Accumulation of undigested molecules would most directly be an indication of dysfunction.

enzyme dysfunction 

Digested food means that the food complex is cut into a smaller molecule that can be absorbed into the body. To digest food, the gastrointestinal tract will release many kinds of enzyme to make the chemical reaction happen. When there are a deficiency or dysfunction of the enzyme then the molecule won't be digested and can't be absorbed.

They became more resistant.

The answer is lymph sinuses.