Vitamin B9 is a B vitamin your body uses to build DNA, produce new cells, and process amino acids. You’ll see it referred to by two names: folate, which is the form found naturally in foods like leafy greens and legumes, and folic acid, which is the synthetic version used in supplements and fortified foods. Both are forms of B9, but they differ in how well your body absorbs them and how stable they are.
What B9 Does in Your Body
Vitamin B9 is central to a process called one-carbon metabolism, which is a biochemical workhorse behind several critical functions. In practical terms, B9 helps your body copy and repair DNA every time a cell divides. This makes it especially important for tissues that turn over quickly: blood cells, the lining of your gut, and developing embryonic tissue.
B9 also helps regulate levels of an amino acid called homocysteine. Your body converts homocysteine into methionine (a useful building block for proteins) using B9 and vitamin B12 as partners. When B9 is low, homocysteine builds up in the blood, and elevated homocysteine is linked to cardiovascular problems. This recycling process also produces molecules your cells need for gene regulation, a process known as methylation that influences which genes are turned on or off throughout your life.
Because B9 works so closely with B12, B6, and riboflavin (B2), a deficiency in any of these can bottleneck the same pathways. Think of them as a team rather than solo players.
Folate vs. Folic Acid
The distinction matters more than most people realize. Folate in food exists in a bulkier chemical form that your gut has to break down before absorbing it. Only about 50% of the folate in food actually makes it into your bloodstream. Folic acid, the synthetic form, is simpler and more stable. About 85% is absorbed when taken with food, and even more when taken on an empty stomach.
To account for this gap, nutrition labels use a unit called Dietary Folate Equivalents (DFE). The conversions work like this:
- 1 mcg DFE = 1 mcg of food folate
- 1 mcg DFE = 0.6 mcg of folic acid taken with food
- 1 mcg DFE = 0.5 mcg of folic acid taken on an empty stomach
So 200 mcg of folic acid in a fortified cereal is equivalent to roughly 333 mcg DFE. If you’re reading a supplement or food label, look for the DFE number to compare apples to apples.
How Much You Need
Most adults need 400 mcg DFE per day. Pregnant women need 600 mcg DFE, and breastfeeding women need 500 mcg DFE. These recommendations come from the Food and Nutrition Board, which sets dietary guidelines in the United States.
The number that gets the most attention is the 400 mcg of folic acid recommended for all women of childbearing age, whether or not they’re planning a pregnancy. More than 50% of pregnancies in the United States are unplanned, so the CDC recommends women capable of becoming pregnant take 400 mcg of folic acid daily on a continuous basis.
B9 and Pregnancy
Folate’s most well-known role is preventing neural tube defects, which are serious birth defects of the brain and spine like spina bifida. The neural tube forms very early in pregnancy, often before a woman knows she’s pregnant, which is why supplementation needs to start before conception.
A landmark randomized trial by the British Medical Research Council found that women who had previously had an affected pregnancy reduced their risk of recurrence by 70% with high-dose folic acid (4 mg per day). For women without that history, taking 400 to 800 mcg of folic acid daily has been estimated to cut the risk of neural tube defects by roughly 50%. The critical window is at least one month before conception through the first three months of pregnancy. Women with a prior affected pregnancy are typically advised to take the higher 4 mg dose during this same period.
Best Food Sources
Dark leafy greens, lentils, chickpeas, asparagus, and beef liver are among the richest natural sources of folate. A cup of cooked lentils delivers around 358 mcg DFE, and a cup of cooked spinach provides roughly 263 mcg DFE. Fortified foods, including many breads, cereals, and pastas, contain folic acid because the U.S. and many other countries require grain products to be fortified.
Because food folate is only about half as bioavailable as folic acid, people who rely entirely on whole foods for their B9 need to eat generous amounts of folate-rich vegetables and legumes daily. Cooking and prolonged storage also degrade natural folate, since it’s less chemically stable than folic acid.
What Deficiency Looks Like
When B9 runs low, the first organ system to suffer is your blood. Without enough folate, your bone marrow produces abnormally large, immature red blood cells that can’t carry oxygen efficiently. This condition, called megaloblastic anemia, causes fatigue, weakness, shortness of breath (especially with physical activity), lightheadedness, and heart palpitations. A physical exam might reveal pale skin, a rapid heart rate, or a swollen, sore tongue.
Folate deficiency can develop relatively quickly compared to B12 deficiency because your body stores only a small reserve of B9. People most at risk include those with poor dietary intake, heavy alcohol use (which impairs folate absorption), inflammatory bowel conditions, and anyone taking medications that interfere with folate metabolism.
The B12 Masking Problem
One important safety concern with folic acid is that it can mask a vitamin B12 deficiency. Here’s why: both B9 and B12 deficiency cause the same type of anemia, and that anemia is often the first warning sign of low B12. If you take enough folic acid to correct the anemia, the blood work looks normal, but the underlying B12 shortage continues unchecked. Over time, B12 deficiency causes progressive nerve damage that can become permanent.
This is why the safe upper limit for folic acid from supplements and fortified foods is set at 1,000 mcg (1 mg) per day for adults. That threshold was chosen specifically to minimize the risk of masking. Folate from whole foods does not count toward this upper limit because it’s absorbed less efficiently and is unlikely to reach problematic levels.
The MTHFR Gene Variant
You may have seen claims that people with a common genetic variant called MTHFR should avoid folic acid and use a different form of folate instead. The MTHFR gene produces an enzyme involved in processing B9, and certain variants slightly reduce its activity. The most studied variant (MTHFR 677 TT) results in blood folate levels only about 16% lower than average when people consume the same amount of folic acid.
The CDC is clear on this point: people with MTHFR variants can process all types of folate, including folic acid. Taking 400 mcg of folic acid daily still raises blood folate levels regardless of your MTHFR genotype, and folic acid remains the only form of B9 proven to prevent neural tube defects. There is not enough evidence to recommend switching to alternative forms like 5-MTHF based on MTHFR status alone.
Medications That Affect B9 Levels
Several common medications interfere with how your body uses folate. Methotrexate, used for autoimmune conditions and certain cancers, works by deliberately blocking folate metabolism. People on methotrexate are often prescribed supplemental folic acid on specific days to reduce side effects without undermining the drug’s purpose.
Some anti-seizure medications, including phenytoin, carbamazepine, and phenobarbital, also have well-documented anti-folate effects and can lower B9 levels over time. The anti-inflammatory drug sulfasalazine and the antibiotic trimethoprim similarly disrupt folate pathways. Certain antimalarial drugs share this mechanism as well. If you take any of these medications long-term, your folate status is worth monitoring.