The human body is capable of digesting a very wide range of foods, including sugars, fats, and proteins.
But the trick to such a varied diet is in the body's ability to stop digesting when the food's all gone so that it doesn't start in on itself.
Start And Stop
One of the ways the stomach avoids digesting itself involves the body's careful handling of the strong chemical called protease.
Protease is a group of enzymes that break down protein. But since the body itself is made of protein, it's important that those enzymes don't go to work on our own bodies.
How Does The Body Produce Protease?
The body produces protease in the pancreas, but the pancreas doesn't produce protease in a working condition. Instead, the protease produced in the pancreas has to be activated by another enzyme found in the intestine.
Only after it is activated by the other enzyme, can the protease go to work breaking down protein. The second, activating enzyme, in turn only does its job when food enters the stomach.
Alcohol, Disease, And Drugs Harm Your Stomach
At night when there's no food in your stomach, the protease is deactivated so it stops working.
Unfortunately, disease, alcohol, and some drugs can override the enzyme that is supposed to be controlling the protease. When that happens, the protease begins to digest the stomach wall and ulcers develop.
The On/Off Switch
So, in a healthy person, the body builds its digestive enzymes with what amounts to an on/off switch and then builds a second enzyme especially designed to operate the switch.
The digestive system also protects itself by being one of the fast growing tissues in the body, constantly discarding old cells and reproducing new ones.
So some of it does get digested, but there's always more to take its place.