Digestion and absorption of nutrients part 2 Flashcards
Dietary fat
Dietary fat
- 90% triglycerides
- Long chains
- Acyl esters of glycerol
Structure
-Vary in chain length
-Saturation
-Double bonds
Lipids
Dietary fat
5% come from cell membranes
Phospholipids
Like a triglyceride with 2x FA’s
1x phosphate (esterified to head group)
PC, PE, PS
Sphingolipid
Membrane lipid
Serine rather than glycerol backbone
E.g. Ceramide & skin
Fatty acids
Essential Faty Acids
Cannot be made endogenously
Linoleic (18:2 – ω6) & Linolenic (18:3 – ω3)
In the absence linoleic also arachidonic acid
Functions
Cell membranes
Erythropoiesis
Eicosanoids
Blood pressure / viscosity
Digestion - lipases ( 3 types)
Lingual lipase
From the tongue
Starts the process
Optimum at low pH
Gastric lipase
Chief cells
Pancreatic lipase
Acinar cells
Von Ebner’s gland in tongue secrete lingual lipiase
Digestion - lipids
Lipids
Digestion begins in the mouth
Continues in the stomach
Lingual & gastric lipase
Process
Release a single fatty acid (Triglyceride > Diglyceride)
Long chains > stay with triglyceride droplets (insoluble at acidic pH)
Shorter chains > remain in solution (pH) > cross gastric mucosa > portal blood
15% digestion
Lingual lipase has a low pH optimum and can thus remain active through the stomach into the duodenum
Digestion - fatty acids , CCK
Fatty acids
Fatty acids in duodenum stimulate CCK release
Same as with proteins
CCK
Stimulates contraction of gallbladder (Gallbladder contains bile)
Stimulate contraction and dilation of ‘sphincter of Oddi’ > bile released into duodenum
Stimulate pancreas to release pancreatic juices > pancreatic lipase
Secretin stimulate HCO3- release > increased pH > optimum environment for fat digestion.
Digestion - bile , colipase
Bile
Neutralises gastric pH
Needed to create an emulsion
Think about washing up liquid > droplets
Large surface area for lipase
Colipase
From the pancreas
Facilitates attachment of lipase to the droplet
Needed for efficient enzyme activity
Prevents inhibitory effect of bile
2-monoacylglycerol + FA
Pancreatic lipase is active only at the oil-water interface of a triglyceride droplet, hence the need for co-lipase.
Digestion - droplets , vesicles
Droplets
Surface triglycerides are hydrolysed
Replaced by core lipid
Get smaller (lipase)
Vesicles ‘bud’ off
Vesicles
Lamella (membrane / layer)
Multi-lamellar > Uni-lamellar vesicle
Get smaller (lipase)
Vesicles > micelles
Absorption 1 - getting into the enterocyte
Unstirred water layer
Short / medium-chain fatty acids
Water soluble > diffuse into enterocyte > blood (8)
Bile reabsorbed
Enterohepatic recirculation
Terminal ileum
Longer chains
Mixed micelle
Low pH at the brush border
Na+/H+ antiporter
Protonation of the lipid
Diffusion
Membrane incorporation
Carrier mediated transport
Absorption 2 - in the enterocyte
- Converted back to triglycerides in the SER
- Fat droplets form in Smooth ER (SER)
- Apoproteins made in Rough ER and transferred to SER and associate with lipids
- ‘New’ chylomicrons & VLDL (Very low density lipoproteins) transferred to Golgi (cis)
- Vesicles of chylomicrons & VLDL bud off Golgi (trans) > move to membrane
- vesicles fuses with membrane releasing chylomicrons & VLDL
- Enter the lymphatic system > blood (thoracic duct)
2 FA + 1 MG -> TG = re-esterification
Chylomicron is quite large, so cannot leave enterocyte via capillary -> lacteal (lymphatic system)
Absorption 3 - in the blood
In the blood
- Lipoprotein lipase (endothelial surface)
- Chylomicrons > FA + Glycerol
- Remnant chylomicrons / glycerol > Liver
Fatty Acids
-Adipocytes & muscles
-Fat storage
-Other processes that we will come to later this week
Lipid functions
lipids -
Fat absorption critical to health
Major energy source in the fasted state
Most concentrated source of energy (~ 9.4 kCal/g)
Nonpolar: can be stored in an anhydrous state
Evolved to be storage depot of choice
Summary
- Bile salts from liver coat fat droplets
- Pancreatic lipase and colipase break down fats into monoglycerides and fatty acids stored in micelles
- Monoglycerides and fatty acids move out of micelles and enter cells by diffusion
- Cholesterol is transported into cells by a membrane transporter
- Absorbed fats combine with cholesterol and proteins in the intestinal cells to form chylomicrons 6. Chylomicrons are released into the lymphatic system
lipids - location
Location
Stored all over the body
Most in sub-cutaneous tissue / adipose
Obese > Visceral deposits around organs
Estimated 43-70 days’ supply of energy !!!
Sub-cutaneous fat = stored under the skin; Visceral fat = stored around organs
lipid functions - vitamins
Vitamin A, D, E and K are fat soluble
Solubilised within micelles > adsorbed
Vitamin A
Retinol (animal sources)
Beta carotene (plants)
Deficiency > Night blindness > Needed for Rhodopsin
Vitamin D
We can get it from sunlight (Vitamin D3)
Humans > Adequate exposure no need from diet
Cats / Dogs > Food only
Deficiency
Poor diet / lack of sun
Bone softening
Demineralisation
Rickets (children) / Osteomalacia (adults)
Vit. D deficiency has been linked to osteoporosis
Vitamin E
Vitamin E
α-tocopherol
Oily foods are the major source
Antioxidant (protection against cardiovascular diseases/cancer)
Deficiency
Rare
Effects are related to not ‘mopping up’ oxidants
Immune system > Creates ROI and RNI
These must be inactivated
Vitamin K
Vitamin K (Koagulation)
α-tocopherol
Green leafy veg are the major source
Produced by intestinal microbiota
Vitamin K is involved in the carboxylation of specificglutamate residues in proteins to form γ-carboxyglutamate residues. These proteins are important for regulating
Blood coagulation: risk of uncontrolled internal bleeding
Bone metabolism: malformation of developing bone
Vascular biology: prevent deposition of Ca salts in arterial vessel walls.
Vitamin KH2 = active; Vitamin KO = inactive
Fat malabsorption
Many of the vitamin deficiencies are due to poor nutrition or intestinal malabsorption.
Fat malabsorption could be due to liver/gallbladder disease, pancreatic disease and/or intestinal disease
Symptoms: steatorrhea (fat in faeces), essential fatty acid deficiency and/or vitamin deficiency
Faecal fat test is the required diagnostic test to evaluate the amount of fat digested and absorbed.
Floaty, greasy, foul-smelling faeces
