Vitamin B12 and folate metabolism

At a glance

• B12 (cobalamin) comes only from animal foods. Daily need is about 2.4 µg. Total body stores of 2 to 5 mg (about half in the liver) last 2 to 5 years.
• Folate comes from leafy greens, legumes and, in Australia, mandatory folic acid fortification of bread-making flour (since 2009). Stores last only 3 to 4 months.
• Both feed one-carbon metabolism. Lack of either stalls thymidylate synthesis, so DNA replicates slowly while RNA and haemoglobin synthesis continue: large cells with immature nuclei (megaloblasts).