What Causes a Decrease in Vitamin B12?

November 14, 2025 •

3 min read

According to the National Institutes of Health, between 3% and 43% of community-dwelling older adults have vitamin B12 deficiency. The key reason for this and many other cases is that a decline in vitamin B12 can stem from various causes, ranging from dietary choices to specific health conditions and medications.

Quick Summary

A decrease in vitamin B12 can result from dietary insufficiency, impaired absorption due to medical conditions, or drug interactions. Common culprits include autoimmune disorders like pernicious anemia, gastrointestinal surgery, and strict vegan diets, which all prevent the body from obtaining or utilizing this essential nutrient.

Key Points

Malabsorption is the Primary Cause: The most common reason for a decrease in vitamin B12 is the body's inability to absorb it, rather than a simple lack of intake.
Pernicious Anemia is a Key Culprit: This autoimmune condition prevents the stomach from producing intrinsic factor, a protein vital for B12 absorption.
Dietary Restrictions Pose a Risk: Vegans and vegetarians are at high risk of deficiency if they do not consistently consume fortified foods or supplements, as natural B12 is found only in animal products.
Medications Can Block Absorption: Long-term use of certain drugs, including metformin for diabetes and acid-reflux medications (PPIs), can interfere with B12 uptake.
Stomach Surgery Impairs Absorption: Procedures like gastric bypass can eliminate the areas of the stomach and small intestine responsible for producing and absorbing intrinsic factor and B12.
Older Adults are More Vulnerable: Reduced stomach acid and intrinsic factor production, often due to atrophic gastritis, make older adults more susceptible to B12 deficiency.

What is Vitamin B12 and How Is It Absorbed?

Vitamin B12, also known as cobalamin, is a vital water-soluble vitamin essential for nerve function, red blood cell formation, and DNA synthesis. Proper absorption involves stomach acid separating B12 from food, binding to intrinsic factor produced in the stomach, and then being absorbed in the small intestine. Issues at any point can lead to deficiency.

Medical Conditions Affecting B12 Absorption

Malabsorption is the most common cause of low vitamin B12. Several health issues disrupt the absorption process:

Pernicious Anemia

Pernicious anemia is an autoimmune disease where the body attacks stomach cells making intrinsic factor. Without intrinsic factor, B12 cannot be absorbed, leading to deficiency. It's a common cause in the UK and in people of Northern European or African descent, often needing lifelong B12 injections.

Gastrointestinal Disorders

Conditions affecting the gut can hinder B12 absorption:

Chronic Atrophic Gastritis: Inflammation in the stomach reduces acid, needed to free B12 from food. This often affects older adults.
Crohn's Disease and Celiac Disease: These damage the small intestine where B12 is absorbed.
Small Intestinal Bacterial Overgrowth (SIBO): Bacteria can use up B12 in the small intestine.

Stomach Surgery

Procedures like gastric bypass reduce stomach size or alter the digestive path, impacting intrinsic factor production and B12 absorption. Lifelong supplementation may be needed.

Dietary Causes of Low Vitamin B12

Since B12 is mainly in animal products, diet plays a role.

Vegan and Vegetarian Diets: Strict vegans and vegetarians risk deficiency if they don't use fortified foods or supplements. This is critical for pregnant or breastfeeding women due to infant development risks.
Poor Diet: Low intake of animal products can deplete the body's B12 stores over time.

Medications and Other Factors

Certain medications can interfere with B12 absorption with long-term use:

Metformin: This diabetes drug can reduce B12 absorption.
Proton Pump Inhibitors (PPIs) and H2 Blockers: Used for acid reflux, they lower stomach acid, hindering B12 release from food.
Nitrous Oxide: This gas can inactivate B12; repeated recreational use is linked to deficiency.

How Do You Know If You Are Deficient?

Symptoms are often subtle and gradual, including fatigue, weakness, a sore tongue, and numbness or tingling in extremities. Diagnosis is usually via blood tests, potentially including methylmalonic acid (MMA) and homocysteine levels for confirmation.

Comparison of B12 Deficiency Causes


Feature	Dietary Insufficiency	Malabsorption (e.g., Pernicious Anemia)	Medication Interference
Cause	Low intake of animal products or fortified foods.	Autoimmune attack on intrinsic factor-producing cells.	Long-term use of specific drugs.
Mechanism	Depletes long-term body stores over several years.	Prevents the absorption of B12 from the digestive tract.	Decreases stomach acid or interferes with absorption.
Risk Groups	Vegans, vegetarians, breastfeeding infants of vegan mothers.	Older adults, individuals with autoimmune conditions.	Patients with diabetes or GERD on long-term medication.
Onset	Gradual, often taking years to manifest.	Insidious, with symptoms appearing slowly over time.	Can be slow, dependent on the duration and type of medication.
Treatment	Dietary changes, oral supplements.	Lifelong injections are often necessary.	Supplementation, alternative medication if possible.

Conclusion

Understanding what causes a decrease in vitamin B12 is vital for prevention and treatment. Causes range from diet and autoimmune issues like pernicious anemia to surgery and long-term medications. Deficiency can have serious health effects if untreated. At-risk groups, including vegans, older adults, and those with GI issues, need monitoring and potentially supplementation. Consult a healthcare professional for diagnosis and treatment.

How to Prevent Vitamin B12 Deficiency

Preventing low B12 involves diet and supplementation. Eat B12-rich foods like meat, fish, eggs, and dairy. Vegans need fortified foods and supplements. Manage medical conditions affecting absorption and discuss medications with your doctor.

Increase dietary intake: Include animal products or fortified plant-based options.
Use supplements: Consider oral supplements or injections for at-risk individuals.
Manage GI health: Treat conditions like Crohn's or gastritis.
Review medications: Discuss long-term drugs like metformin or PPIs with your doctor.
Consider injections: For severe cases or malabsorption, lifelong injections may be needed.

Frequently Asked Questions

The most common cause is malabsorption, often due to a lack of intrinsic factor—a protein needed for absorption—which can be caused by the autoimmune condition pernicious anemia.

Yes, because vitamin B12 is naturally found almost exclusively in animal products, individuals on strict vegan or vegetarian diets are at a high risk of deficiency if they don't use supplements or fortified foods.

Common medications that can affect B12 levels include metformin (for diabetes), proton pump inhibitors (PPIs) and H2 blockers (for heartburn), and certain antibiotics and seizure drugs.

Surgeries that remove parts of the stomach, such as gastric bypass, can prevent the production of intrinsic factor, which is essential for B12 absorption, leading to lifelong deficiency.

As people age, decreased stomach acid production, often from chronic atrophic gastritis, makes it harder to absorb B12 from food, even with an adequate diet.

Early symptoms are often vague and can include fatigue, weakness, a sore tongue (glossitis), and tingling or numbness in the hands and feet.

Diagnosis typically involves a blood test to check B12 levels. Sometimes additional tests measuring metabolites like methylmalonic acid (MMA) and homocysteine are used for confirmation, especially in cases of borderline B12 levels.