Folate and folic acid: the clearest success in nutrition policy, and its one side effect

Folic acid fortification is the closest thing nutrition has to a clean public-health win. It also comes with one complication, and that complication matters a great deal to a specific group of people.

What it does in the body

• Folate works as a coenzyme in building DNA and RNA and in amino acid metabolism. Evidence E source

How much you need

• The RDA for folate is 400 mcg DFE a day for adults and 600 mcg DFE in pregnancy. — adults and pregnancy Evidence E source
• All women and adolescents who could become pregnant are advised to get 400 mcg of folic acid a day from supplements, fortified foods, or both. — women capable of becoming pregnant Evidence E source

How well your body absorbs it

• About 85% of folic acid is absorbed when taken with food, against roughly 50% of the folate naturally present in food. Evidence E source
• The label does not count folate, it converts it: one microgram of folic acid is written down as 1.7 micrograms of food folate. — United States food labelling Evidence E source
• The whole point of the 1.7 factor is the claim that folate from food is worth less than folic acid from a pill. — United States food labelling Evidence E source
• Measured with labelled folate over four weeks, folate from fruit, vegetables and liver came out at about 80% of folic acid, not the roughly 59% the label formula assumes. — 72 healthy adults; one group ate a high-folate diet of 369 ug food folate a day with placebo, three groups ate 73 ug food folate plus 92, 191 or 289 ug folic acid Evidence B source
• The authors of that study say it plainly: eating folate-rich food raises a population's folate status more efficiently than is generally assumed. — 72 healthy adults Evidence B source
• Folate from spinach reached the blood better and faster than folate from wheat germ or Camembert, so the food it sits in matters as much as the number in the table. — 24 healthy adults, each receiving all four test items Evidence B source
• There is no single conversion that works for all foods, which is the quiet problem under every folate table. — 24 healthy adults Evidence B source

What too little does

• The main clinical sign of folate deficiency is megaloblastic anaemia, in which red blood cells are large and abnormal. — people with folate or vitamin B12 deficiency Evidence E source
• Folate deficiency also causes a sore tongue and mouth ulcers, changes in skin, hair and nail colour, digestive symptoms and raised homocysteine. — people with folate deficiency Evidence E source

Who is most likely to fall short

• Alcohol interferes with folate absorption and uptake, speeds its breakdown and increases its loss in urine. — people with alcohol use disorder Evidence E source

What the evidence does and does not show

• Trial evidence shows that enough folic acid around conception prevents a substantial proportion of neural tube defects. — women capable of becoming pregnant Evidence B source
• Since mandatory folic acid fortification began in the United States in 1998, neural tube defect rates have fallen by 28%. — US population Evidence C source
• Pooling ten systematic reviews covering 296,816 women, folic acid before conception cut neural tube defects to less than half. — women taking folic acid or multivitamins before and around conception, 10 reviews Evidence A source
• Across 39 studies of children, mothers who took folic acid had children with better intellectual development and less autism, ADHD and language trouble, and taking far more than recommended did not add anything. — mothers and their children, 32 cohort studies and 7 case-control studies Evidence C source
• On bowel cancer the picture is mixed: more total folate tracks with less of it, while supplements in people who have already had a polyp show no benefit. — general population and people with inflammatory bowel disease or a history of adenoma, 5 meta-analyses Evidence C source
• In women trying to conceive, 4-5 mg of folic acid a day rather than the standard 0.4 mg went with about a quarter fewer miscarriages and a 10 per cent higher chance of becoming pregnant. — non-pregnant women of reproductive age in three randomised trials; 1,488 of them became pregnant and entered the miscarriage analysis Evidence A source
• Adding the other micronutrients to iron and folic acid cuts the most dangerous kind of newborn, preterm and small for gestational age, by nearly 30 per cent. — pregnant women in low- and middle-income countries, mean maternal age 24.3 years, in 14 randomised trials of multiple micronutrients Evidence A source
• The same swap, multiple micronutrients instead of iron and folic acid alone, leaves children longer and heavier for the first six months, and the advantage then fades. — infants born to women in 19 randomised trials, measured at birth and at 3, 6, 12, 18 and 24 months Evidence A source
• Swapping folic acid for methylfolate does cut the unmetabolised folic acid in your blood, on three trials and low-certainty evidence. — participants in trials comparing equimolar 6S-5-methyltetrahydrofolate against folic acid Evidence A source
• Testing your MTHFR genotype to choose a folate supplement is a research question, not advice: the review that looked hardest says so in as many words. — trial participants stratified by MTHFR 677 genotype; TT homozygotes are about 10% of European-ancestry populations and more in Mexican and Mediterranean-ancestry ones Evidence A source
• In pregnancy the two forms ended up the same where it counts: after 24 weeks, total folate in the mother, the cord blood and the placenta did not differ. — 80 first-trimester pregnant participants randomised, 62 completed, 31 per group Evidence B source
• The methylfolate prenatal did what it is sold to do: 7% of mothers had detectable unmetabolised folic acid against 31% on the folic acid version. — pregnant participants from the second trimester onward Evidence B source

