In short: There is insufficient or conflicting evidence about the cancer-protecting effect of a high-fiber diet.
- Colorectal cancer: increased stool bulk and dilution of carcinogens in the colonic lumen, reduced transit time, and bacterial fermentation of fibre to short chain fatty acids [which are supposedly protective for colonic mucosa]
- Breast cancer: Dietary fibre reduce the risk of breast cancer may likely by decreasing the level of estrogen in the blood circulation.
Several systematic reviews suggest that high intake of dietary fiber may protect against various types of cancer:
1. Dietary fibre, whole grains, and risk of colorectal cancer: systematic review and dose-response meta-analysis of prospective studies (PubMed Central, 2011)
A high intake of dietary fibre, in particular cereal fibre and whole
grains, was associated with a reduced risk of colorectal cancer.
2. Dietary fibre intake and risk of breast cancer: A systematic review and meta-analysis of epidemiological studies (PubMed Central, 2016)
...every 10 g/d increment in dietary fibre intake was associated with
a 4% reduction in breast cancer risk...
3. Dietary fiber and breast cancer risk: a systematic review and meta-analysis of prospective studies (PubMed, 2012)
...there was an inverse association between dietary fiber intake and
breast cancer risk
4. Dietary fiber and the risk of precancerous lesions and cancer of the esophagus: a systematic review and meta-analysis (PubMed, 2013)
Dietary fiber is associated with protective effects against esophageal
carcinogenesis, most notably esophageal adenocarcinoma.
5. Linus Pauling Institute:
- ...more recent findings from large prospective cohort studies and four clinical intervention trials do not support an association
between fiber intake and the risk of colorectal cancer.
- Observational studies on dietary fiber intake and breast cancer incidence have reported inconsistent findings.
This review found that increasing fibre in a Western diet for two to
eight years did not lower the risk of bowel cancer.