Intestinal Absorption Table

• Uptake of nutrients and water from the lumen of the digestive tract into the bloodstream and lymphatic vessels.
• Small intestine (specifically the duodenum) is the major site of nutrient absorption.
• Transport occurs on both the apical and basolateral surfaces of the intestinal epithelium for absorption into the bloodstream or lymphatic system.

– Each nutrient utilizes a specific transport mechanism across either surface.

• Glucose and Galactose

– Apical: secondary active transport with Na+ – Basolateral: facilitated diffusion

• Fructose

– Apical: facilitated diffusion – Basolateral: facilitated diffusion

• Free Amino Acids

– Apical: secondary active transport with Na+ – Basolateral: facilitated diffusion

• Di- and Tripeptides

– Apical: secondary active transport with hydrogen exchange. – Basolateral: facilitated diffusion

• Intact proteins

– Apical: endocytosis – Basolateral: exocytosis

• Short-chain fatty acids:

– Apical and basolateral: simple diffusion

• Long-chain fatty acids and glycerol:

– Apical: simple diffusion

• Long-chain fatty acids and glycerol form chylomicron in small intestine epithelial cell.

– Basolateral: facilitated diffusion

• Fat-soluble vitamins (A, D, E, K):

– Absorbed with fats: dissolved in fat droplets, micelles, and chylomicrons.

• Water-soluble vitamins:

– Simple or facilitated diffusion

• Vitamin B-12: Must be bound to intrinsic factor (IF) for B-12 absorption in the ileum

– Apical: endocytosis – Basolateral: exocytosis

• Step 1

– Active sodium absorption across the basolateral surface allows facilitated diffusion across the apical surface and creates a net positive charge in the cell

• Step 2

– Facilitated chloride diffusion across apical membrane down the electrical gradient.

• Step 3

– Water is absorbed via osmosis.

• Fiber

– Converted to short chain fatty acids and absorbed via simple diffusion.

• Vitamins B and K

– Dissolved in water for absorption.

• Commonly used as a weight-loss (bariatric) surgery: it reduces the stomach size and amount of nutrient absorption.

– Creates a small gastric pouch that comprises only the fundus and bypasses rest of stomach (the body and antrum) and the duodenum. – Jejunum is attached to the gastric fundus. – Duodenum attached further down along the jejunum; this allows digestive juices from the stomach and pancreas as well as bile from the gallbladder release into the intestine. – "Roux Limb" = portion of the jejunum between two attachments.

• Following surgery, food passes: esophagus ? gastric pouch ? Roux Limb ? through Roux Limb ? remaining jejunum.

– Because the duodenum is bypassed, these patients lack the absorptive and secretive properties of the duodenum. – Patients supplement diets with: iron, vitamin B12, and other vitamins and mineral supplements (no longer absorbed) – Also: no longer secrete intrinsic factor from bypassed duodenum ? cannot bind to vitamin B-12 in bypassed duodenum ? unable to absorb vitamin B12 in ileum.

• Physicians prescribe vitamin B-12 supplements (nasal spray, sublingual drops, or tablets) ? ensures B-12 directly absorbed into circulation.