From the Guidelines
Secretin, CCK, enterokinase, and brush border enzymes all play crucial roles in macronutrient absorption through different mechanisms, with their combined actions ensuring efficient digestion and absorption of proteins, fats, and carbohydrates.
Mechanisms of Action
- Secretin primarily stimulates pancreatic bicarbonate secretion, which neutralizes stomach acid in the duodenum, creating an optimal pH environment for digestive enzymes to function, as noted in the ESPEN guidelines on nutrition in acute pancreatitis 1. This indirectly supports macronutrient absorption by maintaining proper conditions for enzymatic activity.
- Cholecystokinin (CCK) directly enhances fat absorption by stimulating gallbladder contraction to release bile salts that emulsify fats, while also triggering pancreatic enzyme secretion that breaks down proteins, fats, and carbohydrates.
- Enterokinase, produced by duodenal cells, specifically activates trypsinogen to trypsin, initiating a cascade that activates other pancreatic proteolytic enzymes essential for protein digestion and subsequent amino acid absorption.
- Brush border enzymes, located on the microvilli of intestinal epithelial cells, are responsible for the final stages of carbohydrate and protein digestion, breaking down disaccharides into monoscharides and peptides into amino acids that can be directly absorbed across the intestinal epithelium, as described in the context of nutrient digestion and absorption in the small intestine 1.
Clinical Implications
The coordinated actions of these hormones and enzymes are critical for maintaining proper nutrient absorption, and any disruption in their function can lead to malabsorption, as observed in conditions like acute pancreatitis 1. Understanding the roles of secretin, CCK, enterokinase, and brush border enzymes in macronutrient absorption is essential for managing nutritional deficiencies and optimizing patient outcomes.
From the Research
Hormones and Enzymes Affecting Macronutrient Absorption
- Secretin: There is limited direct information on the effect of secretin on macronutrient absorption. However, it is mentioned that deficiency of secretin and cholecystokinin may interfere with the action of enterokinase 2.
- CCK (Cholecystokinin): CCK plays a critical role in regulating food intake and diet-induced obesity 3. It is released from enteroendocrine I cells and binds to its receptor CCK1R, leading to vagal afferent activation and providing post-ingestive feedback to the hindbrain. CCK also acts along the GI tract to optimize digestion and nutrient absorption.
- Enterokinase: Enterokinase is a glycoprotein that converts trypsinogen to trypsin in the duodenal lumen 2, 4. It is a mosaic protease composed of a distinctive assortment of domains and is essential for initiating intestinal digestion. Congenital enterokinase deficiency can lead to diarrhea, failure to thrive, hypoproteinemia, and edema.
- Brush Border Enzymes: Brush border enzymes are involved in the digestion of carbohydrates, proteins, and fats in the small intestine 5. They hydrolyze specific bonds in these macronutrients, making them available for absorption. The food matrix can modulate the extent and kinetics of macronutrient digestibility, regulating their concentration profile in the blood and their utilization in peripheral tissues 6.