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

Paleo Diet

Over the last million years a group of primates began walking upright. This genus, Homo, (our species, Homo sapiens sapiens, evolved about 200,000 years ago) is distinguished by a number of physiological and morphological adaptations to their environment.   Importantly, a change in diet from our primate relatives appears to have been the key change that drove our recent evolution.

What was the paleo diet of the genus Homo?

Stephan Guyunet provides this analysis:

"All we know is that they ate some meat. Although humans eventually became top-level predators, we also don't know whether these early humans were actively hunting, or simply scavenging what other predators left behind-- perhaps using their tools to access gristle, brain, and marrow inaccessible to other animals.

At the same time as tool-marked bones appear in the archaeological record, early humans began undergoing a remarkable physical transformation, which represented (in large part) a progressive genetic adaptation to a new subsistence strategy. Our brain doubled in volume, our gut became smaller, and the proportion of small intestine to large intestine increased. Our teeth and jaws became smaller and less robust (Daniel Lieberman. The Story of the Human Body. 2013).

What does this signify? The consensus is that these changes occurred in response to a shift toward a so-called "high-quality" diet. This means a diet that has a higher calorie density and contains less fiber, relative to the typical primate diet of leaves and low-calorie fruit (the latter is not at all suitable for a modern human). The small intestine is what breaks down and absorbs protein, carbohydrate, and fat, while the large intestine ferments fiber to extract calories from it. The shift from a large-intestine-dominant gut to a small-intestine-dominant gut signifies a shift from getting most calories from intestinal fiber fermentation, to getting most calories from direct absorption of protein, carbohydrate, and fat."