What too much does

• The upper limit of 1,000 mcg applies to synthetic folic acid only. No limit was set for folate from food, because high intakes from food have not caused harm. — adults Evidence E source
• Large amounts of folate can correct the anaemia of vitamin B12 deficiency while leaving the nerve damage to progress. — people with B12 deficiency Evidence E source
• High-dose folic acid did not prevent pre-eclampsia in twin pregnancy, and the raw rate was in fact higher in the treated group. — women pregnant with twins, aged 18 or older, randomised between 2011 and 2015 Evidence B source
• Nobody can yet separate the effect of unmetabolised folic acid from the effect of simply having a lot of folate, because folic acid raises both at once. — the observational studies relating unmetabolised folic acid to health outcomes Evidence A source
• Unmetabolised folic acid is at most 1 to 2 per cent of the folate in your blood, and a single measurement of it says as much about when you last took a pill as about your dose. — studies measuring circulating unmetabolised folic acid Evidence A source
• The observational case against unmetabolised folic acid does not hold together: the associations were inconsistent, confounded and never replicated. — observational studies of unmetabolised folic acid, total folate, folic acid intake and population fortification Evidence A source

Medicines it interacts with

• People taking methotrexate for cancer should consult their oncologist before folate supplements, which could interfere with the drug's anticancer effect. — people on methotrexate for cancer Evidence E source
• Where malaria is treated with antifolate drugs, taking folic acid at the same time roughly doubles the chance the treatment fails. — people living in malaria-endemic areas taking antifolate antimalarials, mostly sulfadoxine-pyrimethamine; five of the eight trials were treatment trials, most using folic acid doses above the 1 mg a day upper limit Evidence A source

What to do with this

Who you are decides which half of this page matters.

If you could become pregnant, this is one of the few places in nutrition where a supplement is the recommendation rather than a shortcut, and the timing is the point. It has to be in place before a pregnancy is confirmed, because what it prevents happens in the first weeks.

If you are older, or looking after someone who is, the caution runs the other way. High folic acid can tidy up the blood count of a B12 deficiency while the nerve damage underneath continues. Check B12 before treating a corrected count as a solved problem.

For everyone else, lentils, beans, edamame, leafy greens and enriched grains cover it without any thought at all. Heavy drinking is the one habit that reliably works against folate, on several fronts at once.

If the question in your head is whether more than 400 micrograms would be better, the answer is that nobody knows yet. Three trials put 4 to 5 mg a day against the standard 0.4 mg in women trying to conceive, and among those who became pregnant there were about a quarter fewer miscarriages and a 10 per cent better chance of conceiving. Counted the cleaner way, including everyone randomised rather than only the women who conceived, the miscarriage result lost its significance. A finding that changes shape depending on how it is counted is not a reason to multiply your own dose by ten.

