Vitamin B9 (Folate / Folic Acid)Vitamin

• Neural tube defects in early pregnancyLikely helps

  This is the settled one, and no government source disputes it [1][2][3][4].
  
  CDC and the US Preventive Services Task Force advise 400–800 mcg/day of folic acid, from at least a month before conception through the first 2 to 3 months of pregnancy [1][3].
  
  US neural tube defect rates fell about 28% after mandatory folic acid fortification of grain products began in 1998 [1].
  
  The timing is why the advice covers everyone who could become pregnant, not only those planning to — roughly half of pregnancies are unplanned, and folate matters in the first weeks after conception [1]. ^[1]

• Stroke risk — but not heart attackPossibly helps

  A meta-analysis of 19 RCTs (47,921 participants) found folic acid and B-vitamin supplementation cut relative stroke risk by 12% [1].
  
  A Chinese hypertension trial in 20,702 adults found folic acid plus enalapril lowered stroke risk 21% against enalapril alone [1].
  
  The same body of trials shows no effect on myocardial infarction, coronary heart disease or cardiovascular death [1].
  
  That is a well-replicated split, not a disagreement — and it is why 'good for heart health' overstates what was measured. The effect is also larger in populations without food fortification [1]. ^[1]

• Cancer risk — the evidence points both waysPossibly helps

  Dietary folate and supplemental folic acid do not point the same direction. NIH ODS frames this as folate having a possible dual role depending on dose and timing, not as sources disagreeing [1].
  
  In the NIH-AARP cohort of over 525,000 adults, the highest folate intake group (≥900 mcg/day) had 30% lower colorectal cancer risk than the lowest (<200 mcg/day) [1].
  
  But a meta-analysis of 6 RCTs (25,738 men) found folic acid supplementation associated with 24% higher prostate cancer risk versus placebo [1].
  
  A 2015 NIH/National Toxicology Program expert panel concluded folic acid does not lower cancer risk in people whose folate status is already adequate [1].
  
  The same panel flagged possible adverse effects on cancer growth in some contexts, particularly where precancerous lesions already exist. ^[1]

• Cognitive function and dementiaNot shown to help

  Most clinical trials show folic acid does not improve cognitive function or lower dementia and Alzheimer's risk — despite reliably lowering homocysteine [1].
  
  The one open thread: a few trials found slower decline specifically in people whose homocysteine was elevated at baseline [1].
  
  Read the homocysteine drop as a biochemical change that did not translate into a cognitive one. ^[1]

• Depression, as an add-on to SSRIsPossibly helps

  Low folate status tracks with depression and poorer antidepressant response in some studies, though not all [1].
  
  Low-quality evidence suggests folic acid under 5,000 mcg/day, or 5-MTHF around 15,000 mcg/day, might add something on top of an SSRI [1].
  
  Several trials found no benefit at all. Both of those doses sit far above the 1,000 mcg/day upper limit and are prescription territory, not self-supplementation [1]. ^[1]

• Autism spectrum disorder in the offspringPossibly helps

  The cohorts disagree and we are not resolving it for you.
  
  Norwegian and Israeli cohorts and a US case-control study found lower ASD risk with periconceptional folic acid; a large Danish cohort found no association [1].
  
  NIH ODS attributes the split to the observational design and possible confounding rather than a true contradiction [1]. ^[1]

• Preterm birth and congenital heart defectsPossibly helps

  Observational data suggest periconceptional folic acid may be associated with modestly lower preterm birth and congenital heart defect rates [1].
  
  NIH ODS says more research is needed on both. The data are not strong enough to read as causal [1]. ^[1]

• Megaloblastic anemia — abnormally large, immature red blood cells — is the hallmark [1]
• Weakness, fatigue, difficulty concentrating, irritability and headache [1]
• Heart palpitations and shortness of breath, or rapid breathing [1][5]
• A sore, smooth or red tongue, and shallow ulcerations of the tongue and mouth [1]
• Diarrhea, indigestion and loss of appetite [1][5]
• Elevated blood homocysteine [1]
• Pigmentation changes in skin, hair or fingernails [1]
• Cognitive changes affecting memory, understanding or judgement, per NHS [5]
• What folate deficiency does NOT cause: the numbness, pins-and-needles and balance problems of B12 deficiency. That neurological picture is B12-specific [1][5]
• Higher-risk groups: people with alcohol use disorder (over 60% had low folate status in one Portuguese study), women of childbearing age, and pregnant women.
  
  Also people with celiac disease or IBD (20–60% deficient), and carriers of the MTHFR 677C>T variant [1]^[1]

• Food folateEveryday intake. It has no upper limit, because high intakes from food have not been shown to cause harm .

  Elemental content

  Mostly polyglutamate tetrahydrofolate (THF) forms, found in leafy greens, beans, liver and citrus [1]

  Absorption

  Only about 50% bioavailable — that figure is the basis of the DFE conversion [1].
  
  Processed largely in the small intestine and usable by the body straight away, with no liver conversion step [1].
  
  Less stable than folic acid: it breaks down with heat and light, so cooking and storage cost you some [1]

  Best for

  Everyday intake. It has no upper limit, because high intakes from food have not been shown to cause harm [1].

• Folic acidAnyone who could become pregnant.

  Elemental content

  The fully oxidized synthetic monoglutamate form, used in fortified food and most supplements [1]

  Absorption

  About 85% bioavailable taken with food, close to 100% on an empty stomach [1].
  
  It has to be converted first, by the liver enzyme dihydrofolate reductase, before the body can use it [1].
  
