This is the third of a five-part series on Metabolism.
When chyme exits the small intestine through the ileocecal valve and enters the large intestine, it is a fluid. By the time it leaves the large intestine through the anal sphincter it is a solid. Along the way water is absorbed from what is now called feces.
Fiber molecules are too large to be absorbed and humans do not have the enzymes to break them up, so both soluble fiber (dissolved in the chyme) and insoluble fiber exit the small intestine in much the same state as they entered. Another type of carbohydrate called ‘resistant starch’ also reaches the colon.
Human bodies may not have the enzymes to digest the soluble fiber but bacteria do. The bacteria that flourish in the properly seeded colonic garden take the “food” passing though the assembly line and ferment or eat it. If a favorite food is available, there can be a population explosion (much of the dry weight of feces is made up of bacteria) so bacterial bodies now join the assembly line procession. The by-products of bacterial fermentation are short chain fatty acids SCFA. Cells in the colon like to burn SCFA as fuel and they can also be absorbed into the blood where they have a beneficial effect on the body. Since the SCAF are absorbed they are a source of calories.
Finally the insoluble fiber, undigested food, unabsorbed nutrients, bacterial cells, and bile and other wastes leave your body as a bowel movement. This completes the absorption from your meal, it is now time for the second part –metabolism- to begin.