Folic Acid Deficiency

Etiology

Folic acid deficiency can result from a range of biological, dietary, genetic, and pharmacological factors that impair folate intake, absorption, metabolism, or increase overall requirements.

• Increased physiological requirements
• Pregnancy and lactation: Rapid cell division and fetal development significantly elevate folate demands.
• Growth periods: Infants and adolescents require higher folate to support accelerated growth.
• Chronic hemolytic anemia: Continuous RBC turnover increases folate utilization.
• Exfoliative skin disorders: Conditions such as severe burns or dermatitis increase folate requirements due to heightened cellular regeneration.
• Hemodialysis: This leads to folate loss and increased metabolic demands, thereby necessitating higher intake.[8]

• Genetic factors
• Variants in the methylenetetrahydrofolate reductase (MTHFR) gene, such as C677T, can impair folate metabolism and contribute to folic acid deficiency.

• Malabsorption syndromes
• Gastrointestinal conditions such as celiac disease, inflammatory bowel disease, tropical sprue, and surgical resections (eg, short bowel syndrome) can impair folate absorption.
• Achlorhydria: Reduced gastric acid levels hinder folate absorption.
• Bariatric surgery: Procedures such as gastric bypass reduce folate uptake

• Increased folate catabolism or inhibition
• Medications: Certain drugs, such as methotrexate, phenytoin, sulfasalazine, and trimethoprim, interfere with folate utilization or increase breakdown.

• Dietary insufficiency
• Low intake: Insufficient consumption of folate-rich foods (eg, leafy greens, legumes, and fortified grains).
• Cooking loss: Folate is sensitive to heat and can be significantly reduced by prolonged cooking. 
• Limited dietary diversity: Cultural, socioeconomic, or restrictive dietary patterns may contribute to insufficient folate intake.

• Alcoholism
• Chronic alcohol consumption disrupts folate absorption, hepatic storage, and metabolism, making it a major contributor to deficiency.

• Vitamin B12 deficiency (folate trap)
• Vitamin B12 deficiency impairs methionine synthase activity, causing folate to become trapped as methyl-tetrahydrofolate (methyl-THF), the active form of folate. This reduces the availability of folate for DNA synthesis.

• Congenital enzyme deficiencies
• Rare inborn errors in folate metabolism, such as dihydrofolate (DHF) reductase deficiency, can lead to folate deficiency.

Epidemiology

Folic acid deficiency is now uncommon in the United States, largely due to the mandatory fortification of food. However, it remains prevalent in low- and middle-income countries, especially among women of reproductive age and populations without access to fortified foods.[9][10] In many low-income countries, the prevalence of folate deficiency among women of reproductive age exceeds 20%, while folate insufficiency—levels inadequate to prevent neural tube defects—affects over 40% of this population globally.[11] Socioeconomic status, education, dietary diversity, and access to supplements or fortified foods are key factors in determining the risk of folate deficiency. In contrast, high-income countries with established fortification programs typically report a prevalence below 5%.[11]

In the United States, data from the National Health and Nutrition Examination Survey (NHANES) indicate that women of childbearing age—particularly non-Hispanic Black and Hispanic women—are at increased risk of folic acid deficiency due to inadequate intake and limited supplementation. Among nonpregnant women aged 12 to 49, 22.8% had suboptimal RBC folate levels. Risk was significantly higher among women who did not use dietary supplements, relied solely on fortified grains for folate, or were current smokers.[12]

In Korea, data from 8380 women aged 15 to 49 revealed a mean serum folate level of 9.1 ng/mL. The prevalence of folate deficiency was 6.2% using a threshold of less than 3 ng/mL and 14.9% using a threshold of less than 4 ng/mL. The highest rates were observed in younger women aged 15 to 24, with over 30% falling below the 4 ng/mL threshold.[13]

In Australia, folic acid deficiency was more common in remote and indigenous populations. Between 2004 and 2008, the prevalence was 12.2%, which declined to 1.5% by 2010 to 2015 following mandatory food fortification. This represented an overall relative risk reduction (RRR) of 88%, including 91% in remote areas and 92% among indigenous populations. Urban and non-indigenous populations experienced reductions of 79% and 80%, respectively, during the same period.[14]

