Lipid Digestion

Monogastric Digestion

Challenges Lipids are not water soluble Triglycerides too large to be absorbed

Digestive solution Triglycerides mix with bile and

pancreatic secretions Emulsification and digestion

Bile Produced in liver, stored in gallbladder

Except horse Alkaline solution composed of:

Bile salts Cholesterol Lecithin Bilirubin

Responsible for fat emulsification Detergent action

Mixed micelle formed by bile salts, triacylglycerols and  pancreatic lipase.

Digestion of Lipid Bile salts emulsify lipids Pancreatic lipase acts on triglycerides

Triglycerides sn-2 monoglyceride + 2 fatty acids

Pancreatic colipase Activated by trypsin Interacts with triglyceride and pancreatic

lipase Displaces bile to allow recycling Improves activity of pancreatic lipase

Pancreatic Colipase

Secreted from pancreas as procolipase Activated (cleaved) by

trypsin Anchors lipase to the

micelle One colipase to one lipase

(i.e., 1:1 ratio)

Dietary Fat(large TG droplet)

Bile Salts

Lipid emulsion

Lipase 2-Monoglyceride

+ 2 FFA

Emulsification Produces small lipid spheres

Greater surface area Lipases attack TG at 1 and 3

positionsGlycerol

Fatty Acid1

Fatty Acid2

Fatty Acid3

Lipase

Glycerol

Fatty Acid3

Fatty Acid1Fatty

Acid2

Triglyceride 2-Monoglyceride

+

2 Free Fatty Acids

2 H20

Digestion of Lipid

Phospholipase A1 and A2

Hydrolyzes fatty acids from phospholipids

Cholesterol esterase Hydrolyzes fatty acids from

cholesterol esters

Micelle Formation

Complex of lipid materials soluble in water

Contains bile salts, phospholipids & cholesterol

Combines with 2-monoglycerides, free fatty acids and fat-soluble vitamins to form mixed micelles

Micelle Formation

Lipid Absorption

Mixed micelles move to intestinal mucosal cells (enterocytes) and release contents near cell

The bile salts are re-absorbed further down the gastrointestinal tract (in the ileum), transported to the liver, and finally recycled and secreted back into the digestive tract

Nutrient Absorption - Lipids Fatty acids, 2-monoglycerides,

cholesterol, and cholesterol esters move down concentration gradient (passive diffusion)

Repackaged in intestinal cell for transport to liver Some is reformed into triglycerides Chylomicrons

Lipid Absorption

Once in enterocyte Glycerol and short chain fatty acids

directly enter mesenteric blood 2-monoglycerides and longer-chain

free fatty acids reformed into triglycerides, and then packaged with protein to form chylomicrons

Phospholipids hydrolyzed to free fatty acids

Lipid Absorption

Short and medium chain fatty acids

simple diffusion exocytosis

Lipid Absorption (Chylomicrons)

Chylomicrons absorbed from enterocytes into lacteals (lymph vessels) Ultimately enter blood via thoracic duct

Most long chain fatty acids absorbed into lymphatic system

Exception is poultry

Blood lipids transported as lipoproteins

Lipid Absorption (Direct Transfer)

Alternate route is for free fatty acids to enter circulation directly Free fatty acids (FFA) also called non-

esterified fatty acids (NEFA) Mostly less than 12 C (short and

medium chain fatty acids) Non-esterified fatty acids enter the

liver via the portal vein

Overview of Fatty Acid Uptake Short- and medium-chain fatty acids

Enter portal blood directly from enterocytes Bound to albumin in blood

Albumin–FFA complex Oxidized in liver or elongated and used for

triglyceride formation Long-chain fatty acids

Form chylomicrons Drain into the lymphatics via the lacteal in mammals

(no lacteal in avian small intestinal villi) Enter bloodstream at the thoracic duct

Upstream from liver Slow entry into the blood

In the Enterocyte...

Long-chain fatty acids (more than 10–12 carbons) are bound to fatty acid binding protein (FABP) Transport to the endoplasmatic

reticulum (ER) In the ER

Re-formation of triglycerides From two free fatty acids and one 2-

monoacylglycerol

In the Enterocyte...

Newly formed triglycerides accumulate as ‘lipid droplets’ at the endoplasmic reticulum Coated with a protein layer

Stabilizes lipids for transport in lymph and blood(aqueous environment)

At the Golgi apparatus, carbohydrates are attached to the protein coat

The glycoproteins act as signaling moleculesThese protein-coated lipid droplets are called chylomicrons

Overview of Lipid Digestion in Mammals

Overview of Lipid Digestion and Absorption in Avians Portal

blood*

*Lymph in mammals

Fat

ty a

cid

bind

ing

prot

ein

Repackaging in the Liver Lipid is repackaged in the liver to VLDL

or very low density lipoprotein Lipoproteins are classified by density Lipoproteins transport lipid to the rest of

the body

VLDL

TG

LDL

TG

HDL

Lipoproteins Classified by density

Protein:lipid ratio More protein, increased density More lipid, decreased density

Four classes of lipoproteins Chylomicrons VLDL LDL HDL

Formed in liver

The Good, the Bad, and the Ugly

Lipoproteins are similar to chylomicrons

They are lipid wrapped in proteins and cholesterol

When cholesterol is measuredVLDL = “bad” cholesterolHDL = “good” cholesterol

Lipoproteins

Transports cholesterol to cells

Transports cholesterol from cells to liver

Formed in enterocytes 1st formed in liver – “bad” cholesterol

Lipid Transport

Free fatty acids transported as complex with albumin in blood

Lipids rapidly removed from blood Liver Fat depots Other tissue

Lipid Digestion - Ruminants Microbes rapidly modify lipids: Lipolysis

Triglycerides Glycerol + 3 free fatty acids

Biohydrogenation Addition of H to unsaturated fatty acids

Saturation If carried to completion, all double bonds

become single bonds

Biohydrogenation

Weight percent of fatty acids

Fatty acid Diet Abomasal digesta

16:0 (palmitic)18:0 (stearic)18:2 (linoleic)18:3 (linolenic)

2661731

294546

Sheep fed alfalfa hay

Biohydrogenation Reduction of double bonds Result: fatty acids that are more

saturated with hydrogen

Saturated

Unsaturated

Biohydrogenation of Linoleic Acid

Linoleic acid (18:2)

cis-9, trans-11 CLA

trans-11 18:1

Stearic acid (18:0)

isomerase

reductase

reductase

Lipid Digestion and Synthesis by Microbes Rumen microbes

Produce “trans” configured double bonds Alter chain length Change position of double bonds Produce odd-chain and branched-chain FA

Rumen adipose tissue varies greatly from dietary fat Dietary fat must be rumen protected to

affect animal

Effect of Lipid on Rumen Fermentation

Excess amounts of unsaturated fatty acids and triglycerides Decrease methane production Impair fiber digestion Form soaps Alter rumen metabolism towards propionate

production – less acetate Decrease milk fat

Produce trans fatty acids Inhibit lipid synthesis in mammary gland

Decrease milk fat

Lipid Digestion - Ruminant

Digestion and absorption of lipids is similar to monogastrics except Fat enters small intestine in different

form than was presented to animal in diet

Lipids absorbed more slowly More transported as VLDL