Where b12 absorbed
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Absorption of Vitamin B12: Key Insights and Mechanisms
Introduction to Vitamin B12 Absorption
Vitamin B12, also known as cobalamin, is a crucial nutrient that plays a significant role in various bodily functions, including DNA synthesis and red blood cell formation. Understanding where and how vitamin B12 is absorbed in the body is essential for diagnosing and treating deficiencies effectively.
Site of Vitamin B12 Absorption: The Ileum
Primary Absorption in the Ileum
The primary site for vitamin B12 absorption is the ileum, the distal part of the small intestine. This was established through studies using radioactively labelled vitamin B12, which showed concentrated absorption in the ileum 23. The ileum's unique role in B12 absorption is due to the presence of specific receptor sites that bind to the vitamin B12-intrinsic factor complex, facilitating its uptake 23.
Role of Intrinsic Factor
For efficient absorption, vitamin B12 must bind to a glycoprotein known as intrinsic factor, which is produced in the stomach. This complex then travels to the ileum, where it attaches to receptor sites on the brush border of the ileal mucosal cells in the presence of calcium and an alkaline pH 5. This mechanism ensures that vitamin B12 is absorbed efficiently, although only in small amounts due to the limited number of receptor sites 23.
Additional Absorption Sites and Mechanisms
Jejunum Absorption
While the ileum is the primary site, some studies suggest that a modest amount of vitamin B12 absorption can occur in the jejunum, the middle section of the small intestine. This was observed in a subset of experiments where a small percentage of vitamin B12 was absorbed in the jejunum before reaching the ileum 7.
Passive Diffusion
In addition to the active absorption process, a small percentage of vitamin B12 can be absorbed passively through diffusion. This passive absorption is less efficient but can occur throughout the small intestine 5.
Transport and Metabolism Post-Absorption
Transcobalamin II and Plasma Transport
Once absorbed, vitamin B12 binds to transcobalamin II, a plasma protein that facilitates its transport through the bloodstream. This complex is then cleared by the liver and eventually excreted in the urine, as observed during Schilling tests 4. The delay in the appearance of vitamin B12 in the plasma post-absorption is likely due to metabolic processes within the enterocytes 23.
Tissue Distribution
After entering the bloodstream, vitamin B12 is distributed to various tissues. Studies in rats have shown that the absorbed vitamin is rapidly cleared from the plasma and taken up by tissues, indicating a complex and slow passage through the intestinal wall 10.
Conclusion
Vitamin B12 absorption is a highly specialized process primarily occurring in the ileum, facilitated by the intrinsic factor. While the ileum is the main site, some absorption can also occur in the jejunum, and a small amount can be absorbed passively throughout the small intestine. Understanding these mechanisms is crucial for addressing vitamin B12 deficiencies and ensuring proper nutritional health.
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Most relevant research papers on this topic
Vitamin B12 absorption and malabsorption.
Vitamin B12 absorption and malabsorption are unique, requiring a gastric intrinsic factor and specific receptor sites for efficient absorption.
Highly CitedEvidence for intestinal origin of transcobalamin II during vitamin B12 absorption.
Transcobalamin II, derived from the ileal enterocyte, plays a crucial role in vitamin B12 absorption and its subsequent clearance from the bloodstream and urine.
In Vitro StudyThe ileum is the major site of absorption of vitamin B12 analogues.
Serum vitamin B12 analogues are absorbed in the ileum by a mechanism independent of intrinsic factor, and colonic bacteria and endogenous metabolism of vitamin B12 do not significantly contribute to their level.
Observational StudyTHE SITE OF ABSORPTION OF CO58-LABELED VITAMIN B12 IN MAN. AN INVESTIGATION MADE BY INTESTINAL INTUBATION WITH POLYETHYLENE GLYCOL AS A MARKER SUBSTANCE.
Intestinal intubation reveals that vitamin B12 absorption in humans occurs mainly in the middle and anal thirds of the small intestine, with some cases showing absorption beginning in the jejunum.
Non-RCTAbsorption of Vitamin B12-Glutamic Acid Mixture
Heathcote and Mooney's claim that vitamin B12 is absorbed as either small peptide or amino-acid units is a misinterpretation due to lingual difficulty, not a belief in the peptide or amino-acid nature of vitamin B12.
The Site of Absorption and Tissue Distribution of Orally Administered 56Co‐Labelled Vitamin B12 in the Rat
The rat possesses an intrinsic-factor mechanism for vitamin B12 absorption similar to that found in man, with slow and complex intestinal passage and rapid clearance into tissues.
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