How the two vitamins pass one-carbon units
Folate and B12 work as a relay in one-carbon metabolism. Folate carries a methyl group, and the enzyme methionine synthase uses B12 as a cofactor to transfer that group, converting homocysteine back into methionine. Methionine then becomes SAM, the cell's main methyl donor for DNA, RNA, and proteins. Without functional B12, the methyl group stays trapped on folate, a situation called the methyl trap.
This coupling is why deficiency in either vitamin pushes homocysteine upward and slows the methylation reactions that depend on the cycle.
Why the deficiencies look similar but differ in nerve risk
Both deficiencies impair DNA synthesis in rapidly dividing cells, producing the same large, immature red blood cells seen in megaloblastic anemia. The clinical danger is that high folic acid intake can correct the anemia while leaving a B12 deficiency masked, allowing nerve damage to progress unchecked, since B12 also supports the myelin around nerves.
This is general educational information, not medical advice. Because the two deficiencies overlap on blood tests but carry different neurological stakes, testing and treatment should be guided by a qualified professional rather than self-directed supplementation.