Understanding Dietary Fat Digestion and Emulsion Formation

1. Lecture 13 Dietary Fat

2. Dietary Fat • Lipogenesis is not very active in people on a Western diet • Lipogenic enzyme expression is down-regulated by fat consumption • Most of our fat comes from the diet ~100g/day • Most fat in white adipose tissue will have come from dietary fat and not de novo lipogenesis • Fat is hydrophobic • Problems for digestion and transport • Digestive enzymes need fat to be in an emulsion • Fat needs to be carried around the bloodstream within lipoproteins

3. Formation of Emulsions • Needs molecules which have both hydrophilic and hydrophobic characteristics • amphiphilic • amphipathic • Phospolipids that comprise cell membranes are ampiphatic • As is the phosphatidic acid and lysolecithin that you use in salad dressing • Amphiphilic molecules act as detergents which emulsify fat into tiny ‘particles’ • micelles

4. Bilayers and Micelles • In both structures, polar heads are facing the aqueous environment while the hydrophobic tails are buried in the core • Micelles can also be formed using bile salts

5. Fat Digestion • Fat is trapped in the core of micelles formed with bile salts • Churning of dietary fat with bile salts in the intestine • Chyme • Emulsion • Easy for lipase to interact with • Pancreatic Lipase • Hydrolyses fat into FA and glycerol • Plus mixture of mono- and di-acyl glycerols lipase FAT Bile salts Chol

6. Digestion - Pancreatic Lipase

7. Bile Salts • Produced in the liver • Made from cholesterol • Cholesterol itself it not amphiphilic enough to be a detergent  needs modification by addition of polar groups • Stored in the gall bladder • Reabsorbed and taken back to the liver • Hepatic portal vein

8. Bile Salts • Polar groups are added to cholesterol to make it more amphiphilic • The only way to get rid of cholesterol is to make them into bile salts (chol cannot be oxidised)

9. Undigested Fat • If gall bladder is blocked by gall stones, no bile salts can be secreted into the small intestine  no fat digestion • Less calorie intake • But more oily stools • Inhibitors of fat digestion as weight-loss drugs • Orlistat (brand name Xenical) is an inhibitor of lipase • Listed side effects include • oily spotting; • oily or fatty stools; • orange or brown colored oil in your stool; • gas with discharge, an oily discharge; • an urgent need to go to the bathroom; • an inability to control bowel movements, • an increased number of bowel movements. • Olestra (Olean) is a fat substitute • FA attached to sucrose • Not attacked by lipases • Olestra will passes through gut undigested • Strips fat soluble vitamins • D, E, K need to be added as supplements • See anti-Olestra sites at www.cspinet.org/olestra/

10. Lipoproteins • The phospholipid shell can be up to 1 mm in diameter • Apoproteins • Enzymes • Structural • Docking • Different types of lipoproteins characterised by size and by types of apoproteins • First lipoproteins made by intestinal cells • Chylomicrons • Enter lymphatic system