Why Does My B12 Deplete? Understanding the Root Causes

December 29, 2025 •

2 min read

According to the National Institutes of Health, up to 43% of community-dwelling older adults may have a vitamin B12 deficiency. When you ask, "Why does my B12 deplete?" the answer is rarely simple, often involving a complex interplay of dietary intake, digestive health, and other medical factors.

Quick Summary

Low B12 levels result from inadequate dietary intake, poor absorption due to digestive issues, autoimmune conditions like pernicious anemia, or long-term medication use. Treatment focuses on identifying the cause and implementing dietary changes or supplementation.

Key Points

Absorption is complex: A breakdown in the multi-step process of B12 absorption, involving stomach acid and intrinsic factor, is the most common cause of deficiency.
Dietary intake matters: Vegans and vegetarians are at high risk because vitamin B12 is found almost exclusively in animal products.
Age affects absorption: Older adults are more prone to B12 depletion due to atrophic gastritis, which reduces stomach acid needed for absorption.
Pernicious anemia is a common cause: This autoimmune disorder prevents the body from producing intrinsic factor, leading to severe malabsorption.
Medications can interfere: Long-term use of metformin and acid-reducing drugs like PPIs and H2 blockers can deplete B12 levels.
Digestive diseases are a factor: Chronic conditions such as Crohn's disease, celiac disease, and SIBO impair B12 absorption in the intestines.

The Complex Journey of Vitamin B12 Absorption

Understanding why your B12 levels might be low begins with knowing how it's absorbed. The process involves several steps:

Release: Stomach acid and enzymes detach B12 from food proteins.
Binding: B12 then binds to intrinsic factor, made in the stomach.
Absorption: This complex travels to the small intestine for absorption into the blood.
Storage: The liver can store B12 for years.

Problems at any stage can lead to depletion.

Leading Causes of B12 Depletion

Several factors can disrupt B12 absorption or intake.

Dietary Insufficiency

Not eating enough B12-rich foods is a key reason, especially since B12 is mainly in animal products.

Vegan/Vegetarian Diets: Those avoiding meat, eggs, or dairy are at high risk and need supplements or fortified foods.
Poor Diet: Malnutrition or unbalanced diets can also lead to low intake.

Malabsorption and Gut Issues

Problems absorbing B12 in the digestive system are very common causes.

Pernicious Anemia: An autoimmune attack on stomach cells that produce intrinsic factor, preventing B12 absorption. It requires lifelong treatment.
Atrophic Gastritis: Age-related stomach lining thinning reduces acid and intrinsic factor, common in older adults.
GI Surgery: Procedures like gastric bypass can impair intrinsic factor production and absorption.
Digestive Disorders: Conditions like Crohn's, celiac disease, and SIBO damage the intestines and hinder absorption.

Medications and Other Factors

Certain drugs and lifestyle choices can also contribute.

Metformin: Can reduce B12 absorption with long-term use.
Acid Reducers: PPIs and H2 blockers lower stomach acid, needed to release B12 from food.
Alcohol Use: Chronic use can damage the digestive system and affect absorption.
Nitrous Oxide: Can inactivate B12 with repeated exposure.

Causes of B12 Depletion: A Comparison


Cause Category	Primary Mechanism	Affected Population	Treatment Approach
Dietary	Lack of B12 from animal products.	Vegans, vegetarians, those with poor diet.	Dietary changes, oral supplements, fortified foods.
Pernicious Anemia	Immune system attacks intrinsic factor.	Autoimmune conditions, often older adults.	Lifelong B12 injections.
Gastric Issues	Reduced stomach acid/intrinsic factor production.	Older adults, post-surgery patients.	High-dose oral supplements or injections.
Intestinal Disease	Inflammation or damage to the ileum.	Crohn's disease, celiac disease patients.	Treat underlying disease, supplementation.
Medications	Interference with B12 absorption.	Long-term use of metformin, PPIs.	Supplementation, discuss alternatives with doctor.

Conclusion

While diet is a factor, particularly for vegans, the most frequent causes of B12 depletion relate to absorption problems. Conditions like pernicious anemia, atrophic gastritis, intestinal disorders, and certain medications disrupt the absorption process. If you have symptoms or risk factors, consult a healthcare professional. Diagnosis and management, including supplements or injections, can restore B12 levels.

Managing Your Vitamin B12 Deficiency - WebMD

Frequently Asked Questions

Early signs often include fatigue, weakness, a lack of energy, and pale skin. Some people also experience a sore or red tongue and a reduced appetite.

Yes, a severe or long-term B12 deficiency can lead to neurological issues, such as numbness or tingling in the hands and feet, memory problems, confusion, and difficulty with balance.

Because the body, particularly the liver, stores a significant amount of B12, it can take several years—often three to five—for a deficiency to develop and for symptoms to manifest after a change in diet or the onset of an absorption problem.

The best treatment depends on the cause. For absorption issues like pernicious anemia, B12 injections are typically required. For dietary deficiencies, high-dose oral supplements may be effective, and some fortified foods can also help.

Vegetarians and vegans can prevent B12 deficiency by regularly consuming fortified foods, such as cereals, plant-based milk alternatives, and nutritional yeast, or by taking a daily B12 supplement.

Yes, an overgrowth of certain bacteria in the small intestine, a condition known as SIBO, can compete for and consume the available B12 before your body can absorb it.

Long-term use of medications that block stomach acid, such as proton pump inhibitors (PPIs) and H2 blockers, can reduce the amount of stomach acid needed to release B12 from food, leading to malabsorption and depletion.