What can cause poor absorption of B12? A guide to common culprits

December 30, 2025 •

4 min read

According to the National Institutes of Health, up to 43% of older adults with atrophic gastritis, a common cause, have a B12 deficiency. Several underlying issues can cause poor absorption of B12, ranging from dietary choices and age-related changes to serious medical conditions and certain medications.

Quick Summary

An exploration of the many factors that lead to insufficient B12 uptake, covering autoimmune diseases, dietary restrictions, gastrointestinal surgery, and prescription drugs that interfere with the absorption process.

Key Points

Pernicious Anemia: An autoimmune disease where the body attacks stomach cells, preventing the creation of intrinsic factor needed for B12 absorption.
Atrophic Gastritis: Inflammation of the stomach lining, common in older adults, that reduces the stomach acid required to release B12 from food.
Gastrointestinal Surgery: Procedures like gastric bypass can remove parts of the stomach or small intestine necessary for intrinsic factor production and B12 absorption.
Medications: Certain drugs, including metformin for diabetes and proton pump inhibitors for acid reflux, can interfere with B12 uptake.
Dietary Factors: Vegans and strict vegetarians are at high risk because B12 is found almost exclusively in animal products; proper supplementation is necessary.
Intestinal Disorders: Conditions such as Crohn's disease and celiac disease can damage the ileum, the site of B12 absorption.
Chronic Alcohol Use: Excessive alcohol consumption can damage the digestive tract and impair the body's ability to absorb vital nutrients like B12.

The Complex Journey of B12 Absorption

To understand why poor absorption of B12 occurs, one must first grasp its intricate journey through the digestive system. Vitamin B12, or cobalamin, is typically bound to protein in food sources. The first step involves stomach acid (hydrochloric acid) and enzymes detaching B12 from its protein carrier. Next, the freed B12 binds to a protein called intrinsic factor, which is also produced by the stomach. This intrinsic factor-B12 complex then travels to the final section of the small intestine, the ileum, where it is finally absorbed into the bloodstream. Disruptions at any point in this multi-step process can lead to poor absorption of B12.

Medical Conditions That Impair B12 Absorption

Numerous health conditions can interfere with the body's ability to absorb this crucial nutrient. One of the most prominent is pernicious anemia, an autoimmune disorder.

Pernicious Anemia: In this autoimmune condition, the immune system attacks the stomach cells responsible for producing intrinsic factor, without which B12 cannot be absorbed. It is a leading cause of B12 deficiency, particularly in older adults.
Atrophic Gastritis: This common condition, especially among the elderly, involves inflammation and thinning of the stomach lining. It results in a decrease of stomach acid, which is necessary to separate B12 from its food protein, a critical first step in the absorption process. While people with atrophic gastritis can absorb supplemental B12, they struggle to absorb it from food.
Gastrointestinal Disorders: Several digestive diseases directly affect the small intestine, impairing its ability to absorb nutrients. These include:
- Crohn's Disease: A chronic inflammatory bowel disease that can damage the ileum, the specific part of the intestine where B12 is absorbed.
- Celiac Disease: An autoimmune disorder where consuming gluten damages the small intestine lining, which can inhibit B12 uptake.
- Small Intestine Bacterial Overgrowth (SIBO): An excess of bacteria in the small intestine can consume B12 before the body can absorb it.
Infections: Parasitic infections like the fish tapeworm can compete for dietary B12 within the intestine, leading to a deficiency.
Exocrine Pancreatic Insufficiency: The pancreas's inability to produce digestive enzymes can prevent B12 from being released from certain binding proteins in the small intestine, a necessary step before it can attach to intrinsic factor.

Medications and Surgeries Affecting B12 Absorption

Beyond medical conditions, certain treatments and procedures can significantly impact B12 levels.

