Understanding What is the most common cause of vitamin B deficiency?

October 23, 2025 •

4 min read

Functional vitamin B12 deficiency is a common issue, with some reports indicating that up to 20% of older people may have low levels. For B12 in particular, the most common cause of vitamin B deficiency is inadequate absorption due to underlying health problems rather than insufficient dietary intake. This is a crucial distinction, as a diagnosis requires investigating the root cause beyond a simple dietary analysis.

Quick Summary

This article explores how malabsorption, autoimmune issues, medical conditions, medications, and lifestyle factors like alcoholism and veganism contribute to B vitamin deficiencies, primarily affecting the absorption of vitamin B12.

Key Points

Malabsorption is a primary cause: For vitamin B12, the most common issue is not inadequate dietary intake but rather the body's inability to absorb the vitamin properly due to underlying conditions.
Age and autoimmune issues are major factors: Autoimmune atrophic gastritis and age-related decreases in stomach acid production commonly impair vitamin B12 absorption.
Dietary causes are specific: While vegans are at high risk for B12 deficiency due to the lack of animal product consumption, poor diet is a more significant cause for folate and riboflavin deficiencies.
Medications pose a significant risk: Certain medications, including proton pump inhibitors (PPIs) and the diabetes drug metformin, can interfere with vitamin B12 absorption.
Chronic alcohol use is a contributor: Excessive alcohol consumption can damage the stomach lining and impair the absorption, metabolism, and storage of multiple B vitamins.
Diagnosis requires a holistic approach: Since multiple factors can be involved, a proper diagnosis requires evaluating diet, medication history, and testing for underlying medical conditions like autoimmune issues or gastrointestinal disorders.
Treatment depends on the cause: The correct treatment may range from B12 injections for malabsorption issues like pernicious anemia to oral supplements for dietary gaps.

Malabsorption is the Primary Culprit for Vitamin B12

While dietary intake can play a role, particularly for vegans and those with very poor diets, malabsorption is the leading cause of B12 deficiency. The complex process of B12 absorption relies on specific conditions in the stomach and small intestine. Any disruption to this process can lead to a deficiency, even if a person consumes plenty of B12-rich foods.

The Role of Intrinsic Factor and Stomach Acid

Vitamin B12 in food is bound to protein. Stomach acid and the enzyme pepsin are required to release the B12 molecule so it can bind with intrinsic factor (IF), a protein produced by stomach cells. The B12-IF complex then travels to the small intestine where it is absorbed. A breakdown at any stage can prevent absorption, with common issues including:

Autoimmune Atrophic Gastritis: This condition causes the immune system to attack and destroy the stomach cells responsible for producing stomach acid and intrinsic factor. It is the underlying cause of pernicious anemia, a major cause of severe B12 deficiency.
Age-Related Hypochlorhydria: As people get older, their stomach acid production often decreases. This makes it harder to separate B12 from food proteins, leading to impaired absorption. Fortunately, B12 from supplements is not protein-bound and can be absorbed more easily in these cases.
Helicobacter pylori (H. pylori) infection: This bacterial infection can cause atrophic gastritis, leading to reduced gastric acid and subsequently poor B12 absorption.

Other Factors Contributing to B Vitamin Deficiencies

While B12 malabsorption is a prominent concern, deficiencies in other B vitamins can result from a variety of factors. The B vitamins are water-soluble and must be consumed regularly, with the exception of B12 which is stored in the liver.

Dietary Restrictions

Vegan and Strict Vegetarian Diets: Since B12 is found almost exclusively in animal products (meat, fish, eggs, dairy), those following a vegan diet are at high risk of deficiency unless they take supplements or consume fortified foods.
Malnutrition: Inadequate intake of folate (B9) and riboflavin (B2) due to a consistently poor diet is a common cause, especially among older adults and those with alcohol use disorder.

Medical and Surgical Conditions

Gastrointestinal Diseases: Conditions that affect the small intestine, where absorption occurs, are common culprits. These include Crohn's disease, celiac disease, and chronic pancreatitis.
Gastric and Bariatric Surgery: The removal of parts of the stomach or small intestine significantly reduces the body's capacity to produce intrinsic factor and absorb B12.
Exocrine Pancreatic Insufficiency: Problems with the pancreas can lead to B12 malabsorption.

