Integrated Upper Gastrointestinal Response to Food Intake

Abstract

Ingestion of food simultaneously activates gastrointestinal motility, gastric and pancreatic secretion, and release of gastrointestinal hormones that, in turn, modulate motor, secretory, and absorptive functions in the upper gut. The hormones involved in these integrated functions include gastrin, cholecystokinin (CCK), ghrelin, and pancreatic polypeptide from the upper gut and incretins, such as glucagon-like peptide (GLP)-1 and peptide YY (PYY), from the distal small intestine. The responses to ingestion of food are complex and have been traditionally considered in 3 phases: cephalic, gastric, and intestinal. While this allows the different functions to be reduced to manageable descriptions, it is important to note that this is an integrated process with overlap between the different phases. Variations in function result in part from the physical properties of meals, the different proportions of macronutrients, and the rate of emptying from the stomach. In contrast, the enormous functional reserve of the pancreas results in a fairly standard response, influenced mainly by the rate of delivery of nutrients to the duodenum and the nature of nutrients that stimulate the release of enteropancreatic hormones. The ingestion of food leads eventually to a sensation of fullness and cessation of food intake. In this report, the term “satiation” refers to the sensation of fullness developed during the end of meal ingestion or during the early postprandial period. Operationally, this is investigated by the maximum tolerated volume of a nutrient drink ingested at a standard rate and the symptoms 30 minutes postchallenge. On the other hand, satiety refers to the appetite level that is manifested by the timing of the subsequent meal and the number of calories ingested. This is measured in practice by means of ad libitum buffet meals in which the calories and types of nutrients preferred from standard meals are calculated.