Linked from
The 28 pages that link to Vitamin B12, each with the reason it gives.
HomocysteineRelated: Methionine synthase depends on B12 to transfer a methyl group to homocysteine.
IleumRelated: Vitamin B12 bound to intrinsic factor is absorbed in the terminal ileum.
PescetarianismRelated: Seafood, eggs, and dairy can supply B12, depending on the foods included.
LiverRelated: Liver supplies abundant B12, making it a notably nutrient-dense food.