What causes B12 to be really low?: Uncovering the complex factors

October 20, 2025 •

3 min read

According to the National Institutes of Health, vitamin B12 is essential for nerve function, DNA synthesis, and red blood cell production. Therefore, understanding what causes B12 to be really low is critical for maintaining overall health and preventing potential complications.

Quick Summary

Low B12 levels result from either inadequate dietary intake, most notably in vegans and vegetarians, or malabsorption issues stemming from autoimmune conditions like pernicious anemia, age-related changes, and certain medications. This comprehensive overview details the wide range of causes and associated risk factors.

Key Points

Absorption over Intake: The most common reason for a seriously low B12 level is malabsorption, not just insufficient dietary intake.
Pernicious Anemia: This autoimmune disease is a major culprit, preventing B12 absorption by attacking intrinsic factor-producing stomach cells.
Age is a Risk Factor: Older adults are highly susceptible due to a natural decrease in stomach acid, which impairs the release of B12 from food proteins.
Medication Side Effects: Certain drugs, like metformin and proton pump inhibitors, can significantly interfere with B12 absorption.
Serious Neurological Consequences: If left untreated, a B12 deficiency can lead to permanent nerve damage, impacting mobility, cognition, and mental health.
Early Detection is Crucial: Because neurological symptoms can sometimes precede anemia, early testing is vital to ensure timely and effective treatment.

Understanding the Role of Vitamin B12

Vitamin B12 (cobalamin) is a water-soluble nutrient crucial for nerve function, red blood cell production, and DNA synthesis. Humans must obtain it from animal products or fortified foods, as the body does not produce it. The liver stores B12 for several years, but depletion from poor intake or malabsorption can lead to deficiency.

What Causes B12 to Be Really Low?: The Root Causes

While insufficient intake can be a factor, particularly in restrictive diets, the primary cause of significantly low B12 is often malabsorption. Proper B12 absorption requires intrinsic factor, a protein produced in the stomach, to bind with the vitamin before it can be absorbed in the small intestine. Any disruption in this process can result in deficiency.

Inadequate Dietary Intake

Low B12 can be linked to diet, especially for vegans, strict vegetarians, and individuals with unbalanced diets. B12 is mainly found in animal products, requiring those avoiding these foods to use supplements or fortified products.

Malabsorption and Digestive Problems

Malabsorption is the most frequent reason for low B12. Conditions like pernicious anemia (an autoimmune condition affecting intrinsic factor production), age-related reduced stomach acid, gastrointestinal surgeries (e.g., bariatric surgery), and digestive diseases (e.g., Crohn's, celiac disease) can all impair B12 absorption. Small intestinal bacterial overgrowth (SIBO) can also reduce B12 availability.

Medication and Other Contributing Factors

Certain medications like metformin and heartburn drugs (proton pump inhibitors, H2 blockers), chronic alcohol use, and recreational use of nitrous oxide can disrupt B12 absorption or inactivate the vitamin.

Symptoms of a B12 Deficiency

Symptoms, which can worsen gradually, often affect the nervous system and blood. They include fatigue, weakness, shortness of breath, pale skin, sore tongue or mouth ulcers, loss of appetite, weight loss, pins and needles sensation, vision problems, difficulty walking, balance issues, memory loss, cognitive impairment, depression, irritability, and behavioral changes.

Comparison of B12 Deficiency Causes: Dietary vs. Malabsorption


Feature	Dietary Deficiency	Malabsorption Deficiency
Common Cause	Restrictive diets without supplementation.	Conditions preventing proper B12 absorption.
Mechanism	Insufficient intake of B12-rich foods.	Inability to process B12 due to lack of intrinsic factor, stomach acid, or intestinal issues.
Onset Time	Can take years as liver stores are used.	Varies based on the underlying medical issue.
Typical Patient	Vegans, vegetarians, or those with chronically poor diets.	Older adults, post-surgery patients, people with autoimmune conditions.
Treatment	Often responds to oral supplements.	May require long-term B12 injections.

Conclusion: Proactive Monitoring is Essential

Low B12 can have various effects, from fatigue to neurological issues. Treatment is simple, but identifying the cause is key for the right approach, which might include diet changes, supplements, or injections. Prompt diagnosis and treatment are vital to avoid permanent neurological damage. As malabsorption is common, particularly in older adults, and symptoms can be vague, at-risk individuals should consult a healthcare provider for diagnosis and treatment.

For additional, authoritative information on Vitamin B12 deficiency, please consult the National Institutes of Health {Link: NIH Office of Dietary Supplements https://ods.od.nih.gov/factsheets/VitaminB12-Consumer/}.

Frequently Asked Questions

The most common cause of B12 deficiency is malabsorption, or the body's inability to absorb the vitamin properly, rather than insufficient dietary intake.

Yes, because vitamin B12 is almost exclusively found in animal products, vegans and strict vegetarians are at a higher risk of deficiency if they do not regularly consume fortified foods or supplements.

Yes, several medications can interfere with B12 absorption. These include metformin (for diabetes), and long-term use of heartburn medications like proton pump inhibitors (PPIs) and H2 blockers.

Symptoms can vary but often include fatigue, weakness, a sore tongue, pins and needles in the hands and feet, memory problems, and difficulty walking. Severe deficiency can lead to more serious neurological issues.

Diagnosis typically involves blood tests that measure vitamin B12 levels. Other tests, such as those for methylmalonic acid (MMA) and homocysteine, can also be used to confirm a deficiency.

Treatment depends on the cause. For dietary deficiency, oral supplements may be sufficient. For malabsorption issues like pernicious anemia, regular B12 injections are often necessary, potentially for life.

Prevention can involve eating a balanced diet that includes B12-rich animal products or fortified foods. For those at higher risk, such as older adults, regular supplementation may be recommended.