Lipid digestion and the absorption of Ca++, iron and vitamins Flashcards
how much of total daily energy requirement do lipids provide?
55%
what 4 substances do lipids comprise?
fats/oils (TAGs -account for 90%)
phospholipids
cholesterol/cholesterol esters
fatty acids
are lipids soluble in water? what is the consequence of this?
No
insoluble or poorly soluble
causes special problems for digestion and absorption
What is emulsification?
Conversion of solid fat/oil mass into emulsion of small oil droplets suspended in water
What are the phases of emulsification?
mouth - chewing
stomach - gastric churning and squirting through narrow pylorus. content mixed with digestive enzymes from mouth/stomach.
Small intestine - segmentation and peristalsis mix the luminal content with pancreatic enzymes and bile
How does emulsification improve digestion of fats/oils?
increases the surface area to volume ratio that increases the area of oil-water interface at which digestion by lipases and esterases can occur
how are oil droplets stabilised?
by the addition of a coat of amphiphilic molecules that form a surface coat
How are TAGs digested by lipases in the stomach?
Heat and movements in stomach mix food with gastric lipase which begins digestion and forms an emulsion
Speeds up hydrolysis
Emulsified fats ejected to duodenum
How are TAG’s digested by lipases in the duodenum?
main enzyme = pancreatic lipase
Aided by bile salts
HCO3 in pancreatic juice neutralises stomach acid aiding enzyme function
what are the 2 roles of bile salts?
secreted from gall bladder in response to CCK
1) Act as detergents to emulsify large lipid droplets to small droplets
2) increase surface area for attack by pancreatic lipase but block access of the enzyme to the lipid with the hydrophobic core of the small droplets
bile salts are amphipathic, true or false?
true
what might a failure to secrete bile salts result in?
lipid malabsorption - steatorrhoea
secondary vitamin deficiency due to failure to absorb lipid vitamins
what is colipase and what does it do?
amphipathic polypeptide secreted by pancreas with lipase
Binds to bile salts and lipase allowing access by bile salts to tri and di-glycerides
what does digestion of TAG by pancreatic enzyme produce?
2-monoglceride
2 free fatty acids
where are the final products of lipid digestion stored and released from?
mixed micelles
- monoglycerides
- free fatty acids
- cholesterol
- phospholipid
- bile salts
How do free fatty acids and monoglycerides enter enterocyte cells?
transfer between mixed micelles and apical membrane of enterocytes, entering the cell via passive diffusion
What happens to short (<6 carbon), medium (6-12) and long (>12 carbons) chain fatty acids once they enter the enterocyte?
short and medium exit through basolateral membrane and enter villus capillaries
Long chains resynthesized to triglycerides in the endoplasmic reticulum, then incorporated into chylomicrons
How is chylomicron synthesised?
triglyceride formed in ER > cholesterol esters > phospholipid synthesis > nascent chylomicron > joins with apolipoprotein > chylomicron > exocytosis from cell > into central lacteal > carried in lymph vessels to systemic circulation via thoracic duct
How is chylomicron processed?
enters systemic circulation into subclavian vein > distributed to tissues > chylomicron triglyceride metabolised in capillaries by lipoprotein lipase > free fatty acids and glycerol released bind to albumen then taken up by tissues > remaining chylomicron remnant enriched in phospholipids and cholesterol > remnant endocytosed by hepatocytes producing cholesterol > cholesterol released to be stored/secreted in bile or oxidised to bile salts
How is cholesterol absorbed?
Mainly due to transport by endocytosis in clatherin coated pits by Neimann-Pick protein
what drug can prevent lipid absorption and hence treat hypercholesterolaemia?
Ezetimibe
How is calcium absorbed in the small intestine?
Via passive and active transport mechanisms
when is calcium absorption mainly active?
when concentration of calcium in chyme <5mM
How is calcium absorption regulated?
calcitriol and parathyroid hormone (increases calcitriol synthesis)
when is calcium absorption mainly passive?
when high concentration of calcium in the lumen
How is iron absorbed?
oxidised to Fe2+ by Vit C > Fe2+ enters cell via DMT1 > combines with haem to form apoferratin > apoferratin converted to ferratin (storage form of iron) or iron exits at basolateral membrane to form transferrin
How is Vitamin B12 absorbed?
ingested in food > haptocorin secretd by salivary glands > stomach acid released B12 from food > haptocorin binds released B12 > stomach parietal cells release intrinsic factor > pancreatic proteases digest haptocorin in small intestine, B12 released > B12 binds to intrinsic factor > B12-intrinsic factor complex absorbed in terminal ileum by endocytosis
what are the 4 fat soluble vitamins?
A, D, E, K
How are fat soluble vitamins absorbed?
incorporated into mixed micelles > passively transported into enterocytes > incorporated into chylomicrons or VLDLs > distributed by intestinal lymphatics
what are the 3 water soluble vitamins and how are they absorbed?
B vitamins (not B12), C, H process similar to those described for monosaccharides, amino acids and di/tri-peptides
