It’s been estimated that with a typical Western diet, up to 12 grams of protein can escape digestion, and when it reaches the colon, it can be turned into toxic substances like ammonia. This degradation of undigested protein in the colon is called putrefaction; so, a little meat canactually end up putrefying in our colon. The problem is that some of the by-products of this putrefaction process can be toxic.
It’s generally accepted that carbohydrate fermentation—the fiber and resistant starches that reach our colon—results in beneficial effects because of the generation of short-chain fatty acids like butyrate, whereas protein fermentation is considered detrimental. Protein fermentation mainly occurs in the lower end of colon and results in the production of potentially toxic metabolites. That may be why colorectal cancer and ulcerative colitis tend to happen lower down—because that’s where the protein is putrefying.
Probably the simplest strategy to reduce the potential harm of protein fermentation is to reduce dietary protein intake. But the accumulation of these toxic by-products of protein metabolism may be attenuated by the fermentation of undigested plant matter. In my video, Bowel Wars: Hydrogen Sulfide vs. Butyrate, you can see that a study out of Australia showed that if you give people foods containing resistant starch, you can block the accumulation of potentially harmful by-products of protein metabolism. Resistant starch is resistant to small intestine digestion; and so, it makes it down to our colon where it can feed our good bacteria. Resistant starch is found in cooked beans, split peas, chickpeas, lentils, raw oatmeal, and cooled cooked pasta (like macaroni salad). Apparently, the more starch that ends up in the colon, the less ammonia that is produced.
Of course, there’s protein in plants too. The difference is that animal proteins tend to have more sulfur-containing amino acids like methionine, which can be turned into hydrogen sulfide in our colon. Hydrogen sulfide is the rotten egg gas that may play a role in the development of the inflammatory bowel disease, ulcerative colitis (see Preventing Ulcerative Colitis with Diet).