Hormonal Regulation

During the gastric phase, when proteins are detected, the pyloric gland releases gastrin, which induces histamine release. Together, these hormones stimulate hydrochloric acid production, breaking down food into chyme. A negative feedback loop halts both hormone production once stomach pH becomes sufficiently acidic.

Secretin is released when chyme enters the duodenum. It inhibits gastric juice production in the stomach, induces the pyloric sphincter to close, and stimulates the pancreas to release bicarbonate. This bicarbonate neutralizes chyme acidity, protecting the duodenum lining and creating an optimal environment for digestive enzymes to function.

Cholecystokinin (CCK) is released in the duodenum and performs multiple functions. It induces the pyloric sphincter to close, limiting chyme release, and stimulates the pancreas to release digestive enzymes and the gallbladder to release bile. These secretions allow digestion to continue through the small intestine.

Gastric inhibitory peptide (GIP) is released when chyme enters the duodenum. It slows stomach churning, pacing the rate at which chyme moves through the digestive system. This coordinated slowdown allows the duodenum and small intestine adequate time to process and absorb nutrients.

Peptide YY (PYY) is released by the ileum as chyme enters the large intestine. It slows chyme passage into the colon, allowing water and electrolytes to be maximally absorbed. PYY also acts as a satiety signal to the brain, indicating fullness and halting the eating process.

The neuroendocrine system monitors digestion through sensory neurons of the enteric nervous system. When proteins are detected in the stomach, these neurons trigger hormone release. Multiple hormones work together with feedback mechanisms to regulate each digestive stage, ensuring coordinated movement and processing of food through the system.

Low levels of peptide YY have been observed in obese compared to non-obese individuals. Since PYY signals satiety to the brain and slows chyme passage for nutrient absorption, synthetic analogs are being investigated as potential obesity treatments to enhance fullness signals and improve metabolic regulation.