Megaloblastic Anemia, Vitamin B12 and Folate Deficiency
What are vitamin B12 and folate deficiencies?

Vitamin B12 and folate (vitamin B9) are essential for DNA synthesis, cell division, and normal blood cell production. A deficiency occurs when the body does not get, absorb, or use enough of either vitamin.

Vitamin B12 occurs naturally mainly in meat, fish, eggs, milk, and other animal products. Plant-based foods contain it only when fortified. An adult needs approximately 4 micrograms a day. The body maintains substantial stores that usually last for several years, so deficiency often develops gradually.

Folate is found mainly in leafy green vegetables, legumes, fruit, and fortified foods. An adult needs approximately 400 micrograms of dietary folate equivalents a day. Folate stores are smaller and can be depleted within a few months, so deficiency may develop more quickly.

📌 Helpful fact: The body stores these two vitamins very differently. Folate stores are limited and usually last only a few months, approximately 2–4 months. In contrast, vitamin B12 stored in the liver can usually meet the body’s needs for about 3–5 years. These timeframes vary according to a person’s initial stores and the cause of the deficiency.

What are anemia due to vitamin B12 or folate deficiency and megaloblastic anemia?

Anemia due to vitamin B12 or folate deficiency develops when a lack of either vitamin interferes with normal DNA synthesis and the maturation of red blood cells in the bone marrow. Red blood cell production becomes ineffective, the cells entering circulation are usually larger, and the hemoglobin level falls.

Megaloblastic anemia is the characteristic form of anemia caused by impaired DNA synthesis, often due to vitamin B12 or folate deficiency. It is characterized by large, immature precursor cells in the bone marrow called megaloblasts. Red blood cells in circulation are also usually larger, with an increased mean corpuscular volume (MCV).

In other words, vitamin B12 or folate deficiency is the cause; anemia is a consequence; and megaloblastic anemia is the characteristic form resulting from impaired DNA synthesis and ineffective red blood cell production.

Vitamin B12 deficiency should not be ruled out simply because a person has no anemia or enlarged red blood cells. Neurological signs may appear before changes become visible on a complete blood count.

How do vitamin B12 and folate deficiencies develop?

A deficiency develops when the amount of a vitamin available to the body falls below what it needs. Stores gradually decline until they can no longer meet the needs of the cells.

Vitamin B12 deficiency develops mainly because of inadequate dietary intake, poor absorption, or problems with the vitamin’s transport, activation, or use by cells. Initially, the body draws on vitamin B12 stored primarily in the liver. Because these stores are substantial, depletion usually takes several years. Once they are insufficient, vitamin B12 deficiency develops at the cellular level.

Folate deficiency develops mainly because of inadequate dietary intake, poor absorption, increased needs, greater losses, or interference with its metabolism, transport, or use. Folate stores are limited. When intake does not meet the body’s needs, they can be depleted within a few months. Folate deficiency therefore usually develops more quickly than vitamin B12 deficiency.

How does vitamin B12 or folate deficiency lead to megaloblastic anemia?

The condition does not begin as an ordinary anemia and later turn into megaloblastic anemia. Vitamin B12 or folate deficiency directly impairs DNA synthesis and normal red blood cell maturation in the bone marrow. Megaloblasts form, and many are destroyed before entering circulation. As a result, fewer functional red blood cells are produced, hemoglobin falls, and megaloblastic anemia develops. In severe cases, white blood cell and platelet production may also decline, causing pancytopenia. Vitamin B12 deficiency can also damage the nervous system, even without anemia.

Main causes

The causes of deficiency can be grouped according to how they reduce the amount of vitamin available to the body.

Vitamin B12 deficiency

Vitamin B12 deficiency is most commonly caused by:

Inadequate dietary intake – a vegan diet without supplements or fortified foods.

Poor absorption, which can occur through several mechanisms:

Impaired release from food – atrophic gastritis and long-term use of medications that reduce stomach acid can interfere with the release of vitamin B12 from food proteins.
Lack of intrinsic factor – autoimmune gastritis damages the stomach’s parietal cells and reduces intrinsic factor production. This can also occur after gastrectomy or certain bariatric procedures. Vitamin B12 deficiency caused by autoimmune loss of intrinsic factor is associated with pernicious anemia.
Impaired absorption in the terminal ileum – Crohn’s disease, damage to the terminal ileum, or its surgical removal can prevent absorption of the B12–intrinsic factor complex. Long-term metformin use can also reduce B12 absorption.
Impaired processing in the intestine – pancreatic insufficiency can interfere with the preparation of vitamin B12 for binding to intrinsic factor.
Small intestinal conditions – certain bariatric procedures and, less commonly, celiac disease can cause poor absorption.
Consumption before absorption – bacterial overgrowth in the small intestine and certain intestinal parasites can consume vitamin B12 before the body absorbs it.

Impaired transport or metabolism – certain rare inherited disorders can prevent cells from transporting, activating, or using vitamin B12.

Folate deficiency

Folate deficiency can be caused by:

Inadequate dietary intake – a diet low in leafy green vegetables, legumes, and fortified foods.

