What Causes Colon Cancer? More importantly, what prevents it?

Colon cancer incidence --like that of many cancers-- varies several-fold between different countries, indicating that perhaps there are environmental or lifestyle factors that cause or prevent it. Much of this post is based on this excellent review article

Known Influences of Colon Cancer Risk (ranked from most important to most equivocal)

Aspirin: The Women's Health Study of 34,000 women 45 years and older found a 42% reduction in colonrectal cancer in the group that was taking aspirin.

Selenium: Populations with adequate selenium intake (most of the U.S. is OK, but European soils are selenium-depleted so their foods are lower) have lower colorectal cancer.

CLA, or conjugated linoleic acid is protective against cancer and found in grass-fed beef and dairy products, possibly explaining a lower incidence of colorectal cancer among people who eat dairy.

Dietary fat: results are conflicting. While a study based on a large, hospital-based sample of cases and controls, provided no evidence that dietary fat was associated with increased risk15, an equally large and well executed case-control analysis revealed a strong association of dietary fat with colon cancer risk 16.

Fiber: results are conflicting. One of the best studies of diet and colon cancer, the prospective Nurses Health Study, indicated that dietary fiber does not affect the risk of subsequent colon cancer 17. Although some fruits and vegetables, which contribute fiber to the diet, may be protective, dietary fiber alone appears to have no impact on colon cancer risk17.

Vegetarian Diet: while eating a vegetarian diet slightly lowers the risk of all cancers, it has also been found to increase the risk of colon cancer, according to the European Prospective Investigation into Cancer and Nutrition (EPIC) study.

Meat: results are conflicting, and different for different types of meat.  “The available epidemiologic data are not sufficient to support an independent and unequivocal positive association between red meat intake and CRC [colorectal cancer].” - from a review of the evidence regarding red meat consumption and colon cancer in the European Journal of Cancer Prevention [1]. Cited by Dr. Briffa.

However, a number of compounds in meat are known to be harmful.  For example, heterocyclic amines, a known cancer-promoting substance, are produced when meat is grilled or fried at high temperature.  High iron levels may induce oxidative stress and can be harmful.  Meat increases IGF-1, which promotes growth of all cells, esp. cancer. Recently, a sugar in red meat called Neu5Gc has been linked to inflammatory effects.  And even Carnitine, a heart-healthy compound found in red meat, has been found to stimulate bacteria to produce TMAO, a known cancer-causing substance.  However, whether all of these individual compounds, when consumed as part of a healthy diet, cause cancer, has yet to be determined.   Chris Kresser has comprehensive review article that addresses these complexities in greater detail.  

[1] Alexander DD, et al. Meta-analysis of prospective studies of red meat consumption and colorectal cancer. Eur J Cancer Prev. 2011 May 2