Colon cancer risk in Westernized populations may be reduced by decreasing animal product intake and thereby decreasing  “aggressive” factors as animal protein and fat. I’ve explored how animal protein can putrefy and produce the rotten egg gas that may be toxic to DNA (see Putrefying Protein and “Toxifying” Enzymes), but what about the fat? It can stimulate the synthesis and secretion of bile acids into the intestine.


Bile helps the body digest fats. So, more fat in the intestines means more bile in the intestines, which wouldn’t be a problem except bile acids, especially secondary bile acids, have long been suspected as being carcinogenic. Bile acids stimulate the growth of bacteria, which convert the primary bile acids our livers make into secondary bile acids. And, secondary bile acids have been shown to be cancer-causing.

As described in my How to Reduce Carcinogenic Bile Acid Production video (see below), this could help explain why fat-rich diets are correlated with colon cancer. High saturated fat intake is associated with elevated levels of bile, which is what you tend to see in people with colon cancer. As such, they are considered tumor-producing factors in colorectal cancer development and perhaps also breast cancer, as these secondary bile acids can get absorbed into the blood stream and circulate throughout the body. (See Breast Cancer and Constipation for more on this.)


This may help explain the extraordinarily low rates of colon cancer in sub-Saharan Africa, with native Africans putting out just a fraction of the secondary bile acids of African Americans. Well, if a diet high in animal fat stimulates the growth of these toxic and carcinogenic secondary bile salt-producing bacteria, what about diets that don’t include any animal fat? We’ve known for more than 40 years that those eating plant-based diets have less bile in their stools and a reduced capacity to create colon carcinogens. Those eating vegetarian produce just a fraction of some of the secondary bile acids implicated in cancer—about 70% less. Within just one week on a plant-based diet, the bacterial enzyme activity to produce these secondary bile acids is cut in half. Within a month, their presence is cut in half, as well.


One of the most important toxic effects of these bile acids—the BAs in our BMs—is the increased production of free radicals. That’s one of the ways they can damage our DNA and undermine our DNA repair pathways.

Hydroxyl radicals are one of the most destructive free radicals, which may increase colon cancer risk. They only last about a billionth of a second but, in that time, can convert harmless substances in the bowel to DNA-damaging, mutagenic substances, and bile acids are believed to promote this process. But, if you switch people to a vegetarian diet for only 12 days, you can get a 13-fold drop in hydroxyl free radical production.

So, fecal free radicals may activate carcinogens in the colon. On a standard American diet, the amount of free radicals produced in the stool is quite remarkable, corresponding to that which would be produced by a fatal dose of gamma radiation. So, what do we do about it? What’s an achievable, practical measure to decrease free radical formation in our colon? We could attempt to “colonize the colons of high risk patients with genetically engineered, antioxidant-producing bacteria,” but why not just eat a more plant-based diet?

What about fiber? See Fiber vs. Breast Cancer for more information on why fiber may be so protective.

You can find more on the microbiome in Stool pH and Colon CancerPreventing Ulcerative Colitis with Diet, and Why Do Plant-Based Diets Help Rheumatoid Arthritis?.

In health,

Michael Greger, M.D.

*Article originally appeared at Nutrition Facts.