Mandatory folic acid fortification has significantly lowered deficiency rates worldwide. In countries with established programs (such as the United States, Canada, and Australia), prevalence can be as low as 1.7%, compared to rates as high as 23.8% in countries without fortification. While many European nations recommend periconceptional supplementation, fewer have adopted mandatory fortification policies.[15]

Populations at higher risk include individuals with lower socioeconomic status, older institutionalized adults, and those with cognitive impairments, often due to poor intake of folate-rich foods and limited dietary diversity. These groups may also face barriers to healthcare access, which can limit opportunities for early detection and intervention.

Pathophysiology

Folic acid plays a central role in DNA synthesis, repair, methylation, and erythropoiesis through its active form, THF. Folate, the naturally occurring form, is found in green leafy vegetables, citrus fruits, legumes, and some animal products. Folate is primarily absorbed in the jejunum via carrier-mediated transport systems, such as the proton-coupled folate transporter (PCFT) and the reduced folate carrier (RFC), with optimal uptake at a slightly acidic pH (5.5–6.0). Once absorbed, folate is enzymatically reduced to DHF and then to THF. THF is then further metabolized to 5,10-methylene THF and ultimately to L-5-methyl THF, the predominant form circulating in plasma.[16][17]

L-5-methyl THF must be demethylated within the cell to regenerate THF, a process that requires vitamin B12. In the setting of vitamin B12 deficiency, this reaction is impaired, resulting in a "folate trap," where folate becomes sequestered in an inactive form. This leads to functional folate deficiency despite adequate serum folate levels.

Folate deficiency can result from inadequate intake, impaired absorption, or increased metabolic demands, as seen in pregnancy, hemolytic anemia, and periods of rapid growth. Because the body’s folate stores are limited (approximately 0.5–20 mg) and the daily requirement is around 400 mcg, deficiency may develop within 8 to 16 weeks. Reduced folate levels result in a decreased availability of THF, which impairs purine and pyrimidine synthesis and nucleoprotein metabolism. This disruption results in ineffective erythropoiesis, megaloblastic anemia, and impaired cellular proliferation, particularly affecting rapidly dividing cells.

Recent evidence highlights key molecular mechanisms underlying the broad systemic effects of folate deficiency:

• Upregulation of folate transporters: Chronic deficiency triggers the compensatory upregulation of intestinal transporters, such as PCFT and RFC, to enhance folate uptake.[18][19]

• Disrupted DNA methylation: The impaired synthesis of methyl donors, such as S-adenosylmethionine (SAM), results in global DNA hypomethylation. This particularly affects genes involved in immunity and development, such as Brachyury, leading to epigenetic dysregulation.[20]

• Metabolic imbalance: Elevated plasma homocysteine levels and reduced levels of 1C metabolism intermediates (eg, nicotinamide adenine dinucleotide [NAD] and SAM) disrupt redox homeostasis and methylation reactions, thereby adversely affecting vascular and neurodevelopmental health.[21]

• Altered gene expression: Folate deficiency dysregulates the expression of genes involved in chromatin remodeling (eg, HDAC4 and HDAC6), folate transport, and embryogenesis (eg, Fgf8 and Brachyury), leading to impaired developmental processes and cellular repair pathways.[22]

• Increased apoptosis and reduced proliferation: Experimental models demonstrate that folate deficiency leads to increased apoptosis and impaired cellular proliferation, ultimately compromising tissue regeneration and organ function.[21]

In summary, folic acid deficiency is a complex biochemical disorder with systemic effects, especially during periods of increased demand such as fetal development, adolescence, and hematological stress. Its impact extends beyond anemia to include vascular dysfunction, disrupted gene regulation, and compromised cellular integrity.

History and Physical

A thorough history is essential for identifying the cause of folic acid deficiency. This helps differentiate between dietary insufficiency, malabsorption, increased physiological demand, and interference from medications or chronic alcohol use affecting folate metabolism. 

