Digestion and Absorption of Lipids

Digestion and Transport Into Enterocytes

Two simultaneous processes:

(1) Lipolysis – breakdown of triglycerides/phospholipids into fatty acids and monoglycerides/lysophospholipids

Two essential pancreatic enzymes are responsible for this:

(a) pancreatic lipase – triglycerides Þ free fatty acids + monoglycerides – loss leads to fatty stools

colipase – cofactor for pancreatic lipase; prevents inactivation of pancreatic lipase by bile acids
related enzymes:

lingual acid-stabile lipase catalyzes the same reaction – responsible for low level of stomach lipolysis

pancreatic unspecific lipase is responsible for release of cholesterol and vitamin A esters

(b) pancreatic phospholipase – phospholipids Þ fatty acids + lysophospholipids

(2) Formation of Mixed Micelles – solubilization of lipids (substrates AND products of lipolysis)

takes place by means of bile salts, a detergent made from cholesterol in liver

lipolysis occurs in micelles – they also transport lipids through the unstirred water layer to surface of enterocytes

without micelles, diffusion would be so slow that there would be no absorption (cholestasis Þ no lipid digestion)

absorption across cellular apical membrane is by diffusion

Processing Within the Cell

The lipid subunits are reconstituted into triglycerides and phospholipids inside the ER of the enterocytes

they are then lumped with cholesterol onto apolipoproteins to make chylomicrons (very large micelles)
chylomicron exits cell via exocytosis into a lacteal ("milky" intestinal lymphatic channel)
into systemic circulation – lymph vessels empty into subclavian vein, bypassing liver (protect liver from onslaught of fat)

Note: Short chain fatty acids (<10 carbons) are an exception; they diffuse directly into the portal blood, since they are presumably small enough to enter the capillaries.

Bile Salt Metabolism

Bile acids are made in the liver from cholesterol, then conjugated to a polar group (glycine or taurine) to form bile salts

bile salts are secreted into gut, then reabsorbed to be reused – called enterohepatic circulation

most reabsorption occurs at the terminal ileum via Na⁺-dependent secondary active transport

ordinarily all is recovered – otherwise, irritation of the large bowel (Þ diarrhea) and fat malabsorption (Þ steatorrhea)

this is a standard complication of surgical removal of the ileum
also occurs with outgrowth of bacteria (deconjugate bile salts to lithocolic or deoxycolic acid)

The proportion of cholesterol, bile acids, and lecithin (phospholipid) must be carefully controlled to maintain solution

bile acid must be at least 40% of lipids, cholesterol at most 30% – otherwise cholesterol stones (gall stones) will occur
cholesterol precipitates, phospholipids and bile acids form bilayers

Bile is green because of conjugated bilirubin ("bile pigments")

bilirubin is also responsible for making stool brown and urine yellow – different colors from different oxidation states
conjugated is green; unconjugated is yellow; oxidized or reduced forms are brown (bacteria modify bilirubin in the colon)