Digestion And Absorption Of Fats

DIGESTION AND ABSORPTION OF FATS

• Digestion begins in Duodenum
• Activated by Pancreatic Juice & biliary secretions.
• Principle dietary fat – Triglycerides mainly long chain fatty acids (>10 carbon atoms)

Enzymes involved:

1. Pancreatic juice 

• Pancreatic lipase – Most important for fat digestion
• Digests triglycerides (Triacylglycerols) into free fatty acids & 2-monoglycerides (2-monoacylglycerols).
• Pancreatic lipase hydrolyzes 1- & 3-bonds of triglycerides.
• Relatively spares 2-bonds

2. Biliary secretions – Bile salts mainly

3. Lingual lipase – Secreted by Ebner’s glands on dorsum of tongue

4. Gastric lipase – In Stomach

Role of bile salts in fat metabolism:

1. Properties of bile salts:

• Detergent action of bile salts (Lowering surface tension).
• Due to amphipathic property of bile salts.
• Due to presence of polar & non-polar ends

• Bile salts help in digestion by fat emulsification in small intestine.
• Detergent action of bile salts is necessary for both emulsification & micelles formation.

Absorption of digested fat:

• After fat digestion, fatty acids & monoglycerides are absorbed in small intestine (especially jejunum) & in Ileum (Small amount)

Steps:

1. For long-chain triglycerides:

• Inside enterocyte, fatty acids & monoglycerides aggregate forming triglycerides.
• Triglycerides are incorporated into chylomicrons & transported to blood vessels.

• Independent of pancreatic enzymes & bile salts.
• They can be transported into enterocyte intact without getting hydrolyzed.

3. Short-chain fatty acids (SCFAs):

• Produced by bacterial action in colon.
• Reabsorbed back in colon
• These are acetate (60%), propionate (25%) and butyrate (15%).

Emulsification and micellar formation:

1. Emulsification of fat:

• Process of division of large fat globules into smaller droplets about 1 millimeter in diameter.
• i.e., Ingested fats (mainly triglycerides) into smaller droplets so that they can be digested more efficiently
• Emulsification increases surface to volume ratio of lipid droplets facilitating lipases action.
• Hence, exposes larger surface areas of triglycreide to pancreatic lipase action.
• Emulsification facilitates action of lipases & mainly helps in digestion of ingested fats.

• Bile salts help in fat emulsification in small intestine.
• Due to detergent action of bile salts.

Mechanism:

Fat churning:

• Done in pyloric antrum
• Small intestinal segmentation movement fragments large fat globules into smaller droplets.
• Detergent action of bile salts prevents fragments from reaggregating.
• Hydrophilic polar ends separate fat droplets from aqueous phase
• Detergents form coating on small lipid droplets with their polar residues facing outward.
• repelling polar residues prevent reaggregation into larger droplets.
• Phospholipids (especially lecithin), in bile juice & in food, greatly enhance emulsifying action of bile salts.
• Lecithin has a stabilizing action on emulsion.

2. Micellar formation:

• Process in which digested fats (FFAs & monoglycerides) are incorporated into much smaller droplets(micelles)so that they can be absorbed more efficiently.
• Micelles consist of small fat globules, much smaller than emulsified droplets, about 2.5-5 nanometers in diameter.
• Micellar formation solubilizes digested fats & facilitates absorption into enterocytes. 
• I.e., Helps in absorption of digested fats.
• Bile salts have property for micelles formation.

Composition:

• Similar to emulsified droplets. 
• Each micelle contains detergents (Bile salts) & absorbable fats.
• Detergents located on micellar surface.
• Absorbable fats (FFAs & monoglycerides) are present in the hydrophobic micellar center 

• Once in contact with the cell membrane of enterocytes – 
• FFAs & monoglycerides diffuse freely into cell
• Bile salts are released into lumen repeating the cycle.

Other enzymes:

1. “Bile-salt activated lipase”/”Cholesterol esterase”

• Bile salts activated pancreatic lipase
• Catalyzes hydrolysis of cholesterol esters, esters of fat-soluble vitamins, and phospholipids, as well as triglycerides.
• Enzyme involved: Cholesteryl ester hydrolyase.
• Acts on Cholesteryl esters releasing cholesterol in non-esterified free form.

2. Phospholipase A2:

• Hydrolyses phospholipids.