Understanding the Nutritional Impact: What type of anemia is caused by a vitamin B12 deficiency?

October 13, 2025 •

3 min read

According to the MSD Manuals, B12 deficiency is characterized by abnormally large red blood cells, which directly answers the query: What type of anemia is caused by a vitamin B12 deficiency? This deficiency can lead to a condition called megaloblastic anemia and, in specific cases, pernicious anemia.

Quick Summary

A deficiency in vitamin B12 results in megaloblastic anemia, a form of macrocytic anemia where red blood cells are abnormally large. This is often caused by pernicious anemia, an autoimmune disease affecting nutrient absorption, and can lead to serious health complications if untreated.

Key Points

Megaloblastic Anemia: Vitamin B12 deficiency causes megaloblastic anemia, a condition where the body produces abnormally large, immature red blood cells.
Pernicious Anemia: A common and specific cause of B12 deficiency leading to megaloblastic anemia is pernicious anemia, an autoimmune disease.
Intrinsic Factor Deficiency: Pernicious anemia occurs when the immune system prevents the production or function of intrinsic factor, a protein needed for B12 absorption.
Neurological Symptoms: B12 deficiency is unique among nutrient-based anemias for its potential to cause severe and sometimes irreversible neurological damage.
Beyond Diet: While dietary inadequacy is a cause, especially for vegans, many cases stem from malabsorption issues due to conditions, surgeries, or age.
Lifelong Treatment: Individuals with pernicious anemia typically require lifelong vitamin B12 injections, as oral supplements are ineffective due to the absorption issue.

The Primary Form: Megaloblastic Anemia

A vitamin B12 deficiency impairs DNA synthesis during red blood cell production, leading to large, immature red blood cells called megaloblasts and the condition known as megaloblastic anemia. These cells don't function well and die early, reducing healthy red blood cell count. Megaloblastic anemia is a type of macrocytic anemia, which is characterized by large red blood cells.

The Autoimmune Root: Pernicious Anemia

Pernicious anemia is a specific type of vitamin B12 deficiency anemia caused by an autoimmune disorder. The immune system attacks stomach cells producing intrinsic factor, a protein vital for B12 absorption. Without intrinsic factor, B12 cannot be absorbed, leading to megaloblastic anemia.

Common Symptoms of B12 Deficiency

Symptoms develop gradually as the body uses stored B12. Even with severe anemia, symptoms can be mild due to the body's adaptation. Common signs include:

Fatigue and Weakness: Due to insufficient oxygen transport.
Pale Skin: A yellowish tint may be noticeable.
Shortness of Breath and Dizziness: Especially during activity.

Neurological Problems

Vitamin B12 deficiency can also cause neurological issues like numbness or tingling in extremities, memory issues, confusion, and balance problems.

Other Symptoms

Additional symptoms can include a sore, red tongue and gastrointestinal problems like diarrhea or loss of appetite.

Causes of Vitamin B12 Deficiency

While poor diet is a factor, particularly for vegans, malabsorption is a more common cause. Reasons for malabsorption include:

Pernicious Anemia: Autoimmune issue preventing intrinsic factor production.
Gastrointestinal Surgery: Procedures affecting the stomach or small intestine needed for absorption.
Digestive Disorders: Conditions like Crohn's disease or celiac disease can interfere with absorption.
Medications: Some drugs, like metformin and acid reducers, can lower B12 levels with long-term use.
Aging: Reduced stomach acid can make it harder to absorb B12 from food.

Diagnosis and Treatment

Diagnosis involves a blood test for B12 levels and a complete blood count (CBC) to check for large red blood cells. Elevated levels of methylmalonic acid (MMA) and homocysteine can confirm the diagnosis.

Treatment varies. Oral supplements may help with dietary deficiencies. However, malabsorption issues like pernicious anemia typically require lifelong B12 injections. Early treatment is crucial to reverse anemia and prevent potentially irreversible neurological damage.

Comparison: Megaloblastic Anemia vs. Pernicious Anemia


Feature	Megaloblastic Anemia	Pernicious Anemia
Classification	A type of macrocytic anemia.	A specific cause of megaloblastic anemia.
Underlying Cause	Impaired DNA synthesis, most commonly due to B12 or folate deficiency.	An autoimmune condition preventing B12 absorption due to a lack of intrinsic factor.
Blood Smear	Shows large, immature red blood cells (megaloblasts) and hypersegmented neutrophils.	Also shows megaloblasts and hypersegmented neutrophils, as it is a type of megaloblastic anemia.
Absorption Issue	Caused by insufficient B12 or folate for any reason.	Specifically caused by the absence of intrinsic factor.
Commonality	More common, caused by various deficiencies or conditions.	Less common, a specific autoimmune disease.
Diagnosis	Blood tests show low B12 or folate levels and elevated MMA/homocysteine.	Includes tests for anti-intrinsic factor antibodies.
Treatment	Addressing the vitamin deficiency, often with supplements.	Typically requires lifelong B12 injections due to chronic malabsorption.

Conclusion

A vitamin B12 deficiency leads to megaloblastic anemia, characterized by large red blood cells. Pernicious anemia is a common cause of this deficiency, being an autoimmune disease affecting B12 absorption. Recognizing symptoms, including neurological issues, is important for early diagnosis and treatment. Management often involves supplements or lifelong injections. Consulting a healthcare provider for diagnosis and treatment is recommended if a B12 deficiency is suspected.

A Note on B12 and Nutrition

Vitamin B12 is mainly found in animal products. Vegans should use fortified foods or supplements to meet their B12 needs. Those with malabsorption will need medical treatment regardless of diet.

Source for B12 Information

Frequently Asked Questions

Yes, it is possible to have a vitamin B12 deficiency without having anemia, especially in the early stages. Neurological symptoms, such as tingling in the extremities, can appear before blood cell counts are significantly affected.

Macrocytic anemia is a broad term for any anemia characterized by abnormally large red blood cells. Megaloblastic anemia is a specific type of macrocytic anemia caused by a deficiency in vitamin B12 or folate, which impairs DNA synthesis and red blood cell maturation.

Diagnosis typically involves a blood test to measure vitamin B12 levels and a complete blood count (CBC) to check for large red blood cells. Elevated levels of methylmalonic acid (MMA) and homocysteine also help confirm the diagnosis.

Treatment depends on the cause. Dietary changes and oral supplements may be sufficient for mild deficiencies. However, those with malabsorption, such as in pernicious anemia, will need regular vitamin B12 injections.

No, as an autoimmune disease, pernicious anemia cannot be prevented. However, its complications can be managed and mitigated through early detection and consistent treatment with B12 supplementation.

Vitamin B12 is primarily found in animal products. Good dietary sources include meat, fish, shellfish, eggs, and dairy products. Fortified cereals and plant-based milks also contain added B12.

Because the body can store large amounts of B12 in the liver, a deficiency can take several years to develop after a change in diet or absorption begins. Symptoms can be mild at first, making the condition easy to overlook.

Prompt treatment with B12 can resolve the anemia and prevent further neurological damage. However, if severe neurological symptoms have persisted for a long time, some damage may be permanent.