Summary
Folate (vitamin B9) is an essential water-soluble vitamin critical for synthesising and repairing DNA, producing red blood cells, and converting homocysteine — a cardiovascular risk factor — to the harmless amino acid methionine. It is particularly vital before and during early pregnancy, when deficiency causes neural tube defects such as spina bifida.
Low folate is common and is associated with a distinctive form of anaemia (megaloblastic), fatigue, depression, and elevated homocysteine. It is frequently found alongside low B12 — both vitamins work together in the same metabolic pathways, so both should always be assessed together.nnSerum folate reflects recent dietary intake, while red cell folate (tested separately) provides a longer-term picture of tissue stores. For most clinical purposes, serum folate testing is the standard first-line approach.
What It Is
Folate is the naturally occurring form of vitamin B9, found in leafy green vegetables, legumes, nuts, and fortified foods. Folic acid is the synthetic form used in supplements and food fortification. Both are converted in the body to the active metabolite 5-methyltetrahydrofolate (5-MTHF), which participates in one-carbon metabolism.nnFolate and B12 are co-enzymes in the synthesis of thymidine for DNA production and in the remethylation of homocysteine. Without sufficient folate, cells cannot divide normally — this is most visible in rapidly dividing cells like red blood cells and gut epithelium.nnApproximately 10% of the population carries the MTHFR gene variant, which impairs folate conversion to 5-MTHF — these individuals may benefit from methylfolate supplementation.
Functions
DNA synthesis and cell division
Essential cofactor for thymidine synthesis, enabling accurate DNA replication in all dividing cells — from red blood cells to foetal tissue.
Neural tube development
Critical in the first 28 days of pregnancy for closing the neural tube — deficiency causes spina bifida and anencephaly.
Homocysteine regulation
Works with B12 to remethylate homocysteine to methionine, reducing cardiovascular disease risk associated with elevated homocysteine.
Red blood cell maturation
Required for normal red blood cell development — deficiency causes macrocytic megaloblastic anaemia indistinguishable from B12 deficiency.
Reference Ranges
Serum Folate
Measured in nmol/L| Status | Range (nmol/L) | Range (ng/mL) | What it means |
|---|---|---|---|
| Deficient | < 7 | < 3.1 | Clear folate deficiency — risk of megaloblastic anaemia, elevated homocysteine, and impaired DNA synthesis. |
| Borderline | 7–11 | 3.1–4.9 | Sub-optimal — consider dietary review and supplementation, particularly in women of childbearing age. |
| Adequate | 11–45 | 4.9–20 | Sufficient folate for normal cellular function. |
| High | > 45 | > 20 | Elevated — usually from supplementation. High-dose folic acid may mask B12 deficiency; always test both together. |
Serum folate reflects recent dietary intake and can fluctuate with meals. Red cell folate is preferred for assessing long-term tissue stores. Always interpret alongside B12, as high-dose folate can mask the haematological signs of B12 deficiency while neurological damage progresses.
Symptoms of Imbalance
Folate deficiency symptoms overlap significantly with B12 deficiency — both must be tested together.
- Persistent fatigue and weakness
- Mouth ulcers and a sore, inflamed tongue
- Pale skin or slight jaundice from anaemia
- Low mood and depression
- Difficulty concentrating and brain fog
- Shortness of breath on exertion
- Irritability and mood swings
- High folate from supplements is generally harmless
- Very high doses of folic acid may mask vitamin B12 deficiency — a key clinical concern in older adults
Causes of Imbalance
- Poor dietary intake — low consumption of leafy greens, legumes, and fortified foods
- Alcohol excess (impairs folate absorption and metabolism)
- Malabsorption (coeliac disease, Crohn's disease)
- Certain medications (methotrexate, phenytoin, sulfasalazine)
- Pregnancy and lactation (increased demand)
- MTHFR gene variant reducing conversion efficiency
- Older age and reduced absorption
- Folic acid supplementation (standard prenatal supplements, fortified foods)
- Very high-dose supplementation without clinical indication
FAQs
UK guidelines recommend 400 mcg of folic acid daily for at least 12 weeks before conception and for the first 12 weeks of pregnancy. Women at higher risk (those with a history of neural tube defects, on epilepsy medication, or with BMI over 30) are advised to take 5 mg daily — available on prescription.
Folate is the natural form of vitamin B9 found in food. Folic acid is the synthetic, oxidised form used in supplements and food fortification. Both are converted in the body to the active form (5-MTHF), but folic acid requires additional conversion steps. Individuals with the MTHFR gene variant may convert folic acid less efficiently and can benefit from the active methylfolate form.
Yes — this is an important clinical concern. Folate supplementation can correct the megaloblastic anaemia of B12 deficiency, masking the blood count abnormality while neurological damage from B12 deficiency continues to progress. This is why B12 and folate must always be tested together, and why folate supplements should not be taken at high doses without confirming adequate B12 status.
No fasting is required for serum folate. However, serum folate reflects recent intake — a single high-folate meal can temporarily elevate the result. For a longer-term picture, red cell folate can be measured alongside, as it reflects tissue stores over the preceding 3 months (the lifespan of a red blood cell).
Those at highest risk include: women of childbearing age, people with heavy alcohol use, those with coeliac or Crohn’s disease, older adults, people taking methotrexate or certain anticonvulsants, and individuals with the MTHFR gene variant. A balanced diet rich in dark leafy greens, lentils, and fortified cereals helps maintain adequate levels.
References
- Bailey LB, et al. Biomarkers of Nutrition for Development — Folate Review. J Nutr. 2015;145(7):1636S–1680S. View source
- Greenberg JA, et al. Folic acid supplementation and pregnancy. Rev Obstet Gynecol. 2011;4(2):52–59. View source
- Stanger O. Physiology of folic acid in health and disease. Curr Drug Metab. 2002;3(2):211–223. View source