Metabolism

Dietary vitamin B12 reaches the stomach where it combines with intrinsic factor. The complex is essential for the absorption of the vitamin in the terminal ileum: it is otherwise too massive and fat-insoluble for absorption.

After absorption, it is transported by transcobalmin within plasma. It is stored within the liver; the half-life of this store is several years and minimizes the risk of deficiency states developing.

Vitamin B12 is required as a coenzyme in the synthesis of:

• methionine from homocysteine; vital to DNA, purine and folate production
• succinic acid from methylmalonic acid; involved in fatty acid and amino acid metabolism

Reference

1. Sukumar N et al. Investigating vitamin B12 deficiency. BMJ. 2019 May 10;365:l1865