  That step is slower, and in some people leaves detectable unmetabolized folic acid circulating in the blood [1]

  Best for

  Anyone who could become pregnant. It is the only folate form with clinical trial evidence behind neural tube defect prevention, and its heat stability is why it is the fortification form [1][3].

• 5-MTHF (L-methylfolate, methylfolate)People who prefer to skip the conversion step.

  Elemental content

  A natural monoglutamate form — the same active form the body converts folate into [1]

  Absorption

  Reported as equal to or better than folic acid's [1].
  
  No formal mcg-to-DFE conversion factor has been established. The FDA lets manufacturers apply a factor of up to 1.7, or a lower one of their own [1]

  Best for

  People who prefer to skip the conversion step. Flagged conflict: some clinicians suggest MTHFR-variant carriers add 5-MTHF, while CDC and NIH ODS say do not change folate intake by genotype — see the FAQ [1][3].

• High-dose folic acid side effects

  Milder effects reported at high doses include nausea, loss of appetite, bloating and gas, stomach pain, a bitter taste, and sleep or mood changes [1][14].
  
  True allergy to folic acid is rare but documented. An FDA drug label for folic acid tablets lists skin rash and redness, itching, general malaise and bronchospasm [12].
  
  One reported case was suggestive of anaphylaxis after injection [12]. Swelling of the face, lips, tongue or throat, or difficulty breathing, warrants immediate medical help. ^[12]

• Masking a vitamin B12 deficiency

  This is the one to know about. Large amounts of folate or folic acid can correct the megaloblastic anemia that B12 deficiency causes, while B12's nerve damage carries on unnoticed [1].
  
  By the time it surfaces, that damage may already have progressed [1].
  
  CDC states that folic acid at the recommended 400 mcg/day does not delay detection of B12 deficiency — the concern is tied to high intakes, not to normal ones [4]. ^[4]

• Methotrexate

  Methotrexate is a folate antagonist, and which way this cuts depends entirely on why it was prescribed [1].
  
  For cancer, folate supplements could interfere with methotrexate's anticancer effect. NIH ODS says check with the oncologist first [1].
  
  For low-dose methotrexate in rheumatoid arthritis or psoriasis, folate supplementation may actually reduce the drug's gastrointestinal side effects [1]. ^[1]

• Antiseizure medication

  Phenytoin, carbamazepine, valproate and, per one drug reference, barbiturates such as phenobarbital can all lower serum folate [1].
  
  It runs both ways — folate supplements can lower blood levels of those same medications, which is the more consequential direction [1].
  
  NIH ODS advises anyone on these drugs to check with their provider before supplementing [1]. ^[1]

• Sulfasalazine

  Sulfasalazine, used in ulcerative colitis, inhibits folate absorption in the intestine [1].
  
  Long-term use can therefore cause folate deficiency in its own right [1]. ^[1]

• Tetracyclines and folate blood tests

  This is a lab-interference issue rather than a nutritional one.
  
  Tetracycline antibiotics can produce falsely low folate readings on assays that rely on growth of the Lactobacillus casei organism, by suppressing that organism [14].
  
  The folate level itself is not necessarily low — the test result is [14]. ^[14]

Folate turns up across a wide range of foods — vegetables, fruit and juices, nuts, beans and peas, seafood, eggs, dairy, meat, poultry and grains [1].

The richest, per USDA data cited by NIH, in mcg DFE per serving with %DV:

Beef liver, 3 oz braised — 215 mcg (54% DV). Spinach, half a cup boiled — 131 mcg (33%) [1][2].

Black-eyed peas, half a cup boiled — 105 mcg (26%). Fortified breakfast cereal, a 25% DV serving — 100 mcg (25%) [1][2].

Asparagus, 4 spears boiled — 89 mcg (22%). Brussels sprouts, half a cup boiled — 78 mcg (20%) [1][2].

Enriched spaghetti, half a cup cooked — 74 mcg (19%). Romaine lettuce, 1 cup — 64 mcg (16%) [1][2].

Avocado, half a cup — 59 mcg (15%).

Broccoli, half a cup cooked — 52 mcg (13%).

Orange juice, three-quarters of a cup — 35 mcg (9%) [1][2].

Fortification is the quiet giant here. Since January 1998 US law has required 140 mcg of folic acid per 100 g in enriched bread, flour, cornmeal, pasta and rice [1].

The FDA additionally allowed voluntary fortification of corn masa flour in 2016, up to 154 mcg/100g. Together the program raised average US folic acid intake by about 190 mcg/day [1].

Cooking method matters for the natural folate in vegetables. A British Journal of Nutrition study found steaming spinach or broccoli caused no significant folate loss even at the longest times tested — 4.5 minutes for spinach [11].

Boiling leaches folate into the water instead, costing substantially more. Steaming, microwaving, or using the cooking liquid in soups beats boiling and draining [11].

Folate is water-soluble, so it needs no dietary fat and can be taken at any hour, with or without food [1].

Multiple medically reviewed sources agree morning versus evening makes no measurable difference to absorption or effect [6][7].

What the dietitians quoted in those sources emphasise instead is a consistent time each day, because the routine is what actually gets it taken [6][7].

There is a technical wrinkle: folic acid is close to 100% bioavailable fasted against about 85% with food [1][7].

That gap is not considered clinically meaningful enough to override 'take it when you'll remember'. Some people get mild stomach upset fasted, in which case a small snack is the fix [7].

One genuine exception: a combined multivitamin or prenatal also contains fat-soluble vitamins A, D, E and K, and those absorb better with a meal containing some fat [6].