Vitamin b12 absorption and timing
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Vitamin B12 Absorption Mechanisms and Pathways
Vitamin B12 absorption in humans is a complex process involving both active and passive mechanisms. The primary route is active absorption in the ileum, where vitamin B12 bound to intrinsic factor is taken up by a specific receptor complex. This process is saturable, with a maximum active absorption capacity of about 1.2–2.5 µg per dose, and does not increase with higher doses. At higher oral doses, a small amount of B12 is absorbed passively, but this accounts for only about 1% of the dose, indicating that passive absorption is minimal compared to the active pathway 78.
Recent research also shows that a small amount of vitamin B12 can be absorbed in the colon, contributing to overall B12 status, especially in individuals with marginal dietary intake. This colonic absorption is much less efficient than small intestinal absorption, with bioavailability around 7% over four hours .
Timing and Circadian Variation in Vitamin B12 Absorption
The timing of vitamin B12 administration significantly affects its absorption. Studies have found that absorption is highest when B12 is taken around 1:00 PM, compared to early morning (7:00 AM), evening (7:00 PM), or night (1:00 AM). This suggests a circadian rhythm in B12 absorption, with midday being the optimal time for maximal uptake .
Absorption Kinetics and Peak Levels
After oral administration, vitamin B12 appears in the blood with a delayed peak, typically between 8 to 12 hours post-dose. This delayed absorption curve is characteristic of the physiological, intrinsic factor-mediated pathway, as opposed to the rapid rise seen with very high, non-physiological doses . In healthy adults, both gummy and tablet forms of multivitamin B12 show similar absorption profiles, with significant increases in serum B12 and holotranscobalamin (the active form) during the 6–10 hour absorptive phase .
Factors Affecting Vitamin B12 Absorption
Several factors can impair B12 absorption, including gastrointestinal disorders, certain medications, and genetic conditions affecting the intrinsic factor or its receptor. After bone marrow transplantation, for example, B12 absorption can be significantly reduced, especially in the presence of chronic graft-versus-host disease . Inherited disorders such as Imerslund-Gräsbeck syndrome or intrinsic factor deficiency can also lead to malabsorption .
Enhancing Vitamin B12 Absorption
Alternative delivery methods, such as sublingual mucoadhesive tablets, have been developed to improve B12 bioavailability. These formulations increase the residence time of B12 at the absorption site, leading to higher permeability and sustained release, which may enhance overall absorption compared to standard oral tablets .
Conclusion
Vitamin B12 absorption is most efficient through active uptake in the ileum, with a clear saturation point and a minor contribution from passive and colonic absorption. The timing of administration matters, with midday offering the best absorption. Both the form of B12 supplement and the health status of the gastrointestinal tract can influence absorption efficiency. New delivery systems, such as sublingual tablets, may offer improved bioavailability for individuals with absorption challenges.
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