Lipid digestion and the absorption of Ca++, iron and vitamins

Question 1

how much of total daily energy requirement do lipids provide?

Answer 1

55%

Question 2

what 4 substances do lipids comprise?

Answer 2

fats/oils (TAGs -account for 90%)
phospholipids
cholesterol/cholesterol esters
fatty acids

Question 3

are lipids soluble in water? what is the consequence of this?

Answer 3

No
insoluble or poorly soluble
causes special problems for digestion and absorption

Question 4

What is emulsification?

Answer 4

Conversion of solid fat/oil mass into emulsion of small oil droplets suspended in water

Question 5

What are the phases of emulsification?

Answer 5

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

Question 6

How does emulsification improve digestion of fats/oils?

Answer 6

increases the surface area to volume ratio that increases the area of oil-water interface at which digestion by lipases and esterases can occur

Question 7

how are oil droplets stabilised?

Answer 7

by the addition of a coat of amphiphilic molecules that form a surface coat

Question 8

How are TAGs digested by lipases in the stomach?

Answer 8

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

Question 9

How are TAG’s digested by lipases in the duodenum?

Answer 9

main enzyme = pancreatic lipase
Aided by bile salts
HCO3 in pancreatic juice neutralises stomach acid aiding enzyme function

Question 10

what are the 2 roles of bile salts?

Answer 10

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

Question 11

bile salts are amphipathic, true or false?

Answer 11

true

Question 12

what might a failure to secrete bile salts result in?

Answer 12

lipid malabsorption - steatorrhoea

secondary vitamin deficiency due to failure to absorb lipid vitamins

Question 13

what is colipase and what does it do?

Answer 13

amphipathic polypeptide secreted by pancreas with lipase

Binds to bile salts and lipase allowing access by bile salts to tri and di-glycerides

Question 14

what does digestion of TAG by pancreatic enzyme produce?

Answer 14

2-monoglceride

2 free fatty acids

Question 15

where are the final products of lipid digestion stored and released from?

Answer 15

mixed micelles

• monoglycerides
• free fatty acids
• cholesterol
• phospholipid
• bile salts

Question 16

How do free fatty acids and monoglycerides enter enterocyte cells?

Answer 16

transfer between mixed micelles and apical membrane of enterocytes, entering the cell via passive diffusion

Question 17

What happens to short (<6 carbon), medium (6-12) and long (>12 carbons) chain fatty acids once they enter the enterocyte?

Answer 17

short and medium exit through basolateral membrane and enter villus capillaries
Long chains resynthesized to triglycerides in the endoplasmic reticulum, then incorporated into chylomicrons

Question 18

How is chylomicron synthesised?

Answer 18

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

Question 19

How is chylomicron processed?

Answer 19

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

Question 20

How is cholesterol absorbed?

Answer 20

Mainly due to transport by endocytosis in clatherin coated pits by Neimann-Pick protein

Question 21

what drug can prevent lipid absorption and hence treat hypercholesterolaemia?

Answer 21

Ezetimibe

Question 22

How is calcium absorbed in the small intestine?

Answer 22

Via passive and active transport mechanisms

Question 23

when is calcium absorption mainly active?

Answer 23

when concentration of calcium in chyme <5mM

Question 24

How is calcium absorption regulated?

Answer 24

calcitriol and parathyroid hormone (increases calcitriol synthesis)

Question 25

when is calcium absorption mainly passive?

Answer 25

when high concentration of calcium in the lumen

Question 26

How is iron absorbed?

Answer 26

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

Question 27

How is Vitamin B12 absorbed?

Answer 27

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

Question 28

what are the 4 fat soluble vitamins?

Answer 28

A, D, E, K

Question 29

How are fat soluble vitamins absorbed?

Answer 29

incorporated into mixed micelles > passively transported into enterocytes > incorporated into chylomicrons or VLDLs > distributed by intestinal lymphatics

Question 30

what are the 3 water soluble vitamins and how are they absorbed?

Answer 30

B vitamins (not B12), C, H
process similar to those described for monosaccharides, amino acids and di/tri-peptides