The digestive process begins when food is placed in the mouth. Chewing reduces the size of food particles and the saliva moistens food. This release of sugar alters the taste of food. When the food is swallowed, muscle action propels the food downward to the stomach. The stomach fluids are quite acidic in nature and this acidity is beneficial to destroy ingested bacteria, activate some enzymes, and begin the process of protein digestion.
Acid can damage the stomach, so the walls of the stomach have a special lining to protect it.
Increases in stomach acidity can damage the protective lining and lead to a peptic ulcer. Bacterial or viral inflammation, anxiety, and alcohol can all increases the level of stomach acid. The digestive process continues in the small intestine, the major site of digestion and absorption of food. Digestive enzymes assist in the breakdown of food particles to make them smaller for absorption. Lactose intolerance occurs due to an absence or deficiency of the intestinal enzyme lactase.