Only 7.5% of poop comes from dietary fiber.
“Normally, feces are made up of 75 percent water and 25 percent solid matter. About 30 percent of the solid matter consists of dead bacteria; about 30 percent consists of indigestible food matter such as cellulose; 10 to 20 percent is cholesterol and other fats; 10 to 20 percent is inorganic substances such as calcium phosphate and iron phosphate; and 2 to 3 percent is protein. Cell debris shed from the mucous membrane of the intestinal tract also passes in the waste material, as do bile pigments (bilirubin) and dead leukocytes (white blood cells).”
Source: https://www.britannica.com/science/feces
One of the major benefits of longer fasting is to induce autophagy to clear old damaged cells—that’s waste, too.
The idea that you have secret pockets of poop in your gut that are being cleared out several days into your fast is not based on any kind of science. It’s a line people use to sell colonics.
Well, you asked, and the answer is kind of gross, sorry.
Many people have fecal matter backed up inside their colon (this is especially true for those who don’t see the point of drinking lots of water, or eating a lot of fiber). If you’re one of them, your body is going to take the chance to let go of some impacted matter.
Just imagine your turds screaming, “Yay, I’m finally freeeeeee!” as they come out. It doesn’t help with going, but it’ll make you laugh and it will all be less worrying that way. ;)