To appreciate this, consider how those enzymes that digest proteins work. A protein molecule is a large, complex string of amino acids, each linked to the next in a specific order. Suppose there are only six amino acids: 1, 2, 3, 4, 5, and 6. So a particular (imaginary) protein could be structured: 1, 4, 4, 6, 2, 3, 5, 4, 2, 3, 6, 1, 1, 2, 3, etc. Thus you should see that by combining a limited number of amino acids there can be a virtually infinite number of proteins. But proteins are rarely water soluble. As I said a few paragraphs back, digestion consists of rendering insoluble foods into water-soluble substances so they can pass into the blood stream and be used by the body's chemistry. To make them soluble, enzymes break down the proteins, separating the individual amino acids one from the other, because amino acids are soluble. Enzymes that digest proteins work as though they are mirror images of a particular amino acid. They fit against a particular amino acid like a key fits into a lock. Then they break the bonds holding that amino acid to others in the protein chain, and then, what I find so miraculous about this process, the enzyme is capable of finding yet another amino acid to free, and then yet another. So with sufficient churning in an acid environment, with enough time (a few hours), and enough enzymes, all the recently eaten proteins are decomposed into amino acids and these amino acids pass into the blood where the body recombines them into structures it wants to make. And we have health. But when protein chains are heated, the protein structures are altered into physical shapes that the enzymes can't "latch" on to. The perfect example of this is when an egg is fried. The eggwhite is albumen, a kind of protein. When it is heated, it shrivels up and gets hard. While raw and liquid, it is easily digestable. When cooked, largely indigestable. Stress also inhibits the churning action in the stomach so that otherwise digestible foods may not be mixed efficiently with digestive enzymes. For all these reasons, undigested proteins may pass into the gut. Along with undigested starches. When starches convert best to sugars under the alkaline conditions found in the mouth. Once they pass into the acid stomach starch digestion is not as efficient. If starches reach the small intestine they are fermented by yeasts. The products of starch fermentation are only mildly toxic. The gases produced by yeast fermentations usually don't smell particularly bad; bodies that regularly contain starch fermentation usually don't smell particularly bad either. In otherwise healthy people it can take many years of exposure to starch fermentation toxins to produce a life-threatening disease.