Gastrointestinal Lecture 4 Part 2 Digestion and Absorption of Micronutrients

Question 1

digestion of water soluble vitamins

Answer 1

• Vitamin C (ascorbic acid)
• B complex vitamins (B1 – thiamine, B2 – riboflavin, B3 – niacin, B5 – pantothenic acid, B6 – pyridoxine, B7 – biotin, B9 – folate, B12 – cabalamins)
• Majority of water-soluble vitamins are absorbed in the first part of the small intestine
• Absorption coupled to specific sodium co-transporters (except B12 and folate)
  
  • Folate absorption dependent on a proton-coupled folate transporter
  • B12 absorption occurs in the ileum and requires binding to a protein called Intrinsic factor

Question 2

Digestion of fat soluble vitamins

Answer 2

• Vitamin A, D, E and K
• These fat-soluble vitamins are typically consumed bound as esters
  
  • esters must be hydrolyzed in the gut lumen
  • uptake of free vitamin typically occurs via specific transport proteins
  • Free vitamin typically re-esterified and packaged into chylomicrons

Question 3

How is iron present?

Answer 3

70% of the body’s iron is present in hemoglobin, ~25% as ferritin, and ~3% as myoglobin

Question 4

How is iron replaced?

Answer 4

While the amount of Iron lost from the body is small, the process is unregulated and lost iron must be replaced via the diet

Question 5

iron absorption

Answer 5

Tight regulation of iron absorption means the amount taken up matches what is lost

• Thus, while the average Iron intake is ~20 mg, only ~5% is absorbed by the body

Question 6

Most of the iron found in the diet occurs as…?

Answer 6

Most of the iron found in the diet occurs as the ferric form (Fe3+), but it is the ferrous form (Fe2+) that is absorbed.

Question 7

where is Fe3+ reduced to Fe2+?

Answer 7

A Ferric reductase in the brush border epithelium reduces Fe3+ to Fe2+

Question 8

Uptake of Fe2+

Answer 8

• Uptake of Fe2+ into enterocytes is facilitated by DMT1 (divalent metal transporter 1)
• Once inside enterocytes, some Fe2+ is stored bound to ferritin and the remainder is transported out of the cell and into the circulation.
  
  • Export of Fe2+ is facilitated by the basolateral transporter ferroportin 1 (FP)
• Once in the circulation, Fe2+ is converted to Fe3+ , which binds to transferrin

Question 9

Overall digestion summary

Answer 9

Question 10

What happens if you block fat absorption?

Answer 10

Diarrhea >>> steatorrhea

Many pharmacological agents aimed at weight control via controlling fat absorption run into this problem