How Does B12 Deficiency Occur?

December 30, 2025 •

4 min read

According to the National Institutes of Health, B12 deficiency is more common in older adults and those with certain medical conditions. Understanding how does B12 deficiency occur requires examining a complex interplay of dietary intake, gastrointestinal health, and specific medical conditions.

Quick Summary

B12 deficiency arises from inadequate dietary intake, most commonly in vegans, or impaired absorption due to conditions like pernicious anemia, gastric surgery, or digestive diseases. Certain medications and chronic alcohol use also contribute to the deficiency.

Key Points

Malabsorption is key: Most B12 deficiencies are caused by impaired absorption in the digestive tract, not simply low dietary intake.
Pernicious anemia is a major cause: This autoimmune disease attacks the stomach cells needed to produce intrinsic factor, a protein essential for B12 absorption.
Vegan and vegetarian diets carry risk: Since B12 is primarily in animal products, those on strict plant-based diets must use supplements or fortified foods to avoid deficiency.
Medications can interfere: Long-term use of drugs like metformin and acid-reducing medications can significantly lower B12 levels over time.
Surgical procedures increase risk: Gastric bypass and other stomach or intestinal surgeries can drastically reduce the body's ability to absorb B12.
Early diagnosis is crucial: Detecting B12 deficiency early is vital to prevent irreversible neurological damage, as symptoms can progress over time.
Treatment varies by cause: Treatment ranges from oral supplements for dietary issues to lifelong injections for pernicious anemia or severe malabsorption.

Causes of B12 Deficiency: Beyond Diet

Vitamin B12, or cobalamin, is an essential nutrient for red blood cell formation, DNA synthesis, and neurological function. While inadequate dietary intake can cause a deficiency, the most common reason is actually malabsorption. The body's ability to absorb B12 from food relies on a complex multi-step process involving stomach acid and a protein called intrinsic factor. Any disruption to this process can lead to a deficiency, even if your diet contains plenty of B12.

Malabsorption Issues: The Primary Cause

Disruptions to the digestive tract are the most frequent culprits behind B12 deficiency. The normal absorption pathway involves B12 being released from food by stomach acid, binding to a protein (intrinsic factor) produced in the stomach, and then being absorbed in the small intestine (specifically, the ileum). If any part of this process fails, B12 levels can plummet.

Pernicious Anemia: This autoimmune condition is a leading cause, particularly in older adults. It involves the immune system attacking the parietal cells in the stomach that produce intrinsic factor, resulting in a lack of this crucial protein for absorption. Without intrinsic factor, dietary B12 cannot be absorbed.
Gastric Surgery: Procedures that reduce the size of the stomach or bypass sections of the small intestine, such as gastric bypass for weight loss, significantly impair B12 absorption. This is because the surgery may reduce or eliminate the cells that produce intrinsic factor.
Atrophic Gastritis: Inflammation and thinning of the stomach lining, often associated with a chronic Helicobacter pylori infection or autoimmune issues, can reduce the production of stomach acid and intrinsic factor. This condition can significantly hinder the body's ability to free B12 from food proteins.
Intestinal Disorders: Diseases that affect the small intestine, especially the ileum, can prevent proper B12 absorption. Examples include Crohn's disease, celiac disease, and severe infections.
Other Causes: Small intestinal bacterial overgrowth (SIBO) can cause the bacteria to consume the B12 before the body can absorb it. Chronic pancreatitis can also interfere with pancreatic enzymes needed for the absorption process.

Lifestyle and Medication Contributors

While malabsorption is the most common cause, other factors also play a significant role in causing a B12 deficiency.

Strict Vegan or Vegetarian Diet: As B12 is found almost exclusively in animal products, individuals who follow a strict vegan diet without adequate fortified foods or supplementation are at high risk. The body can store B12 for several years, so it may take time for a deficiency to manifest after adopting a plant-based diet.
Long-term Medication Use: Certain medications can interfere with B12 absorption. This includes metformin, commonly used to treat type 2 diabetes, as well as proton pump inhibitors and H2-receptor blockers used to reduce stomach acid.
Excessive Alcohol Consumption: Chronic alcohol use can damage the stomach lining and digestive system, leading to impaired B12 absorption.
Older Age: The body's ability to produce sufficient stomach acid and intrinsic factor can decrease with age, making older adults more susceptible to B12 deficiency.

Comparative Overview of B12 Deficiency Causes

This table provides a quick look at the major categories of B12 deficiency causes, highlighting their primary mechanisms and risk factors.


Cause Category	Primary Mechanism of Deficiency	Common Risk Factors
Malabsorption	Compromised intrinsic factor production or ileum absorption.	Pernicious anemia, gastric surgeries, atrophic gastritis, Crohn's disease.
Dietary	Lack of B12 intake from food sources.	Strict vegan or vegetarian diet without supplementation or fortified foods.
Medication-Induced	Interference with B12 absorption or metabolism.	Long-term use of metformin, proton pump inhibitors, H2 blockers.
Age-Related	Natural decline in stomach acid production.	Adults over 60, especially those with declining digestive function.
Other	Varied, including infections, genetic disorders, and toxins.	SIBO, tapeworm infection, nitrous oxide exposure.

How is B12 Deficiency Diagnosed?

Diagnosis typically begins with evaluating symptoms such as fatigue, neurological issues (numbness or tingling), and memory problems. A doctor will order blood tests to check your B12 and folate levels. A complete blood count (CBC) may also show macrocytic anemia, which features larger-than-normal red blood cells. In ambiguous cases, further enzymatic tests measuring methylmalonic acid (MMA) and homocysteine levels can provide more clarity, as these markers increase during a B12 deficiency. Further evaluation, such as anti-intrinsic factor antibody tests, can help confirm specific causes like pernicious anemia.

Conclusion: A Multi-faceted Problem

Understanding how a B12 deficiency occurs reveals it is a complex health issue stemming from a variety of factors beyond simple dietary intake. While vegans and vegetarians must be mindful of their intake, the majority of cases result from malabsorption issues affecting the digestive process. Conditions like pernicious anemia and atrophic gastritis, as well as factors like gastric surgery and certain medications, are primary drivers. Timely diagnosis through blood tests and effective management via supplementation or injections are crucial to prevent the progression of serious neurological symptoms and other long-term health complications. Consulting a healthcare provider for proper diagnosis and treatment is always the best course of action.

Frequently Asked Questions

The most common cause is impaired absorption due to underlying medical conditions, such as the autoimmune disorder pernicious anemia, atrophic gastritis, or gastrointestinal surgeries.

Yes, as B12 is naturally found in animal products, those on a strict vegan diet who do not consume fortified foods or take supplements are at risk of developing a B12 deficiency.

Intrinsic factor is a protein produced in the stomach that binds to B12, allowing it to be absorbed in the small intestine. Without intrinsic factor, such as in pernicious anemia, B12 cannot be absorbed.

Yes, certain medications like metformin for diabetes and proton pump inhibitors (PPIs) for acid reflux can interfere with B12 absorption with long-term use.

Since the body stores a significant amount of B12 in the liver, it can take several years for a deficiency to develop, especially if the cause is dietary.

Yes, surgery that removes part of the stomach, such as gastric bypass, can prevent the production of intrinsic factor, causing lifelong B12 malabsorption.

Diagnosis typically involves blood tests to check serum B12 and folate levels, along with a complete blood count (CBC). Further tests like methylmalonic acid (MMA) levels may be used for confirmation.