The small intestine's major function is to absorb food and nutrients. When food enters the stomach, acid begins to break it down, and some nutrients are absorbed in the stomach. Then the food passes into the small intestine, where enzymes and sodium bicarbonate flow in from the pancreas to neutralize any remaining stomach acid and break down the remaining nutrients. The large intestine absorbs water and creates fecal matter.
The small intestine also plays a role in the body's immune system. Probiotic gut flora not only aid in the digestion of troublesome compounds, they also increase the body's immune response.
When food enters the small intestine, the body handles each type of molecule differently. Lipids break down into fatty acids and glycerol, while proteins break down into amino acids and peptides for easy absorption into the body. Carbohydrates may break down into simple sugars, but complex carbohydrate molecules may pass through the small intestine relatively intact. Once they reach the large intestine, bacteria break them down and produce intestinal gas as a by-product. This is why foods high in complex carbohydrates, such as beans, can cause intestinal discomfort. To combat this effect, special enzymes break down the complex carbohydrates before the bacteria have a chance to consume them, preventing the release of gas.