Digestion is the process of turning food into nutrients which can be absorbed into the body. Several diseases are the result of the inability of the body to digest food altogether, or the inability to digest certain nutrients.
Digestion starts in the mouth where food is masticated by the teeth and mixed with saliva which contains enzymes that aid the digestive process. Incomplete mastication or lack of saliva will both prevent complete digestion of food.
In the stomach, acid is added to food, fully denaturing the proteins and other foods in the stomach with the added side benefit of destroying any bacteria that might be in the food. The stomach can absorb some nutrients, such as dextrose (a simple sugar) and alcohol directly into the bloodstream at this point. High acidity results in heartburn or acid reflux. Low acidity will also affect digestion as food will not pass through the stomach unless it is acid enough.
When the food has reached the appropriate acidity, it is passed into the duodenum. At this point, alkalines are added to the food to bring its acidity back to normal, and the appropriate enzymes are also added by the liver through the bile duct in order to process most of the complex molecules that pass into the small intestine, such as lactose (milk sugar). A lack of these enzymes results in the food not being processed by the intestines, and often leads to it being processed by intestinal bacteria instead.
Most of the nutrients in the food are then broken down into their components through a combination of enzymatic action and the action of bacteria in the gut and are absorbed by the intestinal walls to provide nutrients to the bloodstream. Waste products, such as cellulose and other fiber that can't be digested, are passed into the large intestine to be expelled as feces.