Folate deficiency is uncommon in healthy adults. When you eat a balanced diet and have no health conditions impairing absorption of folate — also called folic acid — you don't need a supplement. But pregnant women, those planning a pregnancy and individuals with a gene mutation referred to as MTHFR do. Unneeded folate, especially high doses for long periods, can increase risk of some cancers and have other damaging effects. In the short term, you may experience uncomfortable but harmless side effects.
What Is Folate?
Folate is one of a group of eight B vitamins, also including thiamin, riboflavin, biotin, pantothenic acid, niacin and vitamins B6 and B12. As a group, these vitamins play a role in metabolism and the formation of red blood cells. Individually, folate is involved in DNA and RNA synthesis and cell division. Folate also plays a role in lowering levels of an amino acid called homocysteine, high levels of which are associated with increased risk of heart disease, impaired cognitive function and depression.
The B vitamins are water-soluble, so any excess that your body can't immediately use is carried out in urine. Because of this, many people think that taking high-dose supplements isn't harmful, and with some B vitamins — B12, for example — it's not. However folate is different.
Read more: Fruits and Vegetables Containing Folic Acid
High Folate Levels Symptoms
When used at the appropriate dose, folic acid is safe for most adults. But it can still cause side effects, including:
- Bad taste in your mouth
- Loss of appetite
- Sleep disturbance
These side effects may occur at any dose, but large doses could be more likely to cause or worsen side effects. People with allergies may have allergic reactions to folate, symptoms of which include:
- Skin rash
- Difficulty breathing
Taking a large amount of folate could potentially worsen the reaction and have more serious consequences.
High Blood Levels of Folate
While researchers are still investigating how too much folic acid can exacerbate existing conditions and promote disease, studies have uncovered several probable associations:
Exacerbates B12 deficiency: Folate deficiency and B12 deficiency both cause anemias, conditions in which the body is unable to make enough healthy red blood cells. Taking a folic acid supplement for folate deficiency can temporarily alleviate the symptoms of a B12 deficiency, allowing it to go untreated. Untreated B12 deficiency can cause severe neurological problems. According to a 2018 review in Blood Journal, this only happens in the presence of large amounts of excess folic acid.
Increases risk of cancer: Normal folate intake can be cancer-preventive. According to a 2007 review in The American Journal of Clinical Nutrition, higher folate intakes in the absence of precancerous lesions — such as colon polyps — can prevent tumor development and colon and pancreatic cancers. However, in the presence of lesions, folic acid supplements can increase the risk of these cancers.
Reduces immune response: A 2017 study in the Journal of Nutrition found that adults who took a 5 mg (5,000 mcg) folic acid supplement for 90 days had reductions in the number and cytotoxicity of natural killer cells. Natural killer cells are the body's defense mechanism, attaching to and killing cancerous and virus-containing cells.
Recommended Daily Intakes
Healthy adults need 400 mcg of folate per day, and pregnant women need 600 mcg per day. Women who may become pregnant are often encouraged to take a folic acid supplement. However, there are plenty of rich food sources of the nutrient, and many foods are fortified with folic acid. In fact, there's concern that people who eat a balanced diet including fortified foods and who also take a multivitamin may regularly exceed the daily upper tolerable intake of 1,000 mcg set by the National Academy of Medicine.
MTFHR Gene Mutation
People with a genetic mutation of an enzyme called methylenetetrahydrofolate reductase (MTHFR) have trouble converting folate from food and supplements to its active form, 5-MTHF. Decreased levels of 5-MTHF can raise homocysteine levels and increase the risk for neural tube defects in babies. These individuals may need to take a supplement containing the active form of folate. Consult your doctor before you decide to take a folic acid supplement.
- NIH: Folate
- Harvard Health Publishing: Folic Acid: Too Much of a Good Thing?
- NIH: MTHFR Gene
- MedlinePlus: B Vitamins
- Mayo Clinic: Anemia
- Harvard Health Publishing: Vitamin B12 deficiency can be sneaky, harmful
- Blood Journal: Vitamin B12 deficiency from the perspective of a practicing hematologist
- The American Journal of Clinical Nutrition: Folate and cancer prevention: a closer look at a complex picture
- British Society for Immunology: Natural Killer Cells
- NIH: MTHFR Gene Variant
- Nutritional Supplements Health Guide: Are There Potential Risks of Folic Acid Overdose?
- The Vitamin and Nutrition Center: Vitamin B9 (Folic Acid)