Medication Use and Lifestyle Choices

Medications: Certain drugs are known to interfere with B vitamin absorption. Prolonged use of acid-suppressive agents like Proton Pump Inhibitors (PPIs) and H2-receptor antagonists inhibits gastric acid production, harming B12 absorption. The diabetes medication metformin can also lower B12 levels. Other drugs, including some anticonvulsants and methotrexate, can affect folate absorption.
Chronic Alcohol Use: Excessive alcohol consumption can inflame the stomach and intestinal lining, damaging the cells needed for absorption. It also interferes with the metabolism and storage of several B vitamins.

Comparing Major Causes of B Vitamin Deficiency


Factor	Primary Impact	Most Affected B Vitamins	Who is at Risk	Diagnosis Approach
Malabsorption	Impairs the digestive process needed for nutrient uptake, particularly B12 release from food protein.	B12, occasionally folate.	Older adults, individuals with autoimmune diseases, those with GI disorders, post-surgery patients.	Investigate gut health (e.g., test for intrinsic factor antibodies, H. pylori), assess medication history, check for underlying conditions.
Inadequate Diet	Insufficient intake of B vitamin-rich foods.	B12 (for vegans/vegetarians), Folate, Riboflavin.	Vegans, chronic alcohol users, individuals with poor overall diets.	Dietary recall, assessment of food group intake, potentially testing for folate/B12 levels.
Medication Effects	Direct interference with absorption or metabolism of B vitamins.	B12 (PPIs, Metformin), Folate (Methotrexate, Sulfasalazine).	Patients on long-term acid-suppressive medication, type 2 diabetics on metformin, individuals with certain autoimmune conditions.	Review medication history and adjust treatment plan with a doctor.

Conclusion

While a poor diet can certainly lead to a B vitamin deficiency, particularly for folate and riboflavin, the most common and often more complex cause of a vitamin B12 deficiency is malabsorption. This can be triggered by age-related changes in stomach acid production, autoimmune diseases like pernicious anemia, or the long-term use of specific medications. Diagnosing the precise cause is essential, as the correct treatment—whether injections for malabsorption or supplements for dietary gaps—depends on addressing the underlying issue. A healthcare professional can determine the best course of action through blood tests and a thorough medical evaluation, ensuring the deficiency is effectively managed and potential health complications are avoided.

It is important to remember that for older adults, even with adequate intake, absorption can be impaired. Regular check-ups and open communication with a doctor about medications and digestive health are key steps in preventing and managing vitamin B deficiencies.

Frequently Asked Questions

In older adults, the most common cause of vitamin B12 deficiency is malabsorption due to reduced stomach acid (hypochlorhydria), often caused by atrophic gastritis. This impairs the release of B12 from food proteins, preventing it from being absorbed effectively.

Yes, a strict vegan diet can cause a vitamin B12 deficiency because B12 is found almost exclusively in animal products. Vegans must consume fortified foods or take supplements to prevent this deficiency.

Medical conditions that commonly cause vitamin B malabsorption include autoimmune atrophic gastritis (leading to pernicious anemia), Crohn's disease, celiac disease, and chronic pancreatitis.

Certain medications interfere with B vitamin absorption or metabolism. For example, proton pump inhibitors (PPIs) and H2-receptor antagonists reduce stomach acid, inhibiting B12 release from food. Metformin can also cause B12 deficiency.

Chronic alcohol use can damage the gastrointestinal lining, impairing the absorption of several B vitamins. It can also interfere with their metabolism and storage, leading to deficiencies, particularly of thiamine (B1), folate (B9), and B6.

Pernicious anemia is an autoimmune disease where the body's immune system attacks the stomach cells that produce intrinsic factor. Without intrinsic factor, the body cannot absorb vitamin B12, regardless of dietary intake.

No. While malabsorption is a primary cause for B12 deficiency, deficiencies in other B vitamins like folate and riboflavin are often primarily linked to inadequate dietary intake. For these, a consistently poor or unbalanced diet is the main issue.