Fat Digestion and Segmentation

Fat Digestion and Absorption

• Fats are emulsified by bile salts in the lumen of the small intestine
• Lipase hydrolyzes triglycerides into monoglycerides (MG) and free fatty acids (FFA)
• These water-insoluble products are carried by water-soluble micelles
• MG and FFA diffuse from the micelles and through the luminal membrane
• They are re-esterified to form triglycerides inside the epithelial cell
• These aggregates are coated with lipoprotein which aids absorption
• Short or medium chain fatty acids are passively absorbed

Segmentation

• The primary means of motility in the small intestine
• It is initiated by pacesetter cells in the intestinal wall
• Segmentation contractions occur in the duodenum due to local distension
• Gastrin initiates ileum segmentation, via the gastroileal reflex
• Gastrin is released by the presence of chyme in the stomach
• It takes 3-5 hours for chyme to travel through the small intestine
• Segmentation contractions in the duodenum are faster than those in the ileum, proximal to distal

Migrating Motility Complex (MMC)

• MMC occurs after the absorption of a meal
• This is an in-between-meal motility pattern
• It involves repeated peristaltic waves that occur every 1.5 hours
• The MMC is a clean-up type motility
• It helps to clear the intestine after the meal

Innervation of the Small Intestine

• The Parasympathetic Nervous System (PNS) increases intestinal smooth muscle contraction
• The PNS releases Acetylcholine (Ach) and motilin
• Motilin is secreted from the upper duodenum during fasting and increases GI motility
• The Sympathetic Nervous System (SNS) decreases intestinal smooth muscle contraction

Carbohydrate Digestion

• The small intestine absorbs dietary carbohydrates, which make up around 50% of a typical US diet
• Digestion of carbohydrates in the mouth is facilitated by salivary amylase which breaks down starch
• Salivary amylase is inactivated in the stomach
• Pancreatic amylase further digests starch into disaccharides
• Disaccharides are broken down into monosaccharides by brush border enzymes (BBE) at the brush border membrane. These include Lactase, Maltase, and Sucrase.
• The monosaccharides are absorbed at the small intestine BBM
• Glucose and galactose are transported by active transport.
• Glucose and Fructose are transported by facilitated diffusion

Brush Border Membranes

• Enterokinase, also known as enteropeptidase, activates pancreatic enzyme trypsinogen
• This produces trypsin, chymotrypsin, and carboxypeptidase
• Aminopeptidases complete protein digestion by breaking down smaller peptide fragments into amino acids.

Protein Digestion and Absorption

• The small intestine houses brush border enzymes that further digest proteins
• Trypsin, chymotrypsin, and carboxypeptidase digest the proteins.
• Amino acids are passively absorbed through the basal membrane (BLM) via the Na+ and amino acid co-transporters
• Di and tripeptides are transported through the BLM in an H+ dependent manner via H+-dependent cotransporters.
• Oligopeptides require further digestion via BBE before absorption
• Facilitated diffusion is used to transport amino acids through the basolateral membrane

True or False

• The PNS increases and the SNS decreases contraction of the small intestine. True
• The rate of duodenum and ileum segmentation is the same. False
• The ileocecal sphincter contracts more with pressure on the cecal side to prevent bacterial-laden LI contents from entering the small intestine. True
• Dietary disaccharides are broken down at the BBM by BBEs. True
• Salivary amylase is the only enzyme that digests polysaccharides. False
• Single amino acid absorption requires dependent active transport, di and tri peptides H+ dependent active transport. True
• Within SI epithelial cells, MGs and FFAs re-esterify to form a TG. True

This quiz covers the processes of fat digestion and absorption, emphasizing the role of bile salts, lipase, and the formation of micelles. It also explores the motility of the small intestine, particularly segmentation contractions and their regulatory mechanisms. Test your understanding of these essential digestive processes!