Vitamin B12 is the one vitamin plants do not make, and a required cofactor for methionine synthase (methylation) and methylmalonyl-CoA mutase (energy metabolism). It underwrites nerve function and red blood cell formation. The guide flags vegans, people on metformin, and anyone over 60 as the groups it is most often used for. Response is trackable on standard labs: reticulocyte count, serum B12, and MMA.
- Cofactor for methionine synthase, the enzyme that runs the methylation cycle.Peptide Guide (PubMed)
- Cofactor for methylmalonyl-CoA mutase, tying it directly to energy metabolism, nerve function, and red blood cell formation.Peptide Guide (PubMed)
- Chronic metformin use depletes B12, which is why the guide calls for annual B12 and MMA checks in that group.Peptide Guide (PubMed)
- Long-term PPI and H2-blocker use cuts B12 absorption in the gut, a bottleneck the injectable form bypasses entirely.Peptide Guide (PubMed)
- Response is objectively trackable: reticulocytes climb by day 3 to 10 once repletion starts, confirming macrocytic anemia is resolving.Peptide Guide (PubMed)