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Folate (Vitamin B9)
Folate is two different stories wearing one label. For anyone who could become pregnant it is close to settled: the neural tube closes by week six, before most positive tests, so the supplement has to be on board first. Start it a month before trying and severe birth defects drop by half to two-thirds, at pennies a day MRC 1991. For everyone else the answer is smaller: fix a real deficiency, yes, but the heart-and-brain benefits sold in the aisle mostly aren't real.
Do ยท Daily Evidence Strong Chapter Supplements

The pregnancy case is the real one. Two randomised trials point the same way: 72% fewer affected pregnancies in the MRC trial 1, and zero neural-tube defects versus six on placebo in the second 2. When the US started fortifying flour, population rates fell about 28% 3.

The heart-and-brain case mostly collapsed. Folate reliably drops homocysteine by a quarter, so the obvious bet was fewer heart attacks and strokes. Across 37,485 patients in eight trials it did nothing for heart events, cancer, or death, only a modest stroke signal 4. The wins that survive are all in deficient people: stroke prevention where folate is scarce 5, and SSRI-resistant depression in patients who are also low on folate 6.

Which situation are you in? The right dose depends entirely on that, so find your row.

The food route works too: a cup of cooked lentils gives 358 ยตg, near the whole daily target, and fortified grain gets most people there without trying 9.

What you get depends on who you are. Preconception, the payoff is an outcome you never see: a pregnancy that just goes normally, months later 2. From actual deficiency, new red cells appear on the smear in three to four days and the fatigue lifts over a month or two 10. Adding L-methylfolate to a stalled SSRI, the window is eight to twelve weeks to see if you respond 6. Folate-replete and none of the above: nothing, and the aisle won't tell you that.

The fine print โ€” when to skip it, and what people get wrong

Treating large-cell anemia or fatigue in an older adult: check B12 first. Folate fixes the blood count while the nerve damage of a B12 deficiency keeps running silent 10.

The MTHFR gene doesn't require expensive methylfolate: routine testing is advised against and ordinary intake gets carriers to normal status 11. "Start once you see two lines" is too late; the closure window has shut by then 7.

Had a colorectal polyp removed? One trial found more new adenomas on 1 mg/day folic acid, so favour the food route 12. Don't stop at the first trimester; folate stays load-bearing through pregnancy and lactation.

References
  1. 1MRC Vitamin Study Research Group (1991). Prevention of neural tube defects: results of the Medical Research Council Vitamin Study. The Lancet. link
  2. 2Czeizel AE, Dudรกs I (1992). Prevention of the first occurrence of neural-tube defects by periconceptional vitamin supplementation. New England Journal of Medicine. link
  3. 3Williams J, Mai CT, Mulinare J, et al. (2015). Updated estimates of neural tube defects prevented by mandatory folic acid fortification โ€” United States, 1995โ€“2011. MMWR Morbidity and Mortality Weekly Report. link
  4. 4Clarke R, Halsey J, Lewington S, et al. (2010). Effects of lowering homocysteine levels with B vitamins on cardiovascular disease, cancer, and cause-specific mortality: meta-analysis of 8 randomized trials involving 37 485 individuals. Archives of Internal Medicine. link
  5. 5Huo Y, Li J, Qin X, et al. (2015). Efficacy of folic acid therapy in primary prevention of stroke among adults with hypertension in China: the CSPPT randomized clinical trial. JAMA. link
  6. 6Papakostas GI, Shelton RC, Zajecka JM, et al. (2012). L-methylfolate as adjunctive therapy for SSRI-resistant major depression: results of two randomized, double-blind, parallel-sequential trials. American Journal of Psychiatry. link
  7. 7USPSTF (2023). Folic Acid Supplementation to Prevent Neural Tube Defects: US Preventive Services Task Force Recommendation Statement. JAMA. link
  8. 8Wilson RD, et al. (2015). Pre-conception folic acid and multivitamin supplementation for the primary and secondary prevention of neural tube defects and other folic acid-sensitive congenital anomalies. Journal of Obstetrics and Gynaecology Canada. link
  9. 9Bailey RL, Dodd KW, Gahche JJ, et al. (2010). Total folate and folic acid intake from foods and dietary supplements in the United States: 2003-2006. American Journal of Clinical Nutrition. link
  10. 10Institute of Medicine (1998). Dietary Reference Intakes for Thiamin, Riboflavin, Niacin, Vitamin B6, Folate, Vitamin B12, Pantothenic Acid, Biotin, and Choline. link
  11. 11Hickey SE, Curry CJ, Toriello HV (2013). ACMG Practice Guideline: lack of evidence for MTHFR polymorphism testing. Genetics in Medicine. link
  12. 12Cole BF, Baron JA, Sandler RS, et al. (2007). Folic acid for the prevention of colorectal adenomas: a randomized clinical trial. JAMA. link
Related in the handbook (8)
Works well with
  • โ€” Beyond the famous pregnancy role, low folate raises homocysteine and is linked to faster hearing loss.
Related
  • โ€” If your B12 looks low or symptoms linger, get folate checked too โ€” the two share a pathway and one can hide trouble in the other.
  • โ€” Both are pregnancy non-negotiables โ€” check that your prenatal covers iodine the way it covers folate.
  • โ€” Folate deficiency drives the same large-cell pattern as B12 โ€” and the two can mask each other.
  • โ€” A positive MTHFR result almost always kicks off a methylfolate habit the evidence doesn't support โ€” folate is the supplement actually worth knowing about.
  • โ€” Beef liver is the richest whole-food source of folate there is โ€” one serving covers a big chunk of the day's need.
  • โ€” Iodine is half the preconception nutrition story; folate is the other half. Check both before trying to conceive.
  • โ€” B12 and folate work as a pair, and a folate dose can mask a B12 deficiency that's still damaging nerves.
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