What Causes B12 Deficiency? Understanding the Root Causes

October 4, 2025 •

2 min read

According to the National Institutes of Health, up to 20% of adults over 60 in the US and UK are deficient in vitamin B12. This common condition, known as cobalamin deficiency, arises from various factors, including inadequate intake and malabsorption problems. Understanding what causes B12 deficiency is the first step toward effective management and prevention.

Quick Summary

Several factors, including restrictive diets, malabsorption disorders like pernicious anemia, and certain medications, can lead to a B12 deficiency. Age, stomach surgeries, and digestive conditions also play a significant role. Recognition of these underlying causes is key to proper diagnosis and treatment.

Key Points

Inadequate intake: Vegans and strict vegetarians are most at risk due to B12's natural presence only in animal products, requiring supplementation.
Malabsorption: Autoimmune conditions like pernicious anemia, which prevents intrinsic factor production, are major causes of poor absorption.
Age-related factors: Older adults often have reduced stomach acid, hindering the release and absorption of B12 from food.
Surgical alterations: Bariatric surgery or gastrectomy can bypass the sites where B12 is absorbed, necessitating lifelong supplementation.
Medication side effects: Long-term use of drugs like metformin and proton pump inhibitors can interfere with B12 absorption.
Digestive disorders: Conditions like Crohn's and Celiac disease can damage the intestinal lining, impairing the ability to absorb B12.
Early detection is key: Treating a deficiency early is critical to prevent permanent nerve damage and other serious complications.

The Importance of Vitamin B12

Vitamin B12, or cobalamin, is an essential water-soluble vitamin vital for red blood cell formation, DNA synthesis, and a healthy nervous system. A deficiency can lead to fatigue, nerve damage, and cognitive issues. Since the body doesn't produce B12, it must come from the diet, mainly animal products, or supplements.

Leading Causes of B12 Deficiency

Several factors contribute to B12 deficiency, primarily related to insufficient intake or malabsorption.

1. Inadequate Dietary Intake

While many get enough B12 from food, certain diets increase risk. Vegans and strict vegetarians are particularly vulnerable as B12 is mainly in animal products. Fortified foods or supplements are essential for these groups. Infants breastfed by vegan mothers with low B12 levels are also at risk.

2. Malabsorption Problems

Issues with absorbing B12 are the most common cause of deficiency.

Pernicious Anemia: An autoimmune disease where the body attacks cells producing intrinsic factor, a protein needed for B12 absorption in the small intestine.
Atrophic Gastritis: A thinning stomach lining, often in older adults, reduces stomach acid needed to release B12 from food.
Gastrointestinal Disorders: Conditions affecting the small intestine's ileum, where B12 is absorbed, like Crohn's disease and Celiac disease, can impair absorption.
Intestinal Issues: Parasites or bacterial overgrowth (SIBO) in the small intestine can consume B12, reducing availability for the body.

3. Surgical Procedures

Digestive surgeries, especially for weight loss, significantly impact B12 absorption. Procedures like gastric bypass or gastrectomy reduce intrinsic factor production and bypass absorption sites, often requiring lifelong supplementation.

4. Medications that Affect Absorption

Some long-term medications interfere with B12 absorption. Metformin, for diabetes, and acid-reducing drugs like proton pump inhibitors (PPIs) and H2 blockers can lower B12 levels by affecting absorption pathways or reducing stomach acid necessary for B12 release from food.

Comparison of B12 Deficiency Causes


Cause Category	Primary Mechanism	At-Risk Groups	Key Treatment Approach
Dietary	Lack of B12-rich food sources.	Vegans, strict vegetarians, infants of vegan mothers.	Fortified foods or supplements.
Autoimmune	Immune system destroys intrinsic factor.	Individuals with pernicious anemia, often over 50.	Lifelong B12 injections.
Age-Related	Reduced stomach acid and intrinsic factor production.	Older adults (typically over 60).	Fortified foods or supplements, sometimes injections.
Surgical	Alteration or removal of stomach/small intestine.	Post-bariatric surgery patients, gastrectomy patients.	Lifelong injections or high-dose oral supplementation.
Medical Conditions	Digestive tract damage or impaired function.	Crohn's disease, celiac disease, SIBO patients.	Treat underlying condition, use supplements or injections.
Medication-Induced	Interference with absorption pathways.	Long-term metformin or PPI users.	Monitoring, supplementation, or alternative medication.

Conclusion

Vitamin B12 deficiency stems from various causes, not just diet. Malabsorption, often due to pernicious anemia and age, is a primary factor. Medications and digestive disorders also contribute significantly. Management usually involves dietary changes, supplements, or injections, depending on the cause. Monitoring is vital for at-risk individuals to prevent irreversible neurological damage. If symptoms occur, consult a healthcare provider for diagnosis and treatment. For more details, consult the National Institutes of Health Office of Dietary Supplements.

Frequently Asked Questions

The most common cause of vitamin B12 deficiency is inadequate absorption, often due to a lack of intrinsic factor caused by an autoimmune condition called pernicious anemia. Age-related reduced stomach acid is also a prevalent cause of malabsorption.

Yes, a vegan diet can cause a B12 deficiency because vitamin B12 is naturally found almost exclusively in animal products. Vegans must consume fortified foods or take supplements to ensure adequate intake.

Several medical conditions can cause a B12 deficiency, including autoimmune atrophic gastritis (pernicious anemia), Crohn's disease, celiac disease, and chronic pancreatitis.

Yes, certain long-term medications can interfere with B12 absorption. Examples include metformin, used for diabetes, and proton pump inhibitors (PPIs) and H2 blockers, used to reduce stomach acid.

Weight-loss surgeries, such as gastric bypass, alter the digestive tract by reducing stomach size or bypassing sections of the intestine. This can lead to decreased production of intrinsic factor and reduced absorption of B12.

Yes, B12 deficiency is more common in older adults, partly because the body's ability to absorb the vitamin from food decreases with age due to lower stomach acid levels.

Yes, an overgrowth of certain bacteria in the small intestine (SIBO) or an infection with parasites like fish tapeworm can compete for and consume the body's B12, leading to a deficiency.