Impaired absorption in the digestive tract, including:

Damage to the lining of the small intestine – celiac disease and other small intestinal conditions.
Changes after surgery – certain bariatric procedures can reduce the intestine’s surface area or ability to absorb nutrients.
Effects of alcohol – long-term excessive alcohol use can impair folate absorption in the intestine.
Increased needs – pregnancy, breastfeeding, periods of rapid growth, and chronic hemolysis.
Interference with folate metabolism, transport, or use – methotrexate, trimethoprim, pyrimethamine, and certain anti-seizure medications can interfere with normal folate activation or use. Long-term alcohol use can affect folate metabolism and storage. Less commonly, inherited disorders can impair the transport, activation, or use of folate by cells.
Increased losses – hemodialysis and certain chronic conditions can increase folate loss and reduce its stores.

Who is at risk?

Risk factors are conditions, habits, or treatments that increase the likelihood of a deficiency. Having a risk factor does not necessarily mean a deficiency is present.

The main risk factors for vitamin B12 deficiency are:

Older age;
A vegan diet without supplementation;
Highly restrictive diets or malnutrition;
Atrophic or autoimmune gastritis;
Chronic stomach or intestinal conditions;
Gastrectomy, bariatric surgery, or removal of the terminal ileum;
Long-term use of metformin or medications that reduce stomach acid.

The main risk factors for folate deficiency are:

Inadequate nutrition or highly restrictive diets;
Long-term excessive alcohol use;
Pregnancy and breastfeeding;
Periods of rapid growth;
Conditions that cause poor absorption;
Bariatric surgery;
Chronic hemolysis or conditions with accelerated cell turnover;
Hemodialysis;
Use of medications that interfere with folate metabolism.

Assessing these factors helps identify people who may need laboratory tests, monitoring, or preventive supplementation.

Types of deficiency and anemia

Deficiency and anemia can be classified by the vitamin involved and by how the condition presents.

By the vitamin involved

Vitamin B12 deficiency.
Folate deficiency.
Combined deficiency, when vitamin B12 and folate deficiencies are present at the same time.

By how the condition presents

Deficiency without anemia – tests show a vitamin deficiency, but hemoglobin may still be normal. Neurological signs can occur with vitamin B12 deficiency.
Deficiency with megaloblastic anemia – impaired DNA synthesis causes ineffective red blood cell production and a fall in hemoglobin.
Advanced disease – white blood cells and platelets may also fall, causing pancytopenia.

Forms of anemia

Megaloblastic anemia due to vitamin B12 deficiency – may be accompanied by neurological damage.
Megaloblastic anemia due to folate deficiency – causes similar blood changes but usually lacks the neurological damage characteristic of B12 deficiency.
Pernicious anemia – a specific form of vitamin B12 deficiency associated with autoimmune gastritis and a lack of intrinsic factor.

Symptoms

Deficiency without anemia

In the early stages, a deficiency may cause no symptoms and may only be found through laboratory tests. Sometimes, symptoms include:

Mild fatigue or weakness;
Difficulty concentrating;
A red, smooth, or painful tongue;
Cracks at the corners of the mouth;
Loss of appetite or digestive problems, mainly diarrhea.

Vitamin B12 deficiency may also cause neurological symptoms:

Numbness or tingling in the hands and feet;
Reduced sensation;
Muscle weakness;
Difficulty walking or loss of balance;
Forgetfulness, difficulty concentrating, or mood changes.

Deficiency with anemia

When red blood cell production becomes ineffective and hemoglobin falls, symptoms of the deficiency become more noticeable and are joined by symptoms of anemia:

Marked fatigue and weakness;
Pale skin;
Dizziness or headaches;
Shortness of breath, particularly during physical activity;
A rapid or pounding heartbeat;
Reduced ability to tolerate physical activity.

Paleness and mild yellowing of the skin or eyes may also occur because abnormal cells are destroyed within the bone marrow. Other possible symptoms include a smooth, painful tongue, loss of appetite, weight loss, or diarrhea.

Advanced disease

With severe or prolonged deficiency, white blood cells and platelets may also decline. This can cause:

Marked fatigue, shortness of breath during exertion, and a rapid heartbeat as hemoglobin falls further;
More frequent infections due to a low white blood cell count;
Easy bruising, bleeding from the nose or gums, and small red spots on the skin due to a low platelet count.

With prolonged vitamin B12 deficiency, neurological damage may progress, causing muscle weakness, difficulty walking, loss of balance, and changes in memory or mood.

Neurological symptoms are characteristic of vitamin B12 deficiency and can appear at any stage, even without anemia. They usually do not occur with isolated folate deficiency.

Warning signs and urgent care

Some signs require prompt medical assessment because they may indicate worsening neurological damage, severe anemia, or pancytopenia:

Signs of neurological damage – worsening numbness or weakness; difficulty walking; marked loss of balance or coordination; confusion, disorientation, or sudden changes in behavior.
Signs of severe anemia – shortness of breath at rest or after minimal exertion; chest pain; a very rapid or irregular heartbeat; dizziness or fainting.
Signs of pancytopenia – fever or frequent infections; bleeding into the skin or mucous membranes; unusual bleeding or bleeding that does not stop.

