Which body system converts food into simpler substances?

The answer to your question is the Digestive System.

It goes like this:
Starches are broken down into disaccharides from enzymes in the mouth.

Then protein is broken down into large peptides from enzymes in the stomach.

Enzymes in the small intestines continues to break down starch and protein and breaks down fat.

Then enzymes in the large intestines breaks down the remaining disaccharides into monosaccharides and breaks down dipeptides into amino acids.

I hope this helps! :)