Vitamin B12 absorption operates through two distinct physiological routes, each with fundamentally different characteristics. The first pathway, mediated by intrinsic factor, is highly efficient but capacity-limited, saturating at approximately 1.5 micrograms per dose. The second pathway—passive diffusion across the intestinal lining—is slow and inefficient, absorbing only 1–2% of any given dose. However, at supplement doses of 500–1,000 micrograms, passive diffusion becomes the dominant route, delivering clinically significant quantities of B12 independent of intrinsic factor function.
This dual-pathway model has important clinical implications for patients with pernicious anaemia and other B12 malabsorption disorders. At 1,000 micrograms, passive diffusion alone delivers approximately 10 micrograms of B12—more than four times the recommended daily intake—making high-dose oral supplementation a recognized alternative to intramuscular injection in select cases, according to the NIH Office of Dietary Supplements. While injection remains the first-line treatment, understanding this absorption mechanism offers patients and clinicians a non-invasive option worth considering.
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