Vitamin B9

Folate — the naturally occurring form — and folic acid, the synthetic one used in supplements and fortification. It is needed wherever cells divide quickly, which is why demand rises sharply in pregnancy and why a shortage shows first in the blood.

Required to make and repair DNA and to divide cells — so it matters most where cells divide fastest, including in a developing embryo.

• Dark leafy greens
• Legumes and lentils
• Asparagus
• Avocado
• Fortified flour and cereals
• Liver

Folic acid from supplements and fortified food is absorbed better than natural folate.

400 µg DFE · 600 µg in pregnancy for most adults. The upper limit, past which harm becomes likely, is 1,000 µg from supplements.

Reference intakes are population figures. Pregnancy, illness, age and medication all move an individual’s needs, which is one of several reasons this page is information rather than advice.

A sustained shortage has a name here: Megaloblastic anaemia. That is worth pausing on — nutrients whose deficiency earned a name are the ones where the consequences were severe enough, historically, to be described as a disease.

Most at risk: Anyone who could become pregnant, heavy drinkers, coeliac disease, and people on methotrexate.

Low intake of vegetables and legumes, alcohol, pregnancy, malabsorption in coeliac disease, and medicines that interfere with folate metabolism — methotrexate, some anticonvulsants, sulfasalazine. Folate is also destroyed by prolonged cooking.

Large immature red cells, fatigue, mouth ulcers and irritability. In early pregnancy, shortage causes neural tube defects such as spina bifida — which is why flour is fortified in Canada and why supplementation is advised before conception.

Every sign here has other, often likelier, explanations. A list of symptoms is a reason to look into something, never a diagnosis.

Serum folate reflects recent intake; red cell folate reflects the longer picture and is the more useful of the two. Folate and B12 are usually tested together, because correcting one while missing the other causes problems.

Dark leafy greens, legumes, asparagus, broccoli, citrus and fortified grains. Steaming or eating raw preserves more than boiling.

Folic acid supplementation before conception and in early pregnancy substantially reduces the risk of neural tube defects, which is why it is recommended for anyone who could become pregnant. Outside that, high-dose folic acid can mask a B12 deficiency while nerve damage continues.

High folic acid can mask B12 deficiency by correcting the anaemia while nerve damage continues unchecked. That is the main reason for the upper limit.

Interactions: Methotrexate, some anticonvulsants and sulfasalazine all interfere with it.