Folate is vitamin B9, the water-soluble vitamin the body uses to build and repair DNA and to pass methyl groups around one-carbon metabolism. Folic acid is the stable synthetic form used in supplements and fortified flour. The body cannot make either, so the whole supply comes from diet, and stores empty within a few months of poor intake. The fuller clinical reference, including deficiency signs and the pregnancy case, is the Folate (vitamin B9) entry in the Smart Longevity glossary.
What it measures
Two blood tests are in common use and they answer different questions. Serum folate reflects recent intake: it rises after a folate-rich meal and falls again within days, so a fasting sample is preferred and one low result is not proof of empty stores. Red cell folate, often written RBC folate, is taken up while red blood cells are formed and stays put for their roughly four-month lifespan, so it reads as a longer-term average.
Results come in nanograms per milliliter or nanomoles per liter, and mixing the units confuses comparisons between labs. A third reading is indirect but useful: when folate runs short the body cannot recycle homocysteine back to methionine efficiently, so Homocysteine tends to rise, which is why the two are often ordered together.
Typical values
Cut-offs differ by assay and by laboratory, so read your own report’s reference interval first. The widely published thresholds sit roughly here:
| Test | Commonly cited threshold | Read as |
|---|---|---|
| Serum folate | Below about 4 ng/mL (roughly 10 nmol/L) | Deficiency in WHO’s framing; labs differ |
| Red cell folate | Below about 150 ng/mL (roughly 340 nmol/L) | Depleted stores, not a single low day |
| Red cell folate | Around 400 ng/mL (roughly 900 nmol/L) or above | WHO’s level for lowest neural tube defect risk before pregnancy |
There is no meaningful upper limit on the test, so a high result usually just means a supplement or fortified food. Where grain is fortified, population levels run higher, which shifts what a given number means locally.
Why it matters for longevity
The strongest evidence concerns the start of life rather than the end of it: folic acid before conception and in early pregnancy prevents neural tube defects, a trial result reproduced at population scale wherever fortification of flour became mandatory. Deficiency also causes megaloblastic anemia, in which red cells are produced too few and too large.
For aging itself the picture is thinner. Low folate is associated with raised homocysteine, and higher homocysteine is in turn associated with cardiovascular disease and cognitive decline. But trials that lowered homocysteine with folic acid and other B vitamins did not reduce heart attacks, and the cognitive results were mixed at best. High-dose folic acid has also raised a debated cancer safety question. The defensible summary: correcting a real deficiency is worthwhile; supplementing past that has not been shown to extend healthspan.
What changes it
Food moves it first. Leafy greens, legumes, asparagus, beets, citrus and liver are the dense sources, and in the United States enriched bread, pasta and cereals add a baseline. Natural folate is fragile: heat degrades it and boiling leaches it into the water, so lightly cooked greens deliver more than a long simmer.
Several things drain it. Regular heavy drinking impairs absorption and use; pregnancy and lactation raise the requirement sharply; malabsorption from celiac disease or bowel surgery cuts uptake; and methotrexate, sulfasalazine, trimethoprim and some anticonvulsants block the pathway directly. Common variants in the MTHFR gene make the conversion step less efficient, which matters mainly when intake is already low.
One rule stands above the rest: never take high-dose folic acid without checking Vitamin B12 (cobalamin), because folate can correct the anemia of a B12 deficiency while the nerve damage quietly continues. Test before you supplement, and treat the tolerable upper intake level for folic acid in adults, 1,000 micrograms a day in the United States, as a ceiling rather than a target.
Related reading
- How to Eat for Brain Health: A Cognitive Longevity Guide
- How to Build a Longevity Supplement Plan
- Mediterranean and MIND Diets Cut Dementia Risk
Disclaimer. This article is for information only and does not replace medical advice. Talk to a qualified clinician before changing anything about your health.