Nutrition Diet: What is the problem if folic acid increases?

October 21, 2025 •

3 min read

While critical for preventing serious conditions like neural tube defects during pregnancy, excessive intake of synthetic folic acid, particularly from high-dose supplements, can lead to several health problems. It's a misconception that more of this vitamin is always better; understanding the risks is crucial for maintaining optimal health.

Quick Summary

Excessive intake of synthetic folic acid can mask an underlying vitamin B12 deficiency, allowing neurological damage to progress undetected. It is also linked to potential cognitive decline, certain cancer risks, and negative developmental outcomes for offspring, especially with prolonged high-dose supplementation.

Key Points

Masks Vitamin B12 Deficiency: High folic acid intake can correct anemia symptoms but conceal the progression of severe, irreversible nerve damage caused by a B12 deficiency.
Potential Cancer Link: Some research suggests excessive folic acid may promote the growth of existing cancerous or pre-cancerous cells, a risk that is still debated.
Cognitive and Neurological Effects: Elevated folic acid, particularly when B12 is low, has been linked to worsened cognitive impairment and neurological symptoms in older adults.
Unmetabolized Folic Acid (UMFA): The body’s capacity to convert synthetic folic acid is limited, and chronic high intake can cause UMFA to build up in the blood, with uncertain long-term effects.
Adverse Pregnancy Outcomes: Beyond the proven benefits, some studies indicate excessive maternal folic acid might be associated with adverse outcomes for the offspring, including neurodevelopmental changes.
Drug Interactions: High doses can interfere with medications like anti-epileptic drugs and methotrexate, potentially affecting their efficacy.
General Side Effects: Acute, less severe side effects can include digestive issues, sleep problems, confusion, and skin reactions.

The Double-Edged Sword of High Folic Acid

Folic acid, the synthetic form of vitamin B9, plays a vital role in preventing neural tube defects through food fortification. However, the widespread availability of fortified foods and high-dose supplements has raised concerns about excessive intake, as the body processes synthetic folic acid differently than natural folate. Potential health issues from high folic acid are being studied, with several risks noted, particularly for certain groups.

The Masking of a Vitamin B12 Deficiency

A significant issue with high folic acid intake is its ability to hide a vitamin B12 deficiency. Both nutrients are needed for healthy red blood cells, and a lack of either can cause megaloblastic anemia. High folic acid can improve the anemia symptoms, but it does not address the underlying B12 deficiency. This allows the more serious neurological problems from untreated B12 deficiency to worsen silently, potentially causing permanent nerve damage. This masking effect is especially concerning for older adults, who are more likely to have trouble absorbing B12.

Neurological and Cognitive Impact

High folic acid has also been linked to negative neurological and cognitive effects independently of B12 deficiency. Studies show that high folate levels, especially with low vitamin B12, are associated with poorer cognitive function and a higher risk of cognitive decline in older individuals. This might be due to unmetabolized folic acid (UMFA) building up when synthetic intake is high. The long-term effects of UMFA on brain function and immune responses are still being researched. High doses of folic acid have also been observed to worsen neurological symptoms like seizures in some epilepsy patients.

The Cancer Connection: A Complex Relationship

The link between folic acid and cancer is complex and debated. While folate is essential for DNA health and a deficiency can increase cancer risk, high levels of synthetic folic acid might promote the growth of existing pre-cancerous or cancerous cells. This idea suggests the timing and amount of folic acid matter greatly. Some research found increased rates of colorectal and prostate cancer after fortification, while others did not. Individuals with a history of cancer or high risk should discuss folic acid intake with a doctor.

Adverse Developmental Outcomes

Adequate folic acid is crucial during pregnancy to prevent neural tube defects. However, recent studies suggest very high maternal folic acid might harm offspring, potentially affecting neurodevelopment and metabolism. High intake during pregnancy has been linked to increased risks of insulin resistance and higher body fat in children. These findings emphasize the importance of following recommended intake levels and avoiding unnecessary high-dose supplements during pregnancy unless a doctor advises it.

Other Potential Side Effects

Besides these major concerns, high folic acid intake can cause more immediate, less severe side effects. These can include:

Nausea and stomach upset
Bloating or gas
A bitter or unpleasant taste in the mouth
Irritability and confusion
Sleep disturbances
Allergic skin reactions

Comparing High vs. Low Folate Risks

It's important to recognize that both too much and too little folic acid can be harmful. A deficiency can be as, or more, damaging than an excess.


Aspect	Risk of High Folic Acid Intake	Risk of Low Folate Intake (Deficiency)
Neurological Health	Masks B12 deficiency, leading to irreversible nerve damage; potential cognitive decline.	Fatigue, irritability, cognitive problems, depression; in infants, neural tube defects.
Cancer Risk	Controversial but some studies suggest potential promotion of existing cancers, especially with high doses.	Increased risk of certain cancers due to impaired DNA synthesis and repair.
Developmental Outcomes	Potential links to altered neurodevelopment, increased insulin resistance, and higher birth weight in offspring.	Serious birth defects such as spina bifida and anencephaly.
Immune Function	Accumulation of UMFA may alter immune responses and reduce natural killer cell activity.	Deficiency can impair immune response due to role in cell growth and replication.
Blood Parameters	Can correct megaloblastic anemia symptoms while B12 deficiency persists.	Causes megaloblastic anemia, leading to fatigue and weakness.

Conclusion: Finding the Right Balance

While folic acid fortification has successfully reduced neural tube defects, the potential issues with excess folic acid, particularly from high-dose supplements, should not be ignored. The complex relationship with vitamin B12, the potential link to cancer, and possible developmental effects highlight the need for careful supplementation. For most individuals, a diet rich in natural folate is sufficient. High-dose supplements should only be taken under medical supervision, especially for those at risk of B12 deficiency or with other health conditions. Blood tests can help determine if supplementation is needed.

The adverse effects of an excessive folic acid intake

Frequently Asked Questions

The most significant danger is that high doses of folic acid can correct the megaloblastic anemia associated with a vitamin B12 deficiency, but not the deficiency itself, allowing irreversible neurological damage to advance undetected.

The relationship is complex and debated. Some studies suggest a potential link between high folic acid, especially from supplements, and an increased risk of certain cancers, or acceleration of pre-existing lesions, while other evidence is inconclusive.

While folic acid is crucial for preventing neural tube defects, excessively high maternal levels have been linked in some studies to adverse developmental outcomes in offspring, including altered neurodevelopment and higher rates of insulin resistance.

Symptoms can include digestive upset, nausea, loss of appetite, bloating, sleep problems, irritability, confusion, and skin reactions. These are more common with high-dose supplementation.

It is extremely rare to consume a harmful level of folate from natural food sources alone. Most concerns about excessive intake stem from high-dose supplements or the combination of supplements with fortified foods.

UMFA is synthetic folic acid that the body cannot process into its active form when intake is high. This can accumulate in the bloodstream and has been an area of research for potential links to altered immune function and cognitive impairment.

For adults, the Tolerable Upper Intake Level (UL) is typically set at 1,000 mcg (1 mg) per day for synthetic folic acid from supplements and fortified foods.