Medications: Some drugs are known to inhibit B12 absorption, often by reducing stomach acid.
- Metformin: A common drug for managing type 2 diabetes, long-term use is strongly associated with reduced B12 levels.
- Proton Pump Inhibitors (PPIs) and H2 Blockers: Medications like omeprazole (Prilosec) and ranitidine (Zantac), used to treat acid reflux and ulcers, lower stomach acid, hindering the release of B12 from food.
- Other Drugs: Certain antibiotics, anticonvulsants, and gout medications can also interfere with B12 uptake.
Gastrointestinal Surgeries: Any surgery that alters the structure of the stomach or small intestine poses a risk for poor absorption.
- Gastric Bypass Surgery: This weight-loss procedure often removes or bypasses the parts of the stomach and small intestine that are essential for producing intrinsic factor and absorbing B12.
- Gastrectomy or Small Intestine Resection: Surgical removal of all or part of the stomach or ileum directly reduces or eliminates the body's capacity to absorb B12.

Comparison of B12 Absorption Impairments


Cause	Mechanism of Impairment	Population at Higher Risk	Treatment/Management
Pernicious Anemia	Autoimmune destruction of intrinsic factor-producing stomach cells.	Older adults, those with other autoimmune diseases.	Lifelong B12 injections are often necessary.
Atrophic Gastritis	Decreased production of stomach acid, hindering B12 release from food.	Adults over 65 years old.	Oral B12 supplements may be sufficient, as they don't require stomach acid for the initial release.
Metformin Use	Reduced B12 absorption in the intestine.	People with type 2 diabetes on long-term treatment.	Monitoring B12 levels; oral supplementation may be required.
Gastric Bypass	Surgical removal of parts of the stomach and intestine critical for intrinsic factor production and absorption.	Post-bariatric surgery patients.	Lifelong B12 supplementation, often via injections.
Vegan Diet	Lack of dietary B12, found primarily in animal products.	Vegans, strict vegetarians.	Consistent intake of fortified foods or supplements.

Lifestyle and Other Factors

Apart from medical and surgical causes, certain lifestyle factors can also contribute to poor absorption of B12. Chronic and excessive alcohol consumption, for example, can damage the digestive system, disrupting the delicate absorption process. Furthermore, dietary choices play a significant role. Since B12 is almost exclusively found in animal products, strict vegans who do not consume fortified foods or take supplements are at a high risk of deficiency. Age is another major factor, with older adults experiencing a natural decline in stomach acid production, making food-based B12 absorption more challenging.

Conclusion: Seeking the Root Cause is Key

Understanding what can cause poor absorption of B12 is the first step toward effective management and treatment. Whether the issue is an autoimmune condition like pernicious anemia, a gastrointestinal disorder, a consequence of surgery, or a side effect of medication, a diagnosis is essential for determining the right course of action. While dietary adjustments and oral supplements can work for some, others may require lifelong B12 injections to bypass the absorption issues. Consulting a healthcare professional is crucial for anyone experiencing symptoms like fatigue, tingling, or weakness, as untreated B12 deficiency can lead to irreversible neurological damage. Early diagnosis and treatment are the keys to a positive outcome.

An authoritative resource on this topic is the National Institutes of Health's Office of Dietary Supplements, which provides detailed information on vitamin B12 and its absorption.

Frequently Asked Questions

The most common cause is pernicious anemia, an autoimmune disorder where the body attacks the stomach cells that produce intrinsic factor, a protein essential for B12 absorption.

Yes, as people age, they are more susceptible to atrophic gastritis, which is characterized by decreased stomach acid production. This makes it harder to absorb B12 from food, though absorption from supplements is often unaffected.

While it's not a malabsorption issue, a strict vegan diet can lead to a B12 deficiency because the vitamin is naturally found only in animal products. Vegans must consume fortified foods or supplements to meet their needs.

Long-term use of Metformin can reduce the body's ability to absorb B12, which can lead to low B12 levels over time. Regular monitoring and supplementation may be necessary for those on this medication.

It depends on the cause. For issues like atrophic gastritis, oral supplements may work well since they don't require the same stomach acid process as food-bound B12. However, for conditions like pernicious anemia, injections are typically necessary to bypass the absorption problem.

Any surgery that removes parts of the stomach or small intestine where intrinsic factor is produced or B12 is absorbed can cause malabsorption. This includes gastric bypass surgery and gastrectomy.

Early symptoms are often subtle and can include fatigue, weakness, tingling or numbness in the hands and feet, and forgetfulness. If left untreated, more severe neurological issues can arise.