3. Digestion of LIPIDS Flashcards
What percentage of energy comes from fats? and from carbs?
At rest 60% energy comes from fats, 40% from carbs
Explain digestion and absorption
Digestion
• Chemical digestion of ingested food into absorbable molecules & is due to enzymes secreted into the lumen (salivary, gastric and pancreatic enzymes, bile) or bound to the apical membranes of enterocytes
• Mechanical digestion – peristalsis
Absorption
• Movement of nutrients, water & electrolytes through the epithelial cell lining into the blood or lymph
How much lipid should we eat?
What is the basic structure of lipids?
How do we store lipid in the body?
Lipids
• How much lipid should we eat? 30% diet by weight
o 9 calories/gram
• Structure of lipids: basic units omega end, triglyceride ***
• Storage form in the body: triglyceride
o Glycerol backbone (3C) each with a fatty acid tail (combination of lengths, saturations)
What are the main functions of lipid?
Main functions: • Energy store • Phospholipids – cell membrane • Cholesterol • Omega 3, 6 (carbon 18 molecules) - essential fatty acids o Bonds every 3 carbons
How do lipase enzymes act?
o Preferentially remove fatty acids at C1 and C3 of the TG
o Lingual lipase & gastric lipase do about 20%
• Lingual lipase – parotid glands
• Gastric lipase – gastric mucosa
• Churning & mixing in stomach as well as presence of proteins helps to digest some of the TG
o Pancreatic lipase – secreted into SI
Why is the digestion of TG more complex than protein or starch digesiton?
- Digestion of TG is more complex than protein or starch digestion, due to very low solubility of TG & Fatty acids in aqueous medium
- Due to the hydrophobic nature of lipids, they aggregate together to form globules, and enzymes (lipase) can only attack the surface of the molecule
In order for TG to be digested and absorbed adequately, 2 processes must occur.. what are these?
o Emulsification (in small intestine) o Enzymatic digestion
Describe Emulsification
Emulsification
Is not digestion, because there are no bonds to be broken - pulling the molecules apart
• Bile – made in liver, stored in gall bladded
o Alkaline solution
o Cholesterol
o Phospholipids
o Electrolytes
o Pigments
o Bile acids (cholesterol derivatives)
• Consist of sterol ring (chenodeoxycholic acid & cholic acid), with a side chain to which a molecule of glycine or taurine is covalently attached
• One side hydrophobic, one side is hydrophilic
• Amphipathic – therefore have both a hydrophobic and hydrophilic region. The hydrophobic side will be attracted to hydrophobic fat globules
• Bile acids (and phospholipids) surround lipid globules (aggregates of TG)
• Fat is physically broken down into ‘blobs’ and held in suspension
• Greatly increases the surface area of fat, making it available for digestion by lipases which cannot access the inside of lipid droplets
Emulsification + Enzymatic Digestion
- Co-lipase is the mediator => It pulls the lipase into the fat molecules where it can act
- Pancreatic lipase can then effectively enzymatically digest TG into FFA & monoglycerides
Describe absorption: the Micelle
• Triglycerides are still hydrophobic, so they will try and reform the globules once they have been broken down.
• Bile salts will again surround them to form a micelle
• The mixed micelle carries products of digestion through the aqueous lumen & the unstirred aqueous layers adjacent to the enterocyte
o Micelle = digested products in the middle, surrounded by bile salts, phospholipids (heads are polar) and they solubilize
o Micelles are far more soluble
Describe absorption of lipids
• Upon reaching the brush border, the fatty acids & monoglycerides leave the micelle & move through the lipid phase of the membrane by simple diffusion
o Cholesterol (relatively poorly absorbed) is transported across the apical membrane on specific energy dependent membrane transporters
• Hence can target these to decrease cholesterol
• Micelle solubilize the lipids, then must get close enough to layer so fats can diffuse across membrane & into intestine
In the Enterocyte what happens?
• In the enterocyte must repackage them (fatty acids must be in simplest form to diffuse through membrane, then must solubilize to travel through water environment)
o Monoglycerides + FA reform triglycerides
o FA + Cholesterol form cholesterol esters
• TG & CE are then packaged into lipoproteins called chylomicrons (a lipoprotein)
o Spherical with a core of TG & CE
o Surface is phospholipids, cholesterol & proteins (apolipoprotein B)
• Due to their large size, chylomicra leave the cell via exocytosis an dare absorbed into lacteals
• Chylomicra are exported into lymphatic vessels & enter blood stream via the thoracic duct
What happens to lipids: Chylomicron to cells
• In the blood as chylomicrons
• Triglyceride (TG) cant enter cells; FA can
• Lipoprotein lipase (LPL)
o Enzyme lines capillary walls (mainly in adipose & muscle)
o Converts TG back to FA and glycerol
• FA can enter cell
• Glycerol travels to the liver to be converted to glycerol-3-phosphate (this reaction doesn’t occur in the adipose tissue)
• The rest of the chylomicron (which is not free of TG) is broken down by the liver & repackaged
o VLDLs, LDLs, HDLs
Explain TG synthesis in adipose tissue
• FA enter the adipose cell & combine with glycerol-3-phosphate to form triglycerides
o In adipose cells, glycerol-3-p is made from glucose
• Insulin levels are increased after a meal
o Increased in response to increased blood nutrient concentration
• Insulin promotes TG synthesis by:
o Allowing glucose to enter adipose cells
• stimulates GLUT4 receptor to move to membrane)
• It may then form glycerol-3 phosphate
o Increases lipoprotein lipase levels
o Speeds up release of FA from chylomicrons
o Increases TG formation in adipose cell
Describe the release of FA from adipose tissue
• Hormone-sensitive lipase
o Enzyme in adipose tissue that increases TG breakdown
• TG are broken back to FFA and glycerol
o Activated by glucagon and epinephrine, inhibited by insulin
• When required, adipose cells release FFA into the blood
o FFA are transported in the blood stream by ALBUMIN
