Gastrointestinal Lecture 4 Part 2 Digestion and Absorption of Micronutrients Flashcards
digestion of water soluble vitamins
- 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
Digestion of fat soluble vitamins
- 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
How is iron present?
70% of the body’s iron is present in hemoglobin, ~25% as ferritin, and ~3% as myoglobin
How is iron replaced?
While the amount of Iron lost from the body is small, the process is unregulated and lost iron must be replaced via the diet
iron absorption
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
Most of the iron found in the diet occurs as…?
Most of the iron found in the diet occurs as the ferric form (Fe3+), but it is the ferrous form (Fe2+) that is absorbed.
where is Fe3+ reduced to Fe2+?
A Ferric reductase in the brush border epithelium reduces Fe3+ to Fe2+
Uptake of Fe2+
- 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
Overall digestion summary
What happens if you block fat absorption?
Diarrhea >>> steatorrhea
Many pharmacological agents aimed at weight control via controlling fat absorption run into this problem