The human gastrointestinal (GI) tract is a complex system that ranges from the mouth to the anus and transforms ingested food into nutrients that can be absorbed by the body to maintain the health of each individual. But variations between individuals may lead to large differences in responses during the digestive process based on the enzymatic and pH conditions found in the mouth, stomach, and intestine .
In the mouth, the mastication and shearing action of the tongue causes degradation of the ingested bite of food. Saliva lubricates and the mixing action of the tongue and palate transforms the food into a bolus for swallowing. Saliva is about 99% water with various electrolytes—sodium, potassium, calcium, and bicarbonate—mucins, enzymes, and amylase to begin the breakdown of starch. The bolus is swallowed and passes through the esophagus where it moves via peristaltic contractions until it enters the stomach.
The stomach storage area is where the bolus is mixed with gastric secretions, enzymes (pepsin, gastric lipase), intrinsic factor, and hydrochloric acid at a pH of 1.5-2.0. At the distal end of the stomach, the pylorus acts as a valve to allow particles to enter the small intestine.
In the small intestine, the digested food (now called chyme) is mixed with intestinal secretions to gradually increase the alkalinity and allow pancreatic enzymes (lipase, amylase, amyloglucosidase, trypsin, phospholipase A, chymotrypsin, carboxypeptidase, and elas- tase) to act while bile salts digest lipids [4,5].
Nutrients in the small intestine diffuse through the mucus layer over the gut wall and are transported from the lumen through the cell membrane to the enterocytes and eventually to the blood [4,6].
The large intestine is the body’s fermenter and absorption of water and some vitamins occurs here while waste material is converted into feces. Fermentation of unabsorbed food particles occurs by a large microbial population.