Pulling the other way, women carrying twins who took 4 to 5 mg a day had pre-eclampsia at 17.2 per cent against 9.9 per cent on placebo. Adjustment took the difference below significance and the authors say the raised risk cannot be confirmed. That is not proof of harm. It is a reason to stop treating a big dose as a free bet.

One warning belongs to a particular place. If you are treated for malaria with an antifolate drug such as sulfadoxine-pyrimethamine, folic acid taken alongside it roughly doubles the chance the treatment fails at day 7, on moderate-certainty evidence. Those trials mostly used doses above the 1 mg ceiling, so the 400 microgram tablet before pregnancy is a separate question. What this changes is one thing: tell whoever treats the malaria what you are taking.

If the question in your head is methylfolate rather than folic acid, here is what the trials actually settle. Across three trials, swapping in methylfolate does lower the unmetabolised folic acid circulating in your blood, on low-certainty evidence. In a 24-week pregnancy trial it did that too, with detectable unmetabolised folic acid in 7 per cent of mothers against 31 per cent, and total folate in the mother, the cord blood and the placenta came out the same either way.

The reason to keep that in proportion is that nobody has shown the unmetabolised form does harm in the first place. It is at most 1 to 2 per cent of the folate in your blood, and a single measurement of it reflects when you last took a tablet as much as how much you took. Folic acid also raises it and total folate together, so the observational studies cannot separate the two. Those associations came out inconsistent, confounded and unreplicated.

On MTHFR testing, the review that looked hardest calls it a research question. Participants with the TT genotype may respond to folic acid with a larger drop in homocysteine, the pattern was not consistent within every trial, and no pooled genotype trend was estimated at all. A test result there does not yet choose your supplement for you.

Where to get it from food

The bottom line

If you could become pregnant, 400 mcg of folic acid a day is one of the few supplement recommendations with trial evidence behind it, and it has to start before a pregnancy is confirmed. The complication belongs to older adults. A high folic acid intake can hide a vitamin B12 deficiency by correcting the anaemia while the nerve damage carries on underneath. That is an argument for checking B12, not for avoiding folate. Everything published on folate since this page was written sits beside that recommendation rather than on top of it, and none of it moves the number. Three trials compared 4 to 5 mg a day before conception with the standard 0.4 mg and found about a quarter fewer miscarriages among the women who conceived, an effect that faded once everyone randomised was counted. In twin pregnancies a high dose did nothing for pre-eclampsia, and the rate was higher in the treated group than in the placebo group. Two large analyses swapped iron and folic acid for a full multivitamin and found 29 per cent fewer preterm small-for-gestational-age births and babies slightly longer for six months, which measures what the other micronutrients add rather than what folate does. And where malaria is treated with antifolate drugs, folic acid alongside them roughly doubles the chance the treatment fails. The 400 micrograms before conception comes through all of that untouched. The unit itself is worth one sentence of scepticism. Micrograms of dietary folate equivalents come out of a calculation rather than a measurement, and that calculation multiplies folic acid by 1.7 because food folate was assumed to be worth about 59 per cent of it. Fed to people with labelled folate over four weeks, food folate came out at 78 to 85 per cent. A crossover study of single foods found spinach ahead of wheat germ and camembert, with no one conversion factor fitting all three.

