Where is b12 absorbed
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Vitamin B12 Absorption: Key Insights and Mechanisms
Site of Vitamin B12 Absorption: The Ileum
Vitamin B12, also known as cobalamin, is primarily absorbed in the ileum, the distal part of the small intestine. This was established through studies using radioactively labeled vitamin B12, which showed that patients with lesions in the distal small bowel were unable to absorb normal amounts of the vitamin, leading to conditions such as megaloblastic anemia . Direct evidence from studies in 1959 confirmed that radioactive B12 concentrates in the ileum, highlighting this region as the primary site of absorption .
Role of Gastric Intrinsic Factor in B12 Absorption
The absorption of vitamin B12 is unique because it requires a gastric intrinsic factor, a glycoprotein produced in the stomach. This intrinsic factor binds to vitamin B12 and facilitates its attachment to specific receptor sites on the brush border of the ileal mucosal cells. This process is essential for the efficient absorption of the vitamin . The intrinsic factor-B12 complex is internalized in the ileum by a receptor complex composed of Cubilin and Amnionless, which is crucial for the vitamin's uptake.
Transport and Cellular Uptake of Vitamin B12
Once absorbed in the ileum, vitamin B12 enters the bloodstream attached to transcobalamin II, a plasma binding protein. This complex is then cleared by the liver and excreted into the urine, as observed in Schilling tests. The transport from the intestinal lumen across the enterocyte to the plasma is a delayed process, possibly due to metabolic activities within the enterocyte .
Factors Affecting Vitamin B12 Absorption
Several factors can impair the absorption of vitamin B12, including structural or functional abnormalities in the stomach or ileum. Conditions such as pernicious anemia, ileal resections, and diseases like Crohn's colitis can significantly reduce B12 absorption. Additionally, the bioavailability of vitamin B12 from dietary sources varies, with animal products like meat and fish being more efficiently absorbed compared to others like eggs.
Conclusion
Vitamin B12 absorption is a complex process primarily occurring in the ileum and requiring the gastric intrinsic factor. Understanding the mechanisms and factors affecting this process is crucial for addressing deficiencies and related health issues. The intricate interplay between dietary intake, intrinsic factor, and specific receptor-mediated absorption underscores the importance of maintaining gastrointestinal health for optimal vitamin B12 uptake.
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