Digestion refers to the means through which food is broken down into simpler compounds that can be easily absorbed into the body and it usually starts in the mouth. Teeth breaks food lumps mechanically into smaller particles with the aid of tongue. In the mouth, saliva moistens the food (this improves lubrication) and it also contains an enzyme, salivary amylase, produced by salivary glands (Whitney and Rolfes 57). Salivary amylase is responsible for conversion of starch into simple sugars. The food particles then enter the stomach by passing down the gullet via a process called peristalsis.
Once in the stomach, its epithelial cells produce gastric juice – (PH 1-3) – which contains digestive enzymes pepsin, rennin and lipase, hydrochloric acid and mucus. Enzyme pepsin is produced as a precursor known as pepsinogen. However, hydrochloric acid provides an acidic medium in the stomach that facilitates conversion of pepsinogen into pepsin and enhances its activity. Pepsin hydrolyses proteins into smaller absorbable polypeptides called peptones. Rennin, an enzyme normally produced in the stomach of young children, specifically hydrolyzes the milk proteins.
According to Wardlaw, gastric lipase starts mild digestion of fats into fatty acids and monoglyceride (quoted in Insel et al 37). Besides the enzymatic digestion, the stomach aids in digestion by physically churning and mixing food. Whereas most of the food is not absorbed in the stomach, it’s only simple sugars (glucose), some vitamins (such as vitamin B12) and alcohols (20 %) that are absorbed via stomach lining (Insel et al. 43). Chyme -mixture of food in the stomach- enters into the stomach after about three to four hours and it is in the small intestine that most of digestion takes place.
Small intestine consists of three parts; duodenum –upper portion, middle part –jejunum, lower part –ileum (Whitney and Rolfes 61). In the duodenum, pancreatic enzymes (trypsinogen, carboxypeptidase, pancreatic lipase and pancreatic amylase) and bile are released. Pancreatic enzymes include pancreatic lipase that hydrolyses fats into glycerol and fatty acids; pancreatic amylase which further breaks down starch and other complex carbohydrates into disaccharides (sucrose, lactase and galactose); trypsin breaks down partially and wholly digested proteins (peptones and proteoses) into amino acids.
Bile neutralizes chyme and emulsifies large fat globules into smaller units that can easily be hydrolyzed by pancreatic lipase. In addition to the pancreatic juices, digestion is enhanced by instestinal juices (such as maltase, sucrase and lactase) which further digest carbohydrates. Upon completion of digestion in the small intestine, most of the nutrients are absorbed via mechanisms of active transport, facilitated diffusion, endocytosis and passive diffusion into the lymphatic and circulatory systems (Insel et al. 47).
Absorption largely takes place in jejunum and ileum surfaces known as villi -fingerlike projections- which have microvilli cells. Proteins are absorbed in form of amino acids, dipeptides, and tripeptides. The end products of carbohydrate digestion (glucose, fructose, and galactose) are absorbed in the ileum via active transport (Whitney and Rolfes 77). Lipids are absorbed into the lymphatic system as triglycerides whereas alcohol (80 %), water and salts, and vitamins are normally absorbed in the small intestine (Insel et al.47).
After absorption in the ileum, the remainder –mostly dietary fiber- enters the large intestine where it’s fermented by gut microorganisms (bacteria). Through fermentation, non-starch polysaccharides are broken down into short chain fatty acids (propionic, acetic and butyric acids) to enhance their absorption. A large percentage of fluid content is reabsorbed in the large intestine. The large intestine also serves as storage for waste products –feces before they are excreted through anus.
Central nervous system and hormones control digestion and absorption of food and after complete absorption; water-soluble nutrients are transported via the vascular system while fat-soluble vitamins are transported through the lymphatic system. Works Cited Insel, Paul; Turner, Elaine; and Ross, Don. Nutrition. 2nd edition. Sudbury, MA: Jones and Bartlett. 2004 Whitney, E. N. , and Rolfes, S. R. Understanding Nutrition, 11th edition. Belmont, CA: Wadsworth