Seek immediate medical attention for chest pain, severe difficulty breathing, fainting, sudden confusion, inability to walk, or bleeding that does not stop. Neurological damage due to vitamin B12 deficiency can become irreversible if treatment is delayed.

How is it diagnosed?

Diagnosis involves a clinical assessment and laboratory tests.

Clinical assessment

The doctor reviews symptoms, diet, medications, other medical conditions, and any previous stomach or intestinal surgery. If vitamin B12 deficiency is suspected, the assessment also includes neurological function.

Laboratory tests

Complete blood count (CBC) – MCV is usually increased.
Reticulocyte count – helps assess red blood cell production by the bone marrow.
Microscopic examination of peripheral blood – may show large oval cells and neutrophils with nuclei divided into many segments.
Measurement of vitamin B12 and folate levels in the blood – follow the laboratory’s preparation instructions before testing.

Additional and confirmatory tests may include:

Methylmalonic acid and homocysteine when a vitamin B12 result is borderline or does not match the clinical findings.
LDH, indirect bilirubin, and haptoglobin to help assess the destruction of red blood cell precursors within the bone marrow.
Iron studies to identify a simultaneous iron deficiency, which can keep MCV within the normal range.

Depending on the circumstances, a gastroenterology assessment, including blood tests or gastroscopy, may be needed to find a cause in the digestive tract.

A bone marrow examination (bone marrow aspiration) is not routinely needed. It is reserved for unclear cases, unexplained or atypical pancytopenia, a lack of response to treatment, or suspicion of another blood disorder.

How is it treated?

Treatment aims to replace the missing vitamin, address the cause, and prevent complications.

Treatment of vitamin B12 deficiency

Vitamin B12 can be given by mouth or by intramuscular injection. The choice depends on the cause, severity of the deficiency, neurological signs, ability to absorb the vitamin, and ability to follow the treatment plan.

Intramuscular treatment is preferred when poor absorption is suspected, when neurological symptoms or anemia are severe, or when oral treatment is unsuitable.
Oral treatment can be used for deficiency caused by diet and in selected other cases.

Lifelong intramuscular replacement is usually recommended when the cause is irreversible, such as autoimmune gastritis, total gastrectomy, or complete removal of the terminal ileum. Reversible causes are reassessed once they have been addressed.

Treatment of folate deficiency

Folate deficiency is treated with oral folic acid and correction of the underlying cause. The duration depends on the person’s condition, the cause, and whether increased needs continue.

Vitamin B12 deficiency should be ruled out or treated before folate is used as the sole treatment. Folate may correct the anemia but does not prevent neurological damage from vitamin B12 deficiency and may make the diagnosis harder to recognize.

Follow-up after treatment

The response is assessed through symptom improvement and blood tests. The complete blood count usually improves gradually. Neurological symptoms may take longer to improve and may not resolve fully if treatment was delayed.

Prevention and practical advice

Vitamin B12 and folate deficiencies can sometimes be prevented or detected early through several measures:

A balanced diet – animal products or fortified foods provide vitamin B12; leafy green vegetables, legumes, and fruit provide folate.
Vitamin B12 supplementation according to risk – particularly for people following a vegan diet and after certain gastrointestinal procedures, as advised by a doctor.
Folic acid before and during early pregnancy – the World Health Organization recommends 400 micrograms of folic acid daily from the start of attempts to conceive until the 12th week of pregnancy. Higher doses are used only for specific medical indications.
Limiting alcohol – long-term excessive use can reduce folate intake, absorption, and use.
Monitoring people at risk – periodic assessment may be recommended after gastrointestinal surgery, with conditions that cause poor absorption, and during long-term use of metformin or medications that reduce stomach acid.
Treating the cause – digestive conditions and other causes of poor absorption should be diagnosed and treated.
When the cause of vitamin B12 deficiency is permanent, such as autoimmune gastritis or certain surgical procedures, preventing its return requires replacement treatment and long-term follow-up.

When should you consult a doctor?

When symptoms appear
Consult a doctor if symptoms persist, worsen, or return. Do not delay assessment if neurological symptoms appear: vitamin B12 deficiency can be present even without anemia.

When risk factors are present
Medical assessment may be appropriate even without symptoms, particularly for people with the risk factors described above and before or during pregnancy.

When test results are abnormal
Consultation is recommended if a complete blood count shows anemia, enlarged red blood cells, or a reduction in other blood cells, or if vitamin B12 or folate results are low or borderline.

During or after treatment
Contact your doctor if symptoms do not improve, worsen, or return, or if test results remain abnormal despite treatment.

Why does early diagnosis matter?

Vitamin B12 or folate deficiency can be detected before anemia develops. Early diagnosis helps identify the cause and start appropriate treatment. It is particularly important with vitamin B12 deficiency because neurological damage can appear even without anemia and may not be fully reversible if treatment is delayed.

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📌 Reviewed by Dr. Irma Lapaj, Hematologist, Klinika Kajo.

Megaloblastic Anemia, Vitamin B12 and Folate Deficiency