Investigating food absorption

Dialysis tubing is manufactured for use in kidney machines. It can be used to simulate absorption of food by diffusion in the gut, because it is similar in permeability. The diagram shows how such a simulation could be contrived.
Possible foods to test:

The presence of such foods can be detected using these methods:
● glucose—with urine test strips or with Benedict’s test
● starch—with the iodine test
● amino acids—with universal indicator or a pH probe
● casein—with the biuret test.

Question:

Formulate a hypothesis for the function of the gut (to digest food) and explain it using scientific reasoning.