cancer in a number of prospective studies. Every 10g per day increase in dietary fiber intake was associated with a 7% reduction in breast cancer risk in one analysis. This finding perhaps is not surprising since fiber is found in plant foods with chemopreventive components.
However, fiber also appears to have its own anti-cancer effects. Dietary fiber consumed during adolescence has been found to be associated with reduced incidence of benign breast disease during adulthood. Benign breast disease itself is an indicator of increased breast cancer risk. High consumption of dietary fiber has also been linked to low levels of inflammation (as measured by CRP) in breast cancer survivors. Now a new meta-analysis of previous prospective studies has reported that high fiber consumption is associated with lower risks of both premenopausal and postmenopausal breast cancer.

Types of fiber

Dietary fiber consists of portions of plants that are edible but resistant to digestion. Soluble fibers (pectins, gums, mucilages) can dissolve in water and form a gel, which slows digestion. Insoluble fibers (cellulose, lignins) do not dissolve in water; they add bulk to the diet and speed up the passage of food and waste. Please see our article on fiber for lists of foods with high fiber content.
Fiber is also added to some processed foods in order to qualify the foods as “high fiber” or to improve texture. This isolated or functional fiber is extracted or synthesized from plant sources. Examples include inulin (from chicory root or sugar beets), soluble corn fiber (corn), cellulose (wood pulp), maltodextrin (corn, rice, or potato starch) and polydextrose (corn starch). Such fiber-fortified foods typically lack the beneficial flavonoids and other biologically active components of fruits, vegetables, grains and nuts and may lack nutritional value, however there is no evidence that functional fiber is harmful.

Latest research finds lower BC risk for high fiber diet

The meta-analysis referenced above was designed to investigate the association between dietary fiber intake and risk of breast cancer. To conduct the study, the authors searched the MEDLINE and Excerpta Medica dataBASE (EMBASE) databases through July 2019 for relevant prospective studies reporting on the association between fiber and new breast cancer cases. The meta-analysis included 17 cohort studies, two nested case‐control studies, and one clinical trial study. Relative risks were estimated by comparing the highest and lowest categories of fiber consumption.

Study results

High total fiber (soluble + insoluble) consumption was found to be associated with an 8% lower risk of breast cancer compared to low intake. Soluble fiber intake was found to be associated with lower risk of breast cancer (10%) than insoluble fiber (7%), for which the results were statistically weaker. Higher total fiber intake was also found to be associated with a lower risks of both premenopausal and postmenopausal breast cancers.
The authors also observed an inverse association between intake of total fiber and risk of both hormone receptor positive (ER+/PR+) and hormone receptor negative (ER-/PR-) breast cancer, although this result did not reach statistical significance. The authors conclude that high total fiber consumption, and soluble fiber in particular, is associated with reduced risk of breast cancer for both premenopausal and postmenopausal women.
Please see our article on fiber for more information.