Digestion
Digestion begins in the mouth with the secretion of saliva and its digestive enzymes. Food is formed into a bolus by the mechanical mastication and swallowed into the esophagus from where it enters the stomach through the action of peristalsis. Gastric juice contains hydrochloric acid and pepsin which would damage the walls of the stomach and mucus is secreted for protection. In the stomach further release of enzymes break down the food further and this is combined with the churning action of the stomach. The partially digested food enters the duodenum as a thick semi-liquid chyme. In the small intestine, the larger part of digestion takes place and this is helped by the secretions of bile, pancreatic juice and intestinal juice. The intestinal walls are lined with villi, and their epithelial cells is covered with numerous microvilli to improve the absorption of nutrients by increasing the surface area of the intestine.
Malabsorption
Symptoms can manifest in a variety of ways and features might give a clue to the underlying condition. Symptoms can be intestinal or extra-intestinal - the former predominates in severe malabsorption.The main purpose of the gastrointestinal tract is to digest and absorb nutrients (fat, carbohydrate, protein, micronutrients (vitamins and trace minerals), water, and electrolytes. Digestion involves both mechanical and enzymatic breakdown of food. Mechanical processes include chewing, gastric churning, and the to-and-fro mixing in the small intestine. Enzymatic hydrolysis is initiated by intraluminal processes requiring gastric, pancreatic, and biliary secretions. The final products of digestion are absorbed through the intestinal epithelial cells.