Sources

1. fol-01 · Position of an expert body · expert body statement
   Folate functions as a coenzyme or cosubstrate in single-carbon transfers in the synthesis of nucleic acids (DNA and RNA) and metabolism of amino acids.
   NIH Office of Dietary Supplements, Health Professional Fact Sheet: Folate · checked 2026-09-15 · extracted
2. fol-02 · Position of an expert body · expert body recommendation
   400 mcg DFE for males and females age 19+; 600 mcg DFE/day during pregnancy (value from the source's table, not a sentence)
   NIH Office of Dietary Supplements, Health Professional Fact Sheet: Folate · checked 2026-09-15 · extracted
3. fol-06 · Position of an expert body · expert body recommendation
   consume 400 mcg of folic acid daily from supplements, fortified foods, or both
   NIH Office of Dietary Supplements, Health Professional Fact Sheet: Folate · checked 2026-09-15 · extracted
4. fol-04 · Position of an expert body · expert body statement
   At least 85% of folic acid is estimated to be bioavailable when taken with food, whereas only about 50% of folate naturally present in food is bioavailable.
   NIH Office of Dietary Supplements, Health Professional Fact Sheet: Folate · checked 2026-09-15 · extracted
5. fol-e1-01 · Position of an expert body · federal rulemaking
   As defined by the IOM, mcg DFE is equivalent to mcg food folate + (1.7 x mcg synthetic folic acid) (Ref. 141).
   FDA, Food Labeling: Revision of the Nutrition and Supplement Facts Labels (proposed rule), 79 FR 11879 · checked 2026-09-18 · independently re-checked
6. fol-e1-02 · Position of an expert body · federal rulemaking
   The IOM developed the new term, DFE, to account for the greater bioavailability of synthetic folic acid that is added to fortified foods or dietary supplements than folate that occurs naturally in foods (food folate).
   FDA, Food Labeling: Revision of the Nutrition and Supplement Facts Labels (proposed rule), 79 FR 11879 · checked 2026-09-18 · independently re-checked
7. fol-e1-03 · Randomised controlled trial(s) · randomised controlled dietary intervention
   Bioavailability of food folate relative to that of folic acid was 78% (95% CI: 48%, 108%) according to [(13)C(11)]-labeled folate and 85% (52%, 118%) according to changes in serum folate concentrations.
   Winkels et al., American Journal of Clinical Nutrition, 2007 · checked 2026-09-18 · independently re-checked
8. fol-e1-04 · Randomised controlled trial(s) · randomised controlled dietary intervention
   The consumption of a diet rich in food folate can improve the folate status of a population more efficiently than is generally assumed.
   Winkels et al., American Journal of Clinical Nutrition, 2007 · checked 2026-09-18 · independently re-checked
9. fol-e1-05 · Randomised controlled trial(s) · randomised crossover stable-isotope trial
   When comparing the ratio estimates of AUC and C(max) for the different test foods, highest bioavailability was found for spinach followed by that for wheat germs and Camembert cheese.
   Food & Function, 2015 · checked 2026-09-18 · independently re-checked
10. fol-e1-06 · Randomised controlled trial(s) · randomised crossover stable-isotope trial
    The results underline the dependence of folate bioavailability on the type of food ingested.
    Food & Function, 2015 · checked 2026-09-18 · independently re-checked
11. fol-11 · Position of an expert body · expert body statement
    Megaloblastic anemia, which is characterized by large, abnormally nucleated erythrocytes, is the primary clinical sign of folate or vitamin B12 deficiency.
    NIH Office of Dietary Supplements, Folate - Health Professional Fact Sheet · checked 2026-09-16 · independently re-checked
12. fol-12 · Position of an expert body · expert body statement
    Folate deficiency can also produce soreness in and shallow ulcerations on the tongue and oral mucosa; changes in skin, hair, or fingernail pigmentation; gastrointestinal symptoms; and elevated blood concentrations of homocysteine.
    NIH Office of Dietary Supplements, Folate - Health Professional Fact Sheet · checked 2026-09-16 · independently re-checked
13. fol-09 · Position of an expert body · expert body statement
