One-carbon metabolism

In brief

A set of linked chemical reactions that uses folate, vitamin B12 and vitamin B6 to make DNA and to turn homocysteine into methionine.

• Folic acid supplements lower homocysteine levels, but they do not directly lower the risk of heart disease.
• Large doses of folic acid can correct the anaemia of B12 deficiency but not the nerve damage, so they can hide a B12 deficiency.
• In Australia, wheat flour used in bread making must contain folic acid, according to healthdirect.

How it works

Folate takes part in single-carbon transfers that are needed to make DNA and other genetic material and to turn homocysteine into methionine. Vitamin B12 helps the enzyme methionine synthase do this conversion, and vitamin B6 helps keep homocysteine at normal levels.

Too little folate causes folate deficiency anaemia, in which red blood cells are larger than normal, and a shortage of vitamin B12 can cause megaloblastic anaemia. Taking folic acid before and in early pregnancy helps prevent neural tube defects.

Connections

5 connections on the map. Select a topic to read its page.

Lab tests

• Vitamin B12 and folate testsBlood tests for vitamin B12 and folate, usually ordered after a full blood count shows a problem, or for nerve symptoms.
• Homocysteine testMeasures homocysteine, an amino acid. It is most often used to assess vitamin B12, folate and B6 levels.

Sources

1. Folate: fact sheet for health professionalsNIH Office of Dietary Supplements
2. Vitamin B12: fact sheet for health professionalsNIH Office of Dietary Supplements, last reviewed July 2, 2025
3. Vitamin B6: fact sheet for health professionalsNIH Office of Dietary Supplements, last reviewed June 16, 2023
4. Folate fact sheet for consumersNIH Office of Dietary Supplements, last reviewed November 1, 2022
5. Vitamin B12 fact sheet for consumersNIH Office of Dietary Supplements, last reviewed December 15, 2023
6. Folatehealthdirect Australia, last reviewed February 2024

Each connection above names its own source. See every source AskBioMap uses.

Summarised from the NIH Office of Dietary Supplements and healthdirect. Sources read on 13 October 2026. General education only, not medical advice. Spotted a mistake? Please tell us through the HuBu-Intel contact page.