Understanding What Causes Low B12 and Its Many Triggers

November 13, 2025 •

2 min read

According to the Cleveland Clinic, vitamin B12 deficiency is a treatable condition resulting from either inadequate intake or a failure to absorb the nutrient properly. This article provides a comprehensive look into what causes low B12, exploring the various dietary, medical, and lifestyle factors behind this common deficiency.

Quick Summary

Several factors can lead to low vitamin B12 levels, including restricted diets, medical conditions affecting the digestive system, autoimmune disorders like pernicious anemia, and long-term use of specific medications.

Key Points

Dietary Restriction: A vegan or strict vegetarian diet is a significant cause of low B12, as the vitamin is naturally found in animal products.
Malabsorption is Common: Inadequate absorption, rather than lack of intake, is the most common cause of deficiency, especially in older adults.
Pernicious Anemia: This autoimmune disease is a major cause of malabsorption, as it destroys the stomach cells that produce intrinsic factor, a protein needed for absorption.
Medication Interference: Certain medications, such as metformin and stomach acid reducers, can interfere with the body's ability to absorb vitamin B12.
Underlying Health Conditions: Digestive disorders like Crohn's and celiac disease, as well as gastric surgeries, can damage the intestinal lining and impair absorption.
Age is a Factor: Older adults are at increased risk due to decreased stomach acid production, which is necessary for B12 release from food.

The Crucial Role of Vitamin B12

Vitamin B12, or cobalamin, is an essential water-soluble nutrient vital for many bodily functions. It is necessary for keeping nerve and blood cells healthy, forming red blood cells, and creating DNA. As the body cannot produce B12 on its own, it must be obtained from food or supplements. Absorption primarily occurs in the small intestine and relies on intrinsic factor, a protein made in the stomach. Any disruption in this process can lead to a deficiency.

Key Causes of Low B12

Inadequate Dietary Intake

For many, dietary deficiency is uncommon because B12 is stored in the liver. However, certain diets pose a higher risk.

Vegan and strict vegetarian diets: B12 is mainly found in animal products. Vegans and strict vegetarians are at high risk without supplementation or fortified foods.
Infants of vegan mothers: Breastfed infants are at risk if the mother is deficient and doesn't supplement.
Unhealthy or fad diets: Prolonged restrictive diets may lack sufficient B12.

Poor Absorption (Malabsorption Issues)

Malabsorption is the most common cause of B12 deficiency. It disrupts the absorption process due to various conditions, including pernicious anemia, which is an autoimmune condition. Other causes include atrophic gastritis, gastric surgery, intestinal disorders like Crohn's and celiac disease, bacterial overgrowth, and chronic pancreatitis.

Medications and Other Factors

Medications and lifestyle can also impact B12 levels.

Medications: Stomach acid reducers (proton pump inhibitors, H2 blockers) and metformin can hinder B12 absorption.
Alcohol Abuse: Chronic heavy drinking can damage the digestive system, causing absorption issues.
Nitrous Oxide Abuse: Recreational use inactivates B12.

Malabsorption vs. Dietary Deficiency


Aspect	Malabsorption-Related Deficiency	Dietary-Related Deficiency
Primary Cause	Inability to absorb B12 despite adequate intake.	Insufficient B12 in diet, common in unsupplemented vegan diets.
Mechanism	Digestive process disruptions (intrinsic factor, acid, intestinal damage).	Lack of B12 from diet as it's not in plant foods.
Risk Groups	Older adults, autoimmune conditions, gastric surgery.	Vegans, strict vegetarians, and their infants.
Treatment	Often requires lifelong injections or high-dose oral supplements.	Corrected with diet, fortified foods, and oral supplements if no absorption issue exists.

The Ripple Effect of Low B12

Low B12 can cause fatigue, weakness, nerve damage (numbness, tingling), and memory issues. Developmental delays can occur in infants. Early treatment is vital to prevent lasting neurological damage.

Conclusion: Taking Action to Address Low B12

Low B12 stems from dietary choices, lifestyle factors, or underlying medical conditions. While diet is a risk, particularly for vegans, malabsorption from issues like pernicious anemia or digestive disorders is more common. Medications and alcohol also contribute. Identifying the cause is key to treatment, which may include supplements, injections, or managing the underlying issue. If you suspect a deficiency, consult a healthcare provider for diagnosis and treatment. For more detailed information on Vitamin B12 deficiency, refer to {Link: Merck Manuals https://www.merckmanuals.com/home/disorders-of-nutrition/vitamins/vitamin-b12-deficiency}.

For further reading, consider reviewing the National Institutes of Health's Vitamin B12 Fact Sheet for Health Professionals.

Frequently Asked Questions

Yes, because vitamin B12 is found almost exclusively in animal products, a vegan diet that does not include fortified foods or supplements is a common cause of B12 deficiency.

Pernicious anemia is an autoimmune condition where the body's immune system attacks the stomach cells that produce intrinsic factor, a protein essential for absorbing vitamin B12.

Yes, certain medications, particularly long-term use of stomach acid reducers like proton pump inhibitors and H2 blockers, along with the diabetes drug metformin, can lower B12 levels.

Older adults are at higher risk primarily due to a natural decrease in stomach acid production with age, which impairs the body's ability to extract B12 from food.

Intestinal disorders such as Crohn's disease and celiac disease, as well as bacterial overgrowth in the small intestine and gastric surgery, can all lead to poor B12 absorption.

Because the body stores B12 in the liver for several years, it can take a long time—sometimes years—for a deficiency to develop and for symptoms to become noticeable.

Early signs of a B12 deficiency often include fatigue, weakness, a lack of energy, and sometimes a sore or red tongue.