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What is digestion?

Digestion (from Latin digerere = separate) is breakdown of complex nutrients, such as starch, proteins and fats, into their basic components, such as glucose, amino acids and fatty acids thus making them available for absorption.

Carbohydrate Digestion

Starch is partly broken down into a disaccharide maltose in the mouth with the help of the enzyme salivary amylase, and partly in the small intestine by pancreatic amylase delivered by pancreatic juice.

Disaccharides are broken down into monosaccharides with the help of the enzymes produced in the intestinal lining: sucrose is broken down into glucose and fructose by sucraselactose into glucose and galactose by lactasemaltose into two glucose molecules by maltase, and trehalose in two glucoses by trehalase.

Oligosaccharides, like fructooligosaccharides (FOS), and certain other soluble dietary fibers cannot be digested by human enzymes, but can be broken down (fermented) by normal large intestinal bacteria into short chain fatty acids (SCFA) and monosaccharides.

Insoluble fiber, like cellulose, can be neither digested by human enzymes, nor broken down by bacteria, so it is excreted with the stool practically unchanged.

Protein Digestion

Proteins are broken down into short protein fragments, called peptides, in the stomach with the help of hydrochloric acid (HCl) and the enzyme pepsin, and in the small intestine by the enzymes trypsin and chymotrypsin delivered with the pancreatic juice. Peptides are broken down by peptidases in the small intestinal lining. The end products of the protein breakdown are amino acids.

Lipid (Fat) Digestion

Fats (triglycerides and phospholipids) are broken down into fatty acids, glycerol and other components in the small intestine with the help of bile delivered by the gallbladder (and produced in the liver) and the enzyme lipase delivered by pancreatic juice.

Cholesterol and phytosterols, which appear in the food in the form of cholesterol esters, are broken down into cholesterol and fatty acids with the help of cholesterol esterase, delivered by pancreatic juice.

Regulation of Digestion


Sympathetic nerves arising from the thoracic and lumbar part of the spinal cord innervate all parts of the gastrointestinal tract. Increased sympathetic activity inhibits intestinal motility (peristalsis) and constrict the sphincters, which results in slower food transit, which can improve digestion but can result in constipation [1-p.56].

Parasympathetic nerves, mainly the Vagus nerve (10th cranial nerve), stimulate intestinal motility, relax sphincters and stimulate gastrointestinal reflexes and digestive juice secretions. Increased parasympathetic activity can result in faster food transit, which can impair digestion and result in diarrhea [1-p.56].

Gastrointestinal tract also contains its own enteric nervous system, which either stimulates or inhibits gut motility and digestive juice secretions [1-p.56]. Enteric nervous system is connected and influenced by the central nervous system (brain-gut axis). Enteric nervous system affects gastrointestinal reflexes:

  • Gastroileal reflex. When food enters the stomach, the stomach distends, which stimulates the motility of the ileum.
  • Ileogastric reflex. When food reaches the ileum (the distal part of the small intestine), the muscular sphincter at the end of he stomach will constrict thus reducing the stomach emptying.
  • Gastrocolic reflex. Distension of the stomach by food stimulates colon motility. This is why breakfast can stimulate a bowel movement.

Gastrointestinal Hormones (Neuropeptides)

Gastrointestinal hormones affect gut motility, secretion of digestive juices and nutrient absorption.

Chart 1. Gastrointestinal Hormones

Gastrin [1] Stomach, small intestine Parasympathetic; presence of proteins (amino acids) in the stomach Stimulates gastric acid secretion and intestinal motility
Cholecystokinin (CKK) [3] Small intestine Presence of fatty acids and amino acids in the small intestine Stimulates secretion of pancreatic enzymes and gallbladder contraction
Secretin [4] Small intestine Acidic (usual) pH in the small intestine after entrance of food from the stomach Stimulates secretion of alkaline pancreatic juice into the small intestine and thus neutralizes the acidic food from the stomach
Glucose-dependent insulinotropic peptide or gastric inhibitory peptide (GIP) [2] Small intestine Presence of fat and glucose in the small intestine Stimulates insulin secretion; inhibits gastric acid secretion and motility
Motilin [6] Small intestine Stimulates stomach emptying and gut motility between meals
Somatostatin [5] Pancreas Inhibits secretion of gastric (pepsin) and pancreatic enzymes (lipase) and mobility of the intestine and gallbladder

Chart 1 source: [1-p.57; 2]

Digestion Disorders

Certain gastrointestinal disorders can impair digestion. Main symptoms of impaired digestion include abdominal bloating, excessive gas and diarrhea and, long-term, weight loss and eventual protein deficiency.

Stomach Disorders

H. pylori infection of the stomach can cause chronic gastritis with decreased secretion of gastric acid (hypochlorhydria or achlorhydria) and the enzyme pepsin, which can result in slower digestion of proteins.

Gastric bypass or surgical removal of the stomach can result in faster transit of food through the gut and thus in less efficient digestion.

Small Intestinal Disorders

Infection of the small intestine due to food poisoning can cause inflammation of the stomach and small intestine (gastroenteritis), which can impair the digestion, mainly of carbohydrates.

Congenital or acquired deficiencies of disaccharidases (enzymes that break down disaccharides):

Small intestinal bacterial overgrowth (SIBO), celiac disease, tropical sprue, Crohn’s disease, lymphoma and other diseases that affect the small intestinal lining can also affect the digestion of carbohydrates.

Liver and Gallblader Disorders

Liver inflammation (hepatitis), advanced liver cirrhosis or other severe liver disorders can be associated with decreased bile production and thus impaired digestion of fat.

Bile stones or tumors that block the common bile duct can prevent the delivery of bile in the intestine and thus impair fat absorption. Bile stones that lodge at the end of the common bile duct can block the delivery of both bile and pancreatic enzymes and thus impair the digestion of fats, proteins and carbohydrates.

Pancreatic Disorders

Advanced chronic pancreatitis or pancreatic cancer, cystic fibrosis or other severe pancreatic disorder can be associated with a decreased production of pancreatic digestive enzymes (amylase, trypsin, lipase), which can result in impaired digestion of carbohydrates, proteins and fats.

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