Fruits and vegetables contain soluble and insoluble fiber in varying proportions. When you eat soluble fiber, it absorbs water to form a gel-like mass, which helps you feel full and provides other health benefits. It prevents a surge in blood sugar after you eat by slowing down the absorption of carbohydrates.
If the blood glucose level falls to dangerous levels (as in very heavy exercise or lack of food for extended periods), the Alpha cells of the pancreas release glucagon, a hormone whose effects on liver cells act to increase blood glucose levels.
In a series of studies between 2007 and 2011, researchers at the University of Gothenburg in Sweden demonstrated that the release of ghrelin (the hunger hormone) by the stomach directly increases the release of dopamine in the brain's reward circuit.