What Happens When Folic Acid is High in the Body?

October 24, 2025 •

2 min read

According to the National Institutes of Health, the tolerable upper intake level for adults is 1,000 mcg of folic acid from fortified foods and supplements. When levels exceed this threshold, serious health concerns can arise, including a masked vitamin B12 deficiency.

Quick Summary

Excessive intake of synthetic folic acid can lead to a buildup of unmetabolized folic acid (UMFA) in the blood, potentially masking a vitamin B12 deficiency and impacting neurological function, immune response, and increasing specific cancer risks.

Key Points

Masks B12 Deficiency: High folic acid can hide the symptoms of a vitamin B12 deficiency, delaying treatment and potentially causing irreversible nerve damage.
Increases Unmetabolized Folic Acid (UMFA): The body can't process large amounts of synthetic folic acid efficiently, leading to a buildup of UMFA in the blood.
Potential Cancer Risks: Some research suggests that very high supplemental folic acid might promote the growth of pre-existing cancers, such as colorectal cancer.
Impacts Pregnancy and Offspring: Excessive maternal folic acid intake has been linked to increased risk of childhood obesity, insulin resistance, and some neurological issues in offspring.
Weakens Immune Response: Elevated levels of UMFA have been associated with reduced activity of natural killer (NK) cells, weakening the body's immune system.
Causes Minor Side Effects: High doses can cause symptoms like nausea, bloating, diarrhea, irritability, and sleep problems.

The Difference Between Folate and Folic Acid

Folate is the naturally occurring form of vitamin B9 found in foods, while folic acid is the synthetic version added to fortified foods and supplements. The body processes these differently.

The Danger of Masking a Vitamin B12 Deficiency

A major risk of high folic acid is its ability to hide the symptoms of a vitamin B12 deficiency. High folic acid can correct the anemia associated with B12 deficiency without addressing underlying neurological issues, which is especially concerning for older adults prone to B12 deficiencies and potential nerve damage.

Symptoms and Effects of High Folic Acid

Excessive synthetic folic acid intake may lead to side effects because the body struggles to process large amounts. Potential issues can include digestive problems, confusion, irritability, sleep disruptions, and skin reactions.

Concerns Related to Cancer Risk and Immune Function

Research on the link between high folic acid and cancer risk is ongoing. Some studies suggest high supplemental doses might accelerate the growth of existing pre-cancerous cells. High unmetabolized folic acid levels have also been linked to reduced activity of immune cells.

Special Risks During Pregnancy

While folic acid is crucial in early pregnancy, research is exploring the impact of very high maternal intake. Some studies indicate a potential link between excessive folic acid during pregnancy and increased risks of childhood obesity, insulin resistance, and certain neurological issues.

Comparison: Natural Folate vs. Synthetic Folic Acid Overconsumption


Feature	Excess Natural Folate	Excess Synthetic Folic Acid
Source	Foods like leafy greens and beans.	Fortified foods and supplements.
Body Processing	Easily metabolized.	Requires conversion; high doses can lead to UMFA buildup.
Risk of UMFA	Extremely low.	High with excessive intake.
Masking B12 Deficiency	Not a concern.	Significant risk, especially for older adults.
Side Effects	Not applicable.	Digestive issues, confusion, sleep problems, skin reactions.

How to Address High Folic Acid Levels

If blood tests reveal high folate levels, it's important to review all intake sources. Since folic acid is water-soluble, levels often normalize within weeks of reducing supplementation.

Review all sources: Check labels on supplements and fortified foods.
Choose natural foods: Prioritize folate-rich foods like spinach and lentils.
Consider methylated folate: A healthcare provider might recommend 5-MTHF, especially for individuals with genetic variations affecting metabolism.
Seek professional guidance: Consult a doctor to adjust intake safely, particularly with medical conditions or during pregnancy.

Conclusion

Consuming excessive synthetic folic acid can be harmful. High intake may mask a B12 deficiency, lead to UMFA buildup, and is potentially linked to increased cancer risk and developmental issues. Balancing intake, favoring natural sources, and consulting a healthcare professional are key to maintaining optimal folate status. For recommended daily allowances, refer to sources like the Office of Dietary Supplements at the National Institutes of Health.

Frequently Asked Questions

High folic acid is often indicated by a blood test showing a level greater than 20 ng/mL. Excessive intake typically means consuming over the 1,000 mcg Tolerable Upper Intake Level for adults from supplements and fortified foods.

High folic acid can correct the anemia associated with a B12 deficiency, masking this symptom while neurological damage from the B12 deficiency continues.

Toxic levels from food sources alone are extremely rare. The risk of overconsumption is primarily linked to synthetic folic acid from supplements and fortified foods.

UMFA is synthetic folic acid not fully converted to its active form, 5-MTHF. This buildup occurs when high intake overwhelms the conversion enzyme.

For some individuals, especially those with certain genetic variations, methylated folate (5-MTHF) is easier to process and doesn't carry the same risk of accumulating UMFA.

While vital for preventing neural tube defects, some studies link excessively high maternal levels to potential adverse outcomes in children. Pregnant women should consult their doctor for appropriate dosage.

Review all intake sources, reduce or stop supplementation under medical supervision, focus on natural folate from foods, and consider methylated folate if advised by your doctor.