A groundbreaking biochemical analysis has provided the first direct human evidence of what researchers call the methylfolate trap hypothesis. During B12 deficiency, 94.5% of red blood cell folate becomes sequestered in its 5-methylTHF form—a metabolically inert state that cannot support DNA synthesis or cellular division.
Following B12 repletion, this proportion dropped dramatically to 67.4%, demonstrating the direct mechanistic link between B12 status and folate bioavailability. The enzyme methionine synthase, which requires B12 as a cofactor, serves as the sole metabolic exit from this trapped state.
This 2006 case report, published in the British Journal of Haematology by Smulders and colleagues, represents the first comprehensive demonstration of methylfolate trap dynamics in human tissue—offering clinicians critical insight into why folate supplementation alone cannot reverse B12-deficiency anemia.
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