    alcohol interferes with folate absorption and hepatic uptake, accelerates folate breakdown, and increases its renal excretion
    NIH Office of Dietary Supplements, Health Professional Fact Sheet: Folate · checked 2026-09-15 · extracted
14. fol-05 · Randomised controlled trial(s) · clinical trial evidence cited by expert body
    clinical trial evidence shows that adequate periconceptional folic acid consumption by women prevents a substantial proportion of NTDs.
    NIH Office of Dietary Supplements, Health Professional Fact Sheet: Folate · checked 2026-09-15 · extracted
15. fol-07 · Observational data · population surveillance
    Since 1998, when mandatory folic acid fortification began in the United States, NTD rates have declined by 28%.
    NIH Office of Dietary Supplements, Health Professional Fact Sheet: Folate · checked 2026-09-15 · extracted
16. fol-t1 · Meta-analysis or systematic review · umbrella review
    The random-effects model analysis from 10 included systematic review and meta-analysis revealed that the pooled effect of either folic acid or multivitamin supplementation for the prevention of NTD globally is found to be 0.43 (95% CI: 0.29, 0.58) (I2 = 93.50%; p ≤0.001).
    Tesfa et al., Neuroepidemiology, 2025 · checked 2026-09-16 · independently re-checked
17. fol-t3 · Observational data · meta-analysis of observational studies
    We also found that FA over-supplementation was not associated with an improvement in offspring's brain development, and may have a negative impact on offspring's neurodevelopmental outcomes.
    Liu et al., Critical Reviews in Food Science and Nutrition, 2023 · checked 2026-09-16 · independently re-checked
18. fol-t4 · Observational data · umbrella review
    In the general population, moderate-quality evidence supported an inverse association between total folate intake (from foods and supplements) and CRC risk (RR 0.84; 95% CI 0.80 to 0.90)
    Wang et al., BMJ Open, 2025 · checked 2026-09-16 · independently re-checked
19. fol-t5 · Meta-analysis or systematic review · systematic review and meta-analysis of randomised controlled trials
    In this population, high-dose folic acid supplementation was associated with a statistically significant reduction in miscarriage risk (RR 0.73; 95% CI: 0.56-0.94; I2=0%). An additional intention-to-treat (ITT) analysis, which included all randomized participants regardless of pregnancy status, showed a nonsignificant reduction in miscarriage risk (RR 0.79; 95% CI: 0.61-1.04). Moreover, the pregnancy rate was significantly higher in the high-dose group compared to the standard-dose group (RR 1.10; 95% CI: 1.02-1.18).
    Abdelrahman et al., American Journal of Obstetrics & Gynecology MFM, 2026 · checked 2026-09-17 · independently re-checked
20. fol-t7 · Meta-analysis or systematic review · meta-analysis of individual participant data from randomised trials
    In the four-group categorisation, prenatal MMS reduced the risk of preterm-SGA (0·71, 0·62-0·82; p=0·0002) and term-SGA (0·93, 0·89-0·98; p=0·0066).
    Wang et al., Lancet Global Health, 2025 · checked 2026-09-17 · independently re-checked
21. fol-t8 · Meta-analysis or systematic review · systematic review and meta-analysis of randomised trials
    Infants born to pregnant women consuming MMS were longer at birth (mean difference: 0.05 cm; 95% CI: 0.02, 0.08 cm) and had higher length-for-age z-score at birth (0.09; 95% CI: 0.06, 0.12), 3 mo (0.09; 95% CI: 0.06, 0.12), and 6 mo (0.04; 95% CI: 0.01, 0.07) of age but not thereafter...MMS reduced risk of stunting (risk ratio [RR]: 0.86; 95% CI: 0.82, 0.91), underweight (RR: 0.86; 95% CI: 0.81, 0.90), small HC (RR: 0.84; 95% CI: 0.79, 0.90), and low MUAC (RR: 0.90; 95% CI: 0.82, 0.99) at 3 mo
    Gonçalves et al., American Journal of Clinical Nutrition, 2025 · checked 2026-09-17 · independently re-checked
22. fol-e1-07 · Meta-analysis or systematic review · systematic review and meta-analysis
    5-MTHF substitution reduced plasma UMFA (SMD +0.99; 95% CI +0.20 to +1.79; I2 = 30%; k = 3; GRADE low).
    Nutrients, 2026 · checked 2026-09-18 · independently re-checked
23. fol-e1-08 · Meta-analysis or systematic review · systematic review with genotype-stratified secondary analysis
