3-Digestion and Absorption of Lipids and Vitamins

Lipid Digestion/Absorption Areas of Events (cell wise)

• Lumen of the gut

• Move across apical membrane of intestinal epithelial cells

• Events w/in the cells

• Move across basolateral membrane of epithelial cells

• Events to enter capillaries/lacteals

Lipid Subtypes and Characteristics

• Triacylglycerol (TAG)

  • long term fuel storage, insulation

• Phospholipid

  • modified TAG

  • cell membranes

• Steroid

  • hydrocarbon rings

  • cell membranes

  • hormones

Dietary Fat means:

TAG, phospholipids, steroids

Phospholipids

• Amphipathic

• Make up bilayers and micelles

Steroids

• 27 C molecule with 4 rings

• Membranes, messengers, drugs

• Ingested as steroid esters!

What does it mean when an athlete “bonks”?

• Limited carb storage

• Glycogen stores are decreased → body fatigues and burns fat instead

• Fat takes longer to deplete

Mechanical methods of breaking down fat

• Mastication

• Gastric Churning

• Intestinal Peristalsis

Why do we need to break fats down into increasingly smaller oil droplets?

Fat enzyme (lipase) can only attack external surface

Dietary Protein in the stomach

• Some amphipathic character

Bile Salts and Lecithin

• Made in the liver

• Sometimes stored in gallbladder

• Released via common bile duct → duodenum

Bile Salts

• Cholesterol-derived amphipathic molecules

• 95% reabsorbed before the large intestine (ileum)

• Emulsification and micelle formation

Lecithin

• Phospholipid (diacylglycerol phosphate) derivatives

• Essential in emulsification of fats

What allows bile salts and lecithin to emulsify things?

Amphipathic nature!

Emulsification increases SA by 1000x.

Lipase Reaction

TAG → MAG + 2 FA

Lingual Lipase

• Fat digestion in mouth, continues in stomach

• Works best at pH 5

• Does NOT require bile salts or colipase

Gastric Lipase

• Additional digestion

Pancreatic Lipase

• Works best at pH 8

• Secreted by pancreas → duodenum

• Most important lipase!

• Does require colipase & bile salts

Other Lipases (besides lingual, gastric, and pancreatic)

• Cholesterol Esterase

• Phospholipase A2

Cholesterol Esterase

• Digests most all lipids

• Cuts ester bonds in fats, cholesterols, and phospholipids

Phospholipase A2

• Cleaves fatty acids from middle carbon of glycerol backbone of a phospholipid

Colipase (purpose, secreted as, activated by, functions (2))

• Protein needed for pancreatic lipase to function

• Secreted as procolipase (inactive form)

• Activated by trypsin (in the duodenum)

• Functions:

  • Cofactor

  • Allows pancreatic lipase access to emulsified fat droplets (moves bile salts/lecithin away)

What reverses the reaction of lipases?

The emulsification capacity of bile salts helps pull digested fat away from the main fat droplet.

Packaging of Lipids with Proteins and Lecithin

1. Smooth ER

   1. Lipids/TAG are reconstituted

2. Rough ER

   1. Lipids coated with apolipoprotein and phospholipid

3. Chylomicrons are formed

4. Golgi

   1. Chylomicrons are packaged in vesicles for exocytosis

What happens in packaging of lipids if you don’t have apolipoprotein?

Abetalipoproteinemia: inability to absorb dietary fat/fat soluble vitamins.

Lacteals

• Chylomicrons are too big for intestinal capillaries

• They use lacteals in the villi of the gut instead

• Chylomicrons travel via lymph system through thoracic duct → left subclavian v. → general circulation

Lipemia

Chylomicrons in the Blood After a Fatty Meal

Lipid Digestion/Reabsorption Steps

1: Bile salt secretion

2: Emulsification

3: Enzyme secretion and action

4: Micelle formation

5: Diffusion of lipids into intestinal epithelial cells

6: Chylomicron synthesis

7: Movement of chylomicrons into lymphatic system

What happens if there’s a problem with any portion of lipid digestion/reabsorption?

• Steatorrhea (fat/lipid in feces)

• Insufficient fat absorption

What are the water soluble vitamins?

B’s and C’s are Watery!

• C (ascorbic acid) is for “Collagen not Colds”

• B1 (thiamine) is “To enter The TCA cycle for aTp synthesis”

• B2 (riboflavin) is “Very Vital for FAD and makes PEE TOO yellow”

• B3 (niacin) is “Needed for NADH”

• B5 (pantothenic acid) is for “Pent up CoA entering the TCA”

• B7 (biotin) is for “Biotin binds avidin”

• B9 (folate) is for “Future Fetus”

• B12 (cobalamin)

Vitamin C

C (ascorbic acid) is for “Collagen not Colds”

Vitamin B1

B1 (thiamine) is “To enter The TCA cycle for aTp synthesis”

Vitamin B2

B2 (riboflavin) is “Very Vital for FAD and makes PEE TOO yellow”

Vitamin B3

B3 (niacin) is “Needed for NADH”

Vitamin B5

B5 (pantothenic acid) is for “Pent up CoA entering the TCA”

Vitamin B7

B7 (biotin) is for “Biotin binds avidin”

Vitamin B9

B9 (folate) is for “Future Fetus”

Vitamin B12 (facts, procedure requiring more?)

B12 (cobalamin) requires intrinsic factor (secreted by parietal cells of the stomach) and is absorbed at the ileum.

Gastrectomy requires extra oral B12 or B12 injections to avoid deficiency.

What are the fat soluble vitamins?

Kool ADE man is fat!

• A (retinol) is for “Ayes”

• D (cholecalciforol) is for “Darn strong bones”

• E (tocopherol) is for “Excellent ‘Embranes”

• K (phytonadione) is for “Klotting”

Vitamin A

A (retinol) is for “Ayes”

Vitamin D

D (cholecalciforol) is for “Darn strong bones”

Vitamin E

E (tocopherol) is for “Excellent ‘Embranes”

Vitamin K

K (phytonadione) is for “Klotting”

How are fat soluble vitamins absorbed?

• Similar to lipids!

• Emulsified, micelles, across membrane, absorbed into lacteals!

Any fat digestion/absorption dysfunction is likely to cause what?

Fat soluble vitamin absorption deficiency

Orlistat

Gastric and pancreatic lipase inhibitor

Steatorrhea “treatment effects” are a type of aversion therapy

Pellegra is associated with what? What are the signs to look for?

Pellegra (niacin or B3 deficiency)

3 Ds: dermatitis, dementia, and diarrhea