The Most Common Cause of B12 Deficiency Explained

October 17, 2025 •

4 min read

According to the MSD Manuals, the single most common cause of vitamin B12 deficiency is an issue with the body's ability to absorb the vitamin, rather than inadequate dietary intake. This impaired absorption is often due to underlying medical conditions, with pernicious anemia being a primary culprit in many cases.

Quick Summary

The most common cause of vitamin B12 deficiency is impaired absorption due to conditions like pernicious anemia or atrophic gastritis, not insufficient dietary intake. Pernicious anemia involves an autoimmune attack on the stomach, which hinders the production of intrinsic factor, a protein vital for B12 absorption. Other issues like surgery, digestive disorders, and certain medications can also interfere with proper absorption.

Key Points

Impaired Absorption is the Primary Cause: The most common reason for B12 deficiency is the body's inability to absorb the vitamin, not a lack of it in the diet.
Pernicious Anemia is a Leading Culprit: This autoimmune condition attacks the stomach's parietal cells, stopping the production of intrinsic factor, a protein vital for B12 absorption.
Other Malabsorption Issues Exist: Atrophic gastritis, gastrointestinal surgeries, and certain digestive disorders like Crohn's and celiac disease also disrupt B12 absorption.
Medications Play a Role: Long-term use of specific drugs, such as proton pump inhibitors and metformin, can interfere with B12 absorption.
Dietary Factors Impact Specific Groups: While not the overall most common cause, strict vegans are at high risk due to the lack of B12 in plant-based diets, requiring supplementation.

Understanding Vitamin B12 Absorption

For the body to properly utilize vitamin B12, a complex process must occur. First, vitamin B12 is released from the proteins in food within the acidic environment of the stomach. Then, a substance called intrinsic factor, secreted by the stomach's parietal cells, binds to the vitamin. This complex travels to the small intestine, where the B12 is absorbed into the bloodstream. A breakdown at any stage of this journey can lead to a deficiency, even if a person consumes plenty of B12-rich foods.

The Role of Intrinsic Factor

The intrinsic factor is crucial for this process. Without it, the body cannot absorb vitamin B12 efficiently, regardless of how much is consumed. This is the core mechanism behind pernicious anemia, the most common single cause of B12 deficiency due to malabsorption.

The Culprit: Pernicious Anemia

Pernicious anemia is an autoimmune disorder in which the immune system mistakenly attacks the parietal cells in the stomach that produce intrinsic factor. This autoimmune attack leads to a shortage or complete absence of intrinsic factor, causing severe B12 malabsorption. It is a leading cause of B12 deficiency, especially in older adults. The deficiency develops slowly over years as the body's liver stores of B12 are depleted.

Other Common Causes of Malabsorption

While pernicious anemia is prominent, other conditions can also impair B12 absorption. Many of these are linked to gastrointestinal health.

Atrophic Gastritis: This condition, which can sometimes lead to pernicious anemia, involves the chronic inflammation and gradual destruction of the stomach lining. This reduces stomach acid and intrinsic factor production, hindering B12 separation from food proteins.
Gastrointestinal Surgery: Procedures that remove parts of the stomach (e.g., gastrectomy, gastric bypass) or small intestine (especially the terminal ileum) can severely limit the areas where intrinsic factor is produced and B12 is absorbed.
Chronic Alcohol Use: Excessive alcohol consumption can damage the stomach lining and digestive tract, affecting B12 absorption.
Digestive Disorders: Conditions like Crohn's disease, celiac disease, and severe inflammatory bowel disease can damage the small intestine, impairing B12 uptake.
Medications: Certain drugs, including long-term use of proton pump inhibitors, H2 blockers (antacids), and metformin (for diabetes), can reduce stomach acid and interfere with absorption.

Inadequate Dietary Intake and Other Factors

While absorption issues are the most common cause overall, insufficient dietary intake can lead to B12 deficiency, particularly in specific populations.

Vegan Diet: Since vitamin B12 is found almost exclusively in animal products, strict vegans who do not consume fortified foods or supplements are at high risk of deficiency. The body's large stores can mask a deficiency for a few years, but it will eventually develop.
Older Age: As people get older, their stomach acid levels tend to decrease, making it harder to absorb B12 from food. Though B12 from supplements is often absorbed more effectively.

Comparison of Major B12 Deficiency Causes


Cause	Underlying Mechanism	Primary Effect on B12	Common Risk Groups
Pernicious Anemia	Autoimmune attack on stomach parietal cells, halting intrinsic factor production.	Severe malabsorption, inability to absorb B12 from food.	Older adults, individuals with other autoimmune disorders.
Atrophic Gastritis	Inflammation and thinning of stomach lining, reducing stomach acid and intrinsic factor.	Impaired absorption, difficult to release B12 from food.	Elderly individuals.
Gastric Surgery	Removal of intrinsic factor-producing stomach tissue or terminal ileum.	Malabsorption due to bypassing key absorption sites.	Patients with history of bariatric or stomach surgery.
Strict Vegan Diet	Lack of dietary B12 sources (animal products).	Insufficient intake.	Vegans without proper supplementation.
Medications	Drugs that reduce stomach acid (PPIs, H2 blockers) or interfere with B12 metabolism (metformin).	Impaired absorption due to altered stomach environment.	Individuals on specific long-term medications.

Symptoms and Diagnosis

The symptoms of B12 deficiency can be wide-ranging and often develop gradually, making diagnosis challenging. Common signs include fatigue, weakness, a sore tongue, and neurologic symptoms like numbness or tingling in the extremities. Diagnosis is based on blood tests to check B12 levels. It is important to address the underlying cause to properly manage the condition. For many, this means lifelong B12 supplementation, either orally or via injections.

Conclusion: Malabsorption is Key

In conclusion, while a poor diet is a potential cause, it is malabsorption, particularly from conditions like pernicious anemia and gastritis, that is the most common cause of B12 deficiency. These issues prevent the body from properly processing and absorbing the vitamin from food. For many, especially older adults, addressing this absorption problem is the key to preventing or treating a deficiency. Diagnosis is critical to determine the root cause, as treatment often requires supplementation to bypass the compromised absorption mechanism. Prompt diagnosis and treatment are important to prevent permanent neurological damage.

Frequently Asked Questions

Intrinsic factor is a protein produced by the stomach that binds to vitamin B12, allowing it to be absorbed later in the small intestine. Without sufficient intrinsic factor, the body cannot absorb B12 effectively.

Yes. The most common causes of B12 deficiency relate to malabsorption, which can affect meat-eaters, vegetarians, and vegans alike. Conditions like pernicious anemia prevent the absorption of B12 from food, regardless of how much is consumed.

Early signs can include fatigue, weakness, pale skin, a sore or smooth tongue, and gastrointestinal issues like nausea or diarrhea. Neurological symptoms like numbness or tingling may also appear over time.

Yes, vitamin B12 deficiency is more common in older adults. This is often because stomach acid levels decrease with age, which hinders the release of B12 from food.

Pernicious anemia is typically diagnosed through blood tests that measure B12 levels, as well as checking for antibodies against intrinsic factor and parietal cells. Imaging techniques like endoscopy may also be used.

Treatment depends on the cause. For malabsorption issues like pernicious anemia, treatment often involves lifelong vitamin B12 supplementation, which may be administered through injections or high-dose oral tablets.

No, it is not the most common cause overall. However, it is a significant risk factor within the vegan population because B12 is found almost exclusively in animal products.