Oxidative damage, a byproduct of oxygen-utilizing metabolism, is thought to be involved in the progression of many chronic diseases, including cancers.  The human body has natural antioxidant defenses, but these defenses can be overwhelmed.  Consuming an antioxidant-rich diet can fully enable these defenses.  Many dietary antioxidant molecules have separate beneficial actions in addition to defense against oxidative damage.

Flavonoids are a class of antioxidant molecules that includes isoflavones, flavonols, catechins, and anthocyanins among others.  These molecules are found ubiquitously in plant foods.  The accumulation of evidence implicating a protective effect of fruits and vegetables against colon cancer, and these effects are theorized to be at least in part mediated by flavonoids.  Currently, dietary flavonoid intake in the Western world is quite low, reflecting the low intake of fruits and vegetables.[1] 

A study published in the UK in February compared dietary (excluding tea) flavanoid intake between colorectal cancer patients and healthy controls, and found that high flavonoid intake was associated with a 40% decreased risk of colorectal cancer.  These scientists went on to distinguish between individual flavonoids and cancer sites, and further found that intake of quercetin was associated with a 50% reduced risk of colon cancer.[2]  A similar study in Japan found risk reduction of approximately 25% for colorectal cancer in men and women with high isoflavone intake.[3]  Soybeans and minimally processed soy products like miso, tempeh, soy milk, and tofu are particularly rich in isoflavones.  A cohort study performed in the Netherlands followed men and women for 13 years, and found an association between catechin (another  flavonoid found in berries, fruits, beans and greens)intake and reduced colon cancer risk in women.  In this particular study the risk reduction was only seen in women with a body mass index (BMI) below 25, highlighting the importance of maintaining a healthy weight.[4]

Animal and cell culture studies have shed light on the means by which flavonoids may have these protective effects.  Quercetin, in particular, slowed the development of preneoplastic lesions in mice, suppressed growth and proliferation of colon cancer cells, induced programmed cell death in colon cancer cells, and down-regulated genes and cellular signaling pathways associated with survival of these cells.[5]^,[6],[7]  In other words, even when cells have been damaged earlier in life, flavonoids can offer protecton from those damaged cells advancing to cancer.

Flavonoids may also protect against colon cancer by curbing inflammation.  An inflammatory molecule called NF-κB is elevated in tumors of the colon.  In a recent study, normal mucosa, adenoma, and adenocarcinoma tissue were sampled from colorectal cancer patients, and the patients were interviewed to determine their dietary patterns.  The patients who consumed the most flavonoids had the lowest levels of NF-κB, and their tumors were less invasive.[8]   Polymethoxylated flavones (PMFs) may also have protective effects via reducing inflammation – PMFs decreased the generation of prostaglandin E2 in tumor cells, an inflammatory mediator produced downstream of omega-6 fatty acids.[9]  PMFs are abundant in citrus fruits and their peels.

Since flavonoids are ubiquitously present in plant foods, we can reap the anti-cancer benefits of these antioxidant molecules by eating a diet rich in vegetables, fruits, beans, nuts, and seeds.  Ongoing research is investigating the anti-cancer activities of several different flavonoids in order to identify the foods with the most powerful colon cancer fighting properties. 

Below are the some rich sources of selected flavonoids:[10]

[1] Pierini R, Gee JM, Belshaw NJ, et al. Flavonoids and intestinal cancers. Br J Nutr. 2008 May;99 E Suppl 1:ES53-9.

[2] Kyle JA, Sharp L, Little J, et al. Dietary flavonoid intake and colorectal cancer: a case-control study. Br J Nutr. 2010 Feb;103(3):429-36. Epub 2009 Sep 7.

[3] . Akhter M, Iwasaki M, Yamaji T, et al. Dietary isoflavone and the risk of colorectal adenoma: a case-control study in Japan. Br J Cancer. 2009 Jun 2;100(11):1812-6. Epub 2009 May 5.

[4] Simons CC, Hughes LA, Arts IC, et al. Dietary flavonol, flavone and catechin intake and risk of colorectal cancer in the Netherlands Cohort Study. Int J Cancer. 2009 Dec 15;125(12):2945-52.

[5] Miyamoto S, Yasui Y, Ohigashi H, et al. Dietary flavonoids suppress azoxymethane-induced colonic preneoplastic lesions in male C57BL/KsJ-db/db mice. Chem Biol Interact. 2010 Jan 27;183(2):276-83. Epub 2009 Nov 13.

[6] Shan BE, Wang MX, Li RQ. Quercetin inhibit human SW480 colon cancer growth in association with inhibition of cyclin D1 and survivin expression through Wnt/beta-catenin signaling pathway. Cancer Invest. 2009 Jul;27(6):604-12.

[7] Xavier CP, Lima CF, Preto A, et al.  Luteolin, quercetin and ursolic acid are potent inhibitors of proliferation and inducers of apoptosis in both KRAS and BRAF mutated human colorectal cancer

cells. Cancer Lett. 2009 Aug 28;281(2):162-70. Epub 2009 Apr 2.

[8] Ravasco P, Aranha MM, Borralho PM, et al. Colorectal cancer: Can nutrients modulate NF-kappaB and apoptosis? Clin Nutr. 2010 Feb;29(1):42-46. Epub 2009 Jul 1.

[9] Cai H, Sale S, Schmid R, et al. Flavones as colorectal cancer chemopreventive agents--phenol-o-methylation enhances efficacy. Cancer Prev Res (Phila Pa). 2009 Aug;2(8):743-50. Epub 2009 Jul 28.

[10] U.S. Department of Agriculture, Agricultural Research Service. 2003. USDA Database for the Flavonoid Content of Selected Foods. http://www.nal.usda.gov/fnic/foodcomp/Data/Flav/flav.pdf

U.S. Department of Agriculture, Agricultural Research Service. 2002. USDA-Iowa State University Database on the Isoflavone Content of Foods, Release 1.3 - 2002. Nutrient Data Laboratory Web site: http://www.nal.usda.gov/fnic/foodcomp/Data/isoflav/isoflav.html

U.S. Department of Agriculture, Agricultural Research Service. 2004. USDA Database for the Proanthocyanidin Content of Selected Foods.  http://www.nal.usda.gov/fnic/foodcomp/Data/PA/PA.pdf