    This is a research question rather than a basis for genotype-guided supplementation in practice, and MTHFR 677TT carriers are a plausible early target population for confirmatory substitution trials.
    Nutrients, 2026 · checked 2026-09-18 · independently re-checked
24. fol-e1-12 · Randomised controlled trial(s) · randomised controlled trial
    By week 24 of the study, 5-MTHF and total folate concentrations in maternal and cord serum and the placenta did not differ between groups.
    Frontiers in Nutrition, 2026 · checked 2026-09-18 · independently re-checked
25. fol-e1-13 · Randomised controlled trial(s) · randomised controlled trial
    There were significantly fewer participants (7% vs. 31%) with detectable UMFA and lower average UMFA concentrations in maternal blood of the MTHF-MVI group vs. the FA-MVI group (p < 0.01).
    Frontiers in Nutrition, 2026 · checked 2026-09-18 · independently re-checked
26. fol-03 · Position of an expert body · expert body statement
    The FNB did not establish a UL for folate from food because high intakes of folate from food sources have not been reported to cause adverse effects.
    NIH Office of Dietary Supplements, Health Professional Fact Sheet: Folate · checked 2026-09-15 · extracted
27. fol-08 · Position of an expert body · expert body statement
    Large amounts of folate can correct the megaloblastic anemia, but not the neurological damage, that can result from vitamin B12 deficiency.
    NIH Office of Dietary Supplements, Health Professional Fact Sheet: Folate · checked 2026-09-15 · extracted
28. fol-t6 · Randomised controlled trial(s) · secondary analysis of a randomised controlled trial
    The rate of preeclampsia was significantly higher in the folic acid group compared to the placebo group in crude analyses (17.2 versus 9.9%; relative risk 1.75 [95% CI 1.06-2.88], p = .029). Multivariable analyses attenuated this effect, rendering it not statistically significant (RR 1.58 [95% CI 0.95-2.63], p = .079).
    Wen et al., Journal of Maternal-Fetal & Neonatal Medicine, 2022 · checked 2026-09-17 · independently re-checked
29. fol-e1-09 · Meta-analysis or systematic review · stated evidence gap in a systematic review
    A major methodological limitation of this field is that effects of UMFA cannot readily be identified independently of total circulating folate.
    Nutrients, 2026 · checked 2026-09-18 · independently re-checked
30. fol-e1-10 · Meta-analysis or systematic review · stated evidence gap in a systematic review
    Unmetabolized folic acid also constitutes no more than 1–2% of serum total folate [76], making measurement error proportionally large relative to its dynamic range, and a single measurement conflates dose with time since last ingestion; in the Boston Birth Cohort, time since last FA exposure was not recorded [33,76].
    Nutrients, 2026 · checked 2026-09-18 · independently re-checked
31. fol-e1-11 · Meta-analysis or systematic review · systematic review
    Observational UMFA-outcome associations were inconsistent, confounded by total folate, and unreplicated.
    Nutrients, 2026 · checked 2026-09-18 · independently re-checked
32. fol-10 · Position of an expert body · expert body statement
    Patients taking methotrexate for cancer should consult their oncologist before taking folate supplements because the supplements could interfere with methotrexate's anticancer effects.
    NIH Office of Dietary Supplements, Health Professional Fact Sheet: Folate · checked 2026-09-15 · extracted
33. fol-t9 · Meta-analysis or systematic review · Cochrane systematic review and meta-analysis
    People receiving folic acid (alone or with iron) and antifolate antimalarials were less likely to clear malaria parasites (day 3) compared to individuals who did not receive folic acid (RR 0.89; 95% CI 0.84 to 0.95, 4 trials, 929 individuals, moderate-certainty evidence); and had increased risk of treatment failure on day 7 (RR 2.12; 95% CI 1.41 to 3.19, 4 trials, 1062 individuals, moderate-certainty evidence), day 14 (RR 1.97; 95% CI 1.44 to 2.70, 3 trials; 891 individuals; moderate-certainty evidence), and day 28 (RR 1.35; 95% CI 1.21 to 1.51; 4 trials; 1012 individuals, moderate-certainty evidence).
    Cochrane Database of Systematic Reviews, 2026 · checked 2026-09-17 · independently re-checked