What is the role of the different parts of the small intestine in digestion and nutrient absorption?

Parts of the Small Intestine and Their Digestive Roles

The small intestine consists of three anatomically and functionally distinct segments—the duodenum, jejunum, and ileum—each with specialized roles in nutrient digestion and absorption, with the proximal small intestine (duodenum and jejunum) serving as the primary site for maximal nutrient assimilation. 1

Duodenum: The Digestive Command Center

The duodenum functions as the organ of choice for digestion and absorption of critical nutrients while orchestrating digestive activities throughout the gastrointestinal tract 2:

• Receives approximately 1.5 liters of pancreatico-biliary secretions daily, containing bicarbonate to neutralize gastric acid and digestive enzymes for breaking down carbohydrates, fats, and proteins 3
• Bile salts enter here to emulsify fats, making them accessible for pancreatic lipase digestion 3
• Serves as the primary absorption site for iron and folate, making duodenal resection particularly problematic for these micronutrients 4
• Coordinates motor and secretory functions of the stomach, intestine, pancreas, and biliary system through its endocrine activity 2

Critical Vulnerability in the Duodenum

• Pancreatic lipase is particularly unstable in the duodenum, being rapidly destroyed by acid denaturation and pancreatic proteases (especially chymotrypsin), making fat digestion especially vulnerable in pathologic conditions 1, 3

Jejunum: The Primary Absorption Powerhouse

The proximal small intestine, particularly the jejunum, is the site of maximal nutrient digestion and absorption under normal conditions 1:

• Produces passive secretions to render the lumen isotonic during nutrient passage and digestion 3
• Brush-border enzymes on enterocytes perform final digestion of carbohydrates and proteins 3
• Reabsorbs the majority of gastrointestinal secretions (approximately 4 liters daily from saliva, gastric juice, and pancreatico-biliary sources) 1
• Jejunal mucosa is "leaky" with rapid sodium fluxes, requiring oral rehydration solutions with sodium concentrations of 90-120 mmol/L to prevent net sodium loss into the bowel lumen 1, 5

Postduodenal Digestion Continues

• Enzymatic degradation continues during small intestinal transit, involving both luminal pancreatic enzymes and mucosal brush-border enzymes for protein and carbohydrate digestion 1

Ileum: The Specialized Absorber

The terminal ileum has unique, irreplaceable functions 4:

• Exclusively absorbs vitamin B12, with malabsorption occurring when more than 60-100 cm of terminal ileum is resected 1, 4
• Reabsorbs bile salts, preventing their loss and maintaining the bile salt pool; resection leads to fat malabsorption when hepatic synthesis cannot compensate 1, 4
• Contains enteroendocrine cells producing peptide YY (PYY) and glucagon-like peptide-2 (GLP-2), which slow gastric emptying and small bowel transit (the "ileal brake" mechanism) 1
• Ileal resections are generally worse tolerated than jejunal resections because the jejunum has greater postresection adaptive capacity 4

Coordinated Motor and Secretory Functions

The coupling of motor activity with secretory events maintains intraluminal homeostasis and ensures maximal nutrient assimilation within the proximal small intestine 1:

• Motor activity determines gastric emptying and small intestinal transit rates, while nutrient exposure determines both motor and secretory responses 1
• This coordination is particularly important for fat digestion, economizing the effects of digestive secretions 1
• Inhibitory mechanisms terminate digestive responses through nutrient exposure in both proximal and distal small intestine, mediated by somatostatin, pancreatic polypeptide, peptide YY, and glucagon-like peptide-1 1, 3

Physiologic Malabsorption: A Normal Phenomenon

• Approximately 10% of ingested nutrients (ranging 1-30%) normally escape luminal digestion and are delivered to the colon, independent of pancreatic enzyme presence 1
• This physiologic malabsorption has been demonstrated for carbohydrates, lipids, and proteins, representing normal digestive efficiency limits 1

Clinical Implications of Regional Differences

Understanding these regional differences is essential for predicting nutritional consequences after intestinal resection 4:

• Jejunal resections cause generalized malabsorption but are better tolerated due to ileal adaptive capacity
• Ileal resections cause specific deficiencies (vitamin B12, fat-soluble vitamins, magnesium) that cannot be compensated by the jejunum
• Combined jejunoileal resections with colectomy result in the most severe malabsorption, with massive fluid and electrolyte losses 1, 5