• Diet: Inadequate intake of folate-rich foods (eg, leafy greens, legumes, citrus fruits) is a common cause.
• Medications: Drugs such as methotrexate, phenytoin, trimethoprim, and sulfasalazine can impair folate absorption or metabolism.
• Alcohol use: Chronic alcohol consumption disrupts folate absorption and hepatic metabolism.
• Increased demand: Conditions such as pregnancy, lactation, hemolytic anemia, and exfoliative skin disorders increase folate requirements.
• Malabsorption: A history of gastrointestinal diseases (eg, celiac disease and tropical sprue) or bowel surgery may suggest impaired folate absorption.

As folate and vitamin B12 deficiencies both present with hematological features such as megaloblastic anemia, distinguishing between them is essential. Neurological symptoms, such as paresthesias, ataxia, and loss of proprioception, are typically seen in vitamin B12 deficiency but are generally absent in isolated folate deficiency.

Physical examination findings may include:

• Glossitis: A smooth, red, and painful ("beefy") tongue, often accompanied by oral ulcers.
• Pallor and fatigue: Indicative of underlying anemia.
• Neuropsychiatric symptoms: Although overt neurological deficits are uncommon, symptoms such as irritability, forgetfulness, and mood changes may occur, especially in older adults.
• Icterus: Mild scleral jaundice may be present due to hemolysis of fragile megaloblastic red cells in the bone marrow and circulation.

Inadequate folate during pregnancy has also been associated with an increased risk of diabetes-related congenital anomalies and autism spectrum disorders.[23]

Evaluation

Evaluation of folic acid deficiency involves a combination of clinical evaluation, dietary history, and laboratory testing. Given the overlapping hematologic features of folate and vitamin B12 deficiencies, both should be measured to ensure an accurate diagnosis. Laboratory workup typically includes a complete blood count (CBC), serum folate and vitamin B12 levels, and 1C metabolism markers such as homocysteine and methylmalonic acid (MMA).

Initial Workup

• Complete blood count: Findings may reveal macrocytic anemia, characterized by an elevated mean corpuscular volume (MCV) of more than 100 fL, decreased hemoglobin and hematocrit levels, normal to slightly elevated mean corpuscular hemoglobin (MCH), and typically normal mean corpuscular hemoglobin concentration (MCHC). Mild leukopenia or thrombocytopenia may also be present.

• Reticulocyte count: This is often low, indicating ineffective erythropoiesis

• Peripheral blood smear: This reveals macrocytic RBCs, hypersegmented neutrophils, and possible megaloblasts.

• Serum folate and vitamin B12: A serum folate level of less than 2 ng/mL indicates deficiency, 2 to 4 ng/mL is considered borderline, and more than 4 ng/mL is generally regarded as normal.

• Homocysteine and MMA:
  • Elevated homocysteine with normal MMA and normal vitamin B12 suggests folate deficiency.
  • Elevated levels of MMA and homocysteine, combined with low vitamin B12 levels, indicate a B12 deficiency.

• RBC folate: Reflects long-term folate levels in the body. Low values indicate a chronic deficiency.

• Composite folate deficiency scores: Scoring systems based on hematological parameters may support diagnosis, with demonstrated sensitivity and specificity exceeding 90%.

• Advanced diagnostic assays: Serum total homocysteine and RBC folate are highly sensitive markers, with sensitivities of 94.7% and 96%, respectively.

• Serum folate microbiological assays and liquid chromatography-tandem mass spectrometry: These techniques are used for high-precision folate measurement in clinical and research settings.

• Bone marrow biopsy: Although this procedure is not routinely required, it may show hypercellular marrow with characteristic megaloblastic changes if performed for other indications.[24][25][26][27]

Treatment / Management

Oral folic acid supplementation is the primary treatment for folic acid deficiency. The typical recommended dosage for most adults ranges from 1 to 5 mg daily, adjusted according to the severity of deficiency and underlying cause.

Parenteral administration may be considered for patients unable to tolerate oral therapy due to malabsorption syndromes, severe gastrointestinal conditions, or critical illness. Although current guidelines do not specify standardized protocols for this route, intramuscular or subcutaneous folic acid may be used, with dosing guided by clinical judgment and individual patient response. 

