Originally Posted by mrsD

Chronic diarrhea, gluten intolerance/celiac disease, Crohn's,
GERD (use of acid blocking drugs), all lead to malabsorption of nutrients. Especially B12, but others too. Low B12 damages nerves.

Damage to autonomic nerves that run the GI tract (mostly the stomach) leads to gastroparesis, and failure to digest your food.

It goes both ways.