Vitamin B12 Food Additive

Temova Rakuša, Žane, et al. Molecules, 2022, 28(1), 240.

Vitamin B12, typically bound to proteins in food, initiates active absorption after release in the acidic stomach. Haptocorrin (HC), secreted by salivary and esophageal glands, binds to the freed vitamin, safeguarding it from acid degradation. Pancreatic proteases in the duodenum break down the B12-HC complex, releasing the vitamin for subsequent binding to intrinsic factor (IF) produced by gastric parietal cells. The B12-IF complex then travels to the distal ileum for primary absorption. This complex binds to enterocyte receptors and enters the cells via endocytosis.
A minor fraction (1-2%) of ingested B12 is absorbed passively. This pathway gains importance in IF deficiency (e.g., post-gastrectomy) or when IF capacity is exceeded. The IF system's absorption capacity is limited, decreasing with higher B12 doses: 80% for 0.1 μg, 50% for 1 μg, and less than 10% near or above 20 μg. High oral doses (≥500 μg) result in approximately 1% absorption.
Nasal B12 also undergoes passive absorption. Literature reports varying bioavailability based on the form: 2-5% for OHCbl, 2-6% for CNCbl, and around 20% for MeCbl.

Reinstatler, Lael, et al. Diabetes care, 2012, 35(2), 327-333.

This study investigated the prevalence of biochemical vitamin B12 deficiency in adults with type 2 diabetes on metformin compared to those without metformin and those without diabetes. It also examined the potential impact of vitamin B12 supplementation on this association.
· Results
Biochemical B12 deficiency was significantly higher in individuals with diabetes using metformin (5.8%) compared to those not using metformin (2.4%) and those without diabetes (3.3%). Metformin use was independently associated with an increased risk of B12 deficiency in individuals with diabetes. While B12 supplementation was associated with a reduced risk of deficiency in the non-diabetic group, it did not significantly impact deficiency rates in individuals with diabetes.
· Conclusion
Metformin therapy is associated with a higher prevalence of biochemical B12 deficiency. The recommended daily allowance of B12 (2.4 μg) set by the Institute of Medicine, and the amount typically found in multivitamins (6 μg), may be insufficient to prevent B12 deficiency in individuals with diabetes on metformin.