The mouth and the stomach do a good job with the prep work, but the real serious work of digestion doesn’t get start until the food is in the small intestine.
Bile from the liver and digestive enzymes from the pancreas flow into the small intestine. The pulsating movement of the intestinal walls helps mix them into the food, so it gets broken down into small molecules that are absorbed through the wall of the small intestine and into bloodstream for distribution to the rest of the body.
The watery residue of secretions and food that can’t be broken down pass from the small intestine into the large intestine, which includes the colon and the rectum.
Your colon is recycler: it reabsorbs much of the water added to the food in the small intestine. Feces are the relatively solid material left over after the water has been sponged away by the colon.
When the small intestine is inflamed—as it often is with Crohn’s disease—it can’t fully digest and absorb the nutrients from food. They end up in the colon, along with unabsorbed bile salts.
As a result, people with Crohn’s disease may become malnourished because nutrients pass into the colon instead of being absorbed “upstream” in the small intestine.
In addition, undigested food in the colon interferes with water reabsorption, so diarrhea may result. If the colon is also inflamed, the diarrhea may be more severe.