Why does following a fiber-rich diet help reduce the odds of getting cancer?

Question

Following a fiber-rich diet seems to help reduce the odds of getting cancer, according to some scientific studies. Why would fibers help preventing cancers?

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WebMD indicates an indirect effect: fiber-rich diet -> healthy weight -> lower risk for many kinds of cancer (1), but I'd expect fibers to help prevent cancers in other ways as well:

Almost 70 years later, scientists are still mulling the issue. Some studies have found a link between a fiber-rich diet and lower cancer risks. Others haven't.

Eating lots of fiber mayhelp stave off certain types of cancer -- it just hasn't been proven yet.

Research is clear that eating a high-fiber diet can help you stay at a healthy weight, which in turn, lowers your risk for many kinds of cancer.

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(1) Does being underweight have an influence on cancer risk?

Answer 1

In short: There is insufficient or conflicting evidence about the cancer-protecting effect of a high-fiber diet.

Suggested mechanisms:

• 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.

BUT:

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.

6. Cochrane:

This review found that increasing fibre in a Western diet for two to eight years did not lower the risk of bowel cancer.

Answer 2

No one really knows. A high fibre diet tends, however, to be lower in the foods associated with increased cancer risk. Fibre also reduces the colon transit time so that waste carcinogens have less time to contact the bowel mucosa. High fibre diets also might reduce breast cancer by binding to estrogens, and there's now a suggestion that phytates bind to iron which is helpful since iron is thought to increase the risk for bowel cancer.

Beans, a high fibre vegetable, also have been studied for their anticancer activity

Dry beans contain a wide range of nutrients and nonnutrient bioactive constituents that may be protective against cancer (43,50). The nondigestible carbohydrates are all fermented by colonic microflora into butyrate, a short-chain fatty acid, with demonstrated antineoplastic (51) and anti-inflammatory actions (52,53). Furthermore, dry beans have a low glycemic index (GI), defined as the incremental area under the blood glucose curve induced by a specific carbohydrate-containing food (54), which reduces the rate of the absorption of carbohydrates and lowers the postprandial glycemic and insulinemic responses. A number of epidemiologic studies showed that a low-GI diet is associated with a reduced risk of CRC (55-57). Other bioactive constituents of dry beans that have anticarcinogenic properties and could potentially account for a protective effect include saponins, protease inhibitors, inositol hexaphosphate, γ-tocopherol, and phytosterols (49). It is also possible that the combination of several different constituents of dry beans is most effective in reducing cancer risk.

https://www.pcrm.org/health/cancer-resources/diet-cancer/nutrition/how-fiber-helps-protect-against-cancer

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1713264/