Small Intestine pH and Digestive Enzymes: A Segmental Overview

The small intestine’s pH gradient and enzyme milieu are precisely regulated for sequential digestion and absorption.

pH Profile Along the Small Intestine

Segment	Length	pH Range	Key Determinants
Duodenum	~25 cm	5.5–6.5 (acidic)	Gastric acid chyme; bicarbonate from pancreas/brunner’s glands neutralizes
Jejunum	~2.5 m	6.0–7.0 (near neutral)	Optimal for pancreatic enzymes; continued bicarbonate secretion
Ileum	~3.5 m	7.0–8.0 (alkaline)	Bicarbonate-rich secretions; bile acids; terminal ileum absorbs bile acids

Major Digestive Enzymes (Luminal)

Carbohydrases

• Pancreatic amylase: Starch → maltose, maltotriose, α-limit dextrins (optimal pH 6.7–7.0).
• Brush border enzymes:
  • Maltase-glucoamylase, sucrase-isomaltase → glucose.
  • Lactase → glucose + galactose (declines with age in many).
  • Trehalase, isomaltase.

Proteases

• Pancreatic: Trypsinogen → trypsin (enteropeptidase) → activates chymotrypsinogen, procarboxypeptidases, proelastase.
• Brush border: Aminopeptidases, dipeptidases → amino acids, di/tripeptides (PEPT1 transporter).

Lipases

• Pancreatic lipase: Triglycerides → 2-MAG + 2 FAs (requires colipase, bile salts). Optimal pH 7–8.
• Phospholipase A2: Phospholipids.
• Cholesterol esterase: Cholesterol esters.
• Bile salts: Not enzymes; emulsify fats → micelles for absorption.

Nucleases

• Pancreatic RNase, DNase → nucleotides → brush border phosphatases → bases, sugars, phosphate.

Regulation

• CCK: Fat/protein in duodenum → gallbladder contraction, pancreatic enzyme secretion, gastric emptying ↓.
• Secretin: Acid in duodenum → pancreatic bicarbonate secretion.
• Enteropeptidase (brush border): Activates trypsinogen → cascade.

Clinical Relevance

• Pancreatic insufficiency (CF, chronic pancreatitis): ↓ enzymes → steatorrhea, creatorrhea, malnutrition. Treated with PERT (pancreatic enzyme replacement therapy).
• Celiac disease: Villous atrophy → ↓ brush border enzymes (lactase first) → malabsorption.
• SIBO: Bacterial deconjugation of bile acids → fat malabsorption; carbohydrate fermentation → gas/bloating.
• Short bowel syndrome: Reduced absorptive surface + accelerated transit.

The pH-enzyme axis is exquisitely tuned; disruption causes maldigestion and malabsorption.