When the small intestines have done all they can, the food is passed to the large intestine.
The large intestine contains an incredible amount of bacteria.
There are around 500-1000 different species in there (DiBaise et al., 2008); and about 60% of your stools' dry weight is just bacteria! (Guarner and Malagelada, 2003)
They survive by digesting the stuff you couldn't digest in your stomach and small intestine -- and they produce gas. This is why you get gas after you eat something (like beans) that you aren't good at digesting; the bacteria get more food.
If something interfered with your ability to digest food in your small intestines, all that good stuff would end up in the large intestine. The bacteria would go to town, and produce loads of gas!
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