Not sure of the exact biochemistry here Adam01, but when you smoke weed your body tries to expel the chemicals the same way it would expel alcohol from your system (getting drunk), even though it doesn't directly affect the digestive system.

Dehydration is the main factor, but another is a lack of glucose in the system to process the chemicals in the blood stream, so your body gives the signal to your brain to get more glucose-rich sources, aka you feel hungry.

But if there is glucose in the body the body will process the chemicals faster than on an empty stomach