Digestion is the process by which food is broken down into smaller molecules that can be absorbed into the bloodstream. Absorption is the transport of these nutrients across the intestinal epithelium into the circulation. This process involves coordinated mechanical, chemical, and hormonal activities across the gastrointestinal tract.

Healthy eating plate with balanced food groups — A balanced eating pattern supports optimal digestion and nutrient absorption. The digestive system breaks down food into absorbable molecules through mechanical and chemical processes. Source: Unsplash.

Overview of the GI Tract

The GI tract is a continuous muscular tube from mouth to anus, consisting of the mouth, pharynx, esophagus, stomach, small intestine (duodenum, jejunum, ileum), large intestine (colon, rectum), and anus. The accessory organs — salivary glands, pancreas, liver, and gallbladder — contribute digestive secretions.

Healthy meal bowl for digestion — The digestive tract is a continuous muscular tube that breaks down food into absorbable molecules. A balanced meal provides the nutrients needed for optimal digestive function. Source: Unsplash.

The wall of the GI tract has four layers: mucosa (epithelium, lamina propria, muscularis mucosae), submucosa (connective tissue, blood vessels, nerves), muscularis externa (inner circular, outer longitudinal smooth muscle), and serosa (connective tissue and epithelium). The enteric nervous system — the brain of the gut — controls motility and secretion through the myenteric and submucosal plexuses.

Mouth and Esophagus

Mechanical digestion begins with mastication (chewing), which breaks food into smaller particles and mixes it with saliva. Saliva, secreted by the parotid, submandibular, and sublingual glands, contains water, mucus, electrolytes, salivary amylase (initiates starch digestion), and lingual lipase (initiates fat digestion). Saliva lubricates food for swallowing and contains lysozyme and IgA for antimicrobial defense.

The swallowing reflex moves the bolus from the mouth through the pharynx into the esophagus. The upper esophageal sphincter relaxes to receive the bolus. Peristaltic waves in the esophageal body propel the bolus toward the stomach. The lower esophageal sphincter (LES) relaxes to allow passage, then contracts to prevent gastroesophageal reflux.

Stomach

The stomach has four regions: cardia, fundus, body, and antrum. Gastric function includes storage, mixing, and regulated emptying.

Gastric secretions. Parietal cells secrete hydrochloric acid (HCl) and intrinsic factor. HCl kills microorganisms, denatures proteins, and activates pepsinogen. Chief cells secrete pepsinogen, which is cleaved to pepsin (active protease) by HCl. Mucus neck cells and surface epithelial cells secrete mucus and bicarbonate, forming a protective barrier against gastric acid. G cells in the antrum secrete gastrin, which stimulates acid secretion.

Mechanical digestion. The stomach churns and mixes food through peristaltic waves generated by the smooth muscle of the gastric wall. The pyloric sphincter regulates emptying of chyme (semi-fluid mixture of partially digested food and gastric secretions) into the duodenum, typically 1–4 hours after a meal.

Small Intestine

The small intestine is the primary site of digestion and absorption, with a surface area of 200–300 m² — the size of a tennis court — created by circular folds (valvulae conniventes), villi (finger-like projections), and microvilli (brush border) on enterocytes.

Digestive secretions. The pancreas secretes pancreatic juice (1.5 L/day) containing bicarbonate (neutralizes gastric acid) and digestive enzymes: pancreatic amylase (starch), pancreatic lipase (fats), trypsinogen, chymotrypsinogen, procarboxypeptidase (activated in the duodenum), and nucleases. The liver produces bile (0.5–1 L/day), stored in the gallbladder, which emulsifies fats. Bile salts are essential for fat digestion and absorption. The gallbladder contracts in response to cholecystokinin (CCK) from the duodenum.

Brush border digestion. The enterocyte microvilli contain membrane-bound enzymes: disaccharidases (sucrase, maltase, lactase, isomaltase), peptidases (aminopeptidase, dipeptidyl peptidase), and phosphatases. These complete the digestion of disaccharides and peptides.

Absorption mechanisms. Monosaccharides (glucose, galactose, fructose) are absorbed through SGLT1 (sodium-dependent for glucose and galactose) and GLUT5 (facilitated for fructose). Amino acids and di- and tripeptides are absorbed through multiple amino acid transporters (neutral, acidic, basic, and imino) and PEPT1 (peptide transporter). Monoglycerides and free fatty acids diffuse into enterocytes, where they are reassembled into triglycerides, packaged into chylomicrons with apolipoproteins and lipids, and transported via the lymphatic system. Bile salts are reabsorbed in the ileum and recirculated to the liver (enterohepatic circulation).

Large Intestine (Colon)

The colon absorbs water and electrolytes (sodium, chloride, potassium), producing semi-solid feces from the remaining indigestible material. The colon hosts the gut microbiome — 100 trillion bacteria that ferment undigested fiber into SCFAs (acetate, propionate, butyrate), synthesize vitamin K and some B vitamins, and metabolize bile acids and other compounds. Colonic motility includes haustral contractions (mixing) and mass movements (propulsion, occurring 1–3 times daily). The rectum stores feces until defecation.

Hormonal Regulation

GI hormones coordinate digestion along the tract. Gastrin (stomach) stimulates acid secretion and gastric motility. CCK (duodenum) stimulates pancreatic enzyme secretion and gallbladder contraction, and delays gastric emptying. Secretin (duodenum) stimulates pancreatic bicarbonate secretion. Gastric inhibitory peptide (GIP) and glucagon-like peptide-1 (GLP-1) (incretins from the small intestine) stimulate insulin release from the pancreas. Ghrelin (stomach) stimulates appetite. Peptide YY (PYY) (ileum, colon) suppresses appetite.

ⓘ Information

The human gut microbiome contains approximately 100 trillion bacteria, archaea, viruses, and fungi. The dominant bacterial phyla are Firmicutes and Bacteroidetes, with Actinobacteria and Proteobacteria in smaller proportions. The microbiome is established at birth, shaped by diet, antibiotics, and environment. A healthy microbiome is diverse, resilient, and metabolically active. Dysbiosis — loss of diversity and beneficial organisms — is associated with obesity, inflammatory bowel disease, irritable bowel syndrome, metabolic syndrome, and allergies. Diet is the primary determinant of microbiome composition. Fiber-rich, plant-based diets promote diversity and favor beneficial bacteria. Probiotics (live microorganisms) and prebiotics (non-digestible fibers that stimulate beneficial bacteria) can modulate the microbiome.

Summary

Digestion begins with mastication and salivary enzymes in the mouth. The stomach continues mechanical and chemical digestion. The small intestine is the primary site of nutrient absorption, supported by pancreatic enzymes, bile, and brush border enzymes. The colon absorbs water and hosts the gut microbiome. Hormonal signals coordinate the process from sight and smell of food through to absorption and satiety. Understanding digestive physiology is foundational to nutrition science and the management of GI disorders.