undigested food rots somewhat on the way through and bacteria eats it and makes more gas.  this is particularly true with carbs that don't digest or human-indigestible carbs (sugar alcohols for example).  if it's fully digested (and absorbed) it's mostly fibre coming out.

I had massive problems (celiac) about 20 years ago with digestion, went to a doctor specializing in it.

 

 Aww the truth is not as fun as the speculation sometimes I suppose. Glad you got it sorted, celiacs is fucking gnarly 

 it's complicated though, gas.  what I said is surely not the entire story.

last fall I was super into winter squash.  my husband wasn't into the farts that resulted. 😂 I cut back and it went away.