The colon is not a uniform fermentation chamber. It is a metabolic gradient. In its first region, known as the proximal colon, gut bacteria generally have greater access to resistant starch and other fermentable carbohydrates. As these substrates are consumed, less remains available farther along the colon.
Most dietary protein is digested and absorbed in the small intestine, but some protein-derived material still reaches the colon from incompletely digested food, mucus, digestive secretions, and shed intestinal cells. Proteins are chains of amino acids. When they are partially broken down, they form smaller fragments called peptides. Farther along the colon, these peptides and amino acids can make up a larger proportion of the fuel available to gut bacteria.
This does not mean that bacteria suddenly stop fermenting carbohydrates and begin fermenting protein. Both processes occur throughout the colon, but their relative contribution changes. As protein-derived substrates gain influence, bacteria produce a broader mixture of compounds, including branched-chain fatty acids, ammonia, phenolic compounds, indoles, and sulfur-containing metabolites.
These products cannot be classified collectively as beneficial or harmful. Some can disrupt biological processes at sufficient concentrations, while certain indole derivatives participate in intestinal barrier maintenance and immune signaling. Their significance depends on the specific molecule, its concentration, where it is produced, how much is absorbed, and how the body transforms and clears it.
The full analysis in The Weekly Biome examines how declining carbohydrate availability changes microbial metabolism, what human dietary trials reveal about the protein-fiber balance, why adding fiber does not always produce a predictable correction, and why stool, blood, and urine measurements capture only part of the colonic metabolic gradient.
Continue reading in The Weekly Biome.