In cases of concurrent vitamin B12 deficiency, it is essential to initiate B12 repletion before or alongside folic acid supplementation. Treating folate deficiency alone will not correct the underlying issue and may exacerbate the neurological complications associated with B12 deficiency.

The duration of supplementation depends on the underlying cause. Patients with reversible conditions, such as dietary insufficiency, may only need short-term treatment, whereas those with chronic malabsorption, alcoholism, or long-term use of interfering medications may require lifelong supplementation.[28][29]

Pregnant women with folate deficiency are typically advised to take 500 μg to 5 mg of folic acid 3 times daily. Women of reproductive age should take at least 0.4 mg of folic acid daily, starting before conception and continuing throughout pregnancy. High-risk women, including those with a history of neural tube defect–affected pregnancies or those taking anticonvulsants, are recommended to receive 4 to 5 mg daily during this period.[30]

Adjunctive dietary counseling is essential and should emphasize increasing the intake of folate-rich foods, including leafy green vegetables, legumes, and citrus fruits. At the population level, grain fortification remains an effective public health strategy in countries where it has been implemented.

Differential Diagnosis

The differential diagnosis of folic acid deficiency includes several other causes of macrocytic anemia. Accurate diagnosis relies on clinical context and targeted laboratory testing. The differential includes:

• Vitamin B12 deficiency
• Alcoholic liver disease
• Hypothyroidism
• Myelodysplastic syndromes
• Aplastic anemia
• Drug-induced macrocytosis 
• Hemolytic anemias
• Copper deficiency

Prognosis

With appropriate treatment, the prognosis for folic acid deficiency is favorable, as most clinical and biochemical abnormalities are reversible. Folate supplementation typically raises serum folate levels within 17 days and can reduce the risk of megaloblastic anemia by up to 79%.

Hematological recovery from folic acid deficiency follows a predictable course. Markers of hemolysis normalize within 1 to 2 days, and reticulocytosis appears by days 3 to 4, indicating bone marrow response. Anemia begins to improve within 1 to 2 weeks and typically resolves within 4 to 8 weeks. Hypersegmented neutrophils usually disappear by days 10 to 14, while leukopenia and thrombocytopenia generally resolve within 2 to 4 weeks. Elevated homocysteine levels associated with folate deficiency also normalize with treatment, contributing to improved metabolic function. With early recognition and appropriate management, the long-term prognosis is excellent.

Low-dose folic acid supplementation may also improve blood lipid profiles by reducing total cholesterol and low-density lipoprotein cholesterol (LDL-C) levels while increasing apolipoprotein A-I (apoA-I) concentrations.[31] These effects may contribute to a modest reduction in cardiovascular risk, particularly in individuals with hyperhomocysteinemia or metabolic syndrome.

Complications

Untreated folic acid deficiency can lead to many complications affecting multiple organ systems. Hematological effects include megaloblastic anemia, pancytopenia, leukopenia, and thrombocytopenia. Mucocutaneous and gastrointestinal manifestations may present as glossitis, angular stomatitis, and oral ulcers. Neuropsychiatric symptoms may include depression, irritability, cognitive decline, insomnia, fatigue, and psychosis. In some cases, a folate-responsive neuropathy may occur, which has shown clinical improvement with high-dose folate therapy (10 mg 3 times daily).[32][33][34][35]

In pregnancy, folate deficiency is associated with a significantly increased risk of neural tube defects, preterm delivery (16.94%), fetal growth restriction (27.11%), spontaneous abortion, placental abruption, and severe language delays in offspring.[36] Additionally, impaired methylation due to folate deficiency leads to elevated homocysteine levels, which may contribute to vascular endothelial dysfunction and increased cardiovascular risk.

Excess folic acid intake, especially from supplements and fortified foods, can introduce new concerns, particularly in older individuals. Elevated folate levels may mask vitamin B12 deficiency, potentially worsening anemia and contributing to cognitive decline. Moreover, folate has a dual role in colorectal cancer—while some studies suggest it offers a protective effect, others indicate that high-dose supplementation may promote tumor progression in individuals with a predisposition.[37]