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

1a) Dual functionality of amphipathic property of bile salts:

Helps in digestion & absorption of fats and micelle formation.

Conversely, bile salts inactivate lipase.

– This lipase inactivation by bile salts prevented by colipase

– Colipase, a pancreatic enzyme secreted in precursor form of Pre-colipase

– Activated by trypsin

1b) Functions of bile salts in fat digestion:

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.

2. Medium Chain Triglycerides (MCT):

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.

Activating factors:

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

Fate of end products:

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.

Exam Important