What is the main cause of vitamin B deficiency?

December 30, 2025 •

3 min read

An estimated 20% of adults over 60 have a vitamin B12 deficiency, underscoring the significant impact of malabsorption on overall vitamin B status. While dietary intake is a factor, the primary cause for many individuals is the body's inability to absorb these essential nutrients properly.

Quick Summary

The primary cause of vitamin B deficiency is often malabsorption stemming from medical conditions like pernicious anemia and gut disorders, although inadequate dietary intake is also a contributing factor.

Key Points

Malabsorption is often the main cause: Issues with absorbing vitamins in the gut, rather than just poor diet, are the most frequent cause of deficiency, especially for B12.
Pernicious anemia is a major culprit: This autoimmune disease prevents the production of intrinsic factor, a protein vital for absorbing vitamin B12.
Dietary restrictions are a significant risk: Vegan and vegetarian diets are often low in naturally occurring B12, requiring careful supplementation or fortified foods.
Chronic illness and medications play a role: Conditions like Crohn's disease and celiac disease, along with certain medications such as metformin, can disrupt vitamin B absorption.
Alcohol abuse impairs absorption: Excessive alcohol consumption damages the digestive tract and hinders the body's utilization of B vitamins, particularly thiamine (B1) and folate (B9).

Malabsorption as a Primary Cause

For many, especially older adults, malabsorption is the most significant factor in vitamin B deficiency. This is particularly true for vitamin B12, as its absorption involves several digestive steps. Issues at any stage can lead to deficiency, even with adequate intake.

Pernicious Anemia

Pernicious anemia is a key malabsorption issue for vitamin B12. This autoimmune condition attacks stomach cells that produce intrinsic factor, which is vital for B12 absorption in the small intestine. Lack of intrinsic factor prevents dietary B12 assimilation.

Other Gut and Digestive Issues

Several conditions can hinder B vitamin absorption:

Chronic gastritis: Reduces stomach acid needed to release B12 from food.
Crohn’s and Celiac disease: Damage the small intestine, impairing nutrient absorption.
Gastric surgery: Can bypass sections where B vitamins are absorbed.
Bacterial overgrowth: Bacteria in the small intestine can absorb B vitamins before the body can.

Inadequate Dietary Intake

Poor diet directly causes vitamin B deficiency, particularly for vitamins not stored long. Most B vitamins are water-soluble and need regular dietary replenishment.

Risks for Vegetarians and Vegans

Strict vegetarians and vegans face a high risk of B12 deficiency as it's mainly in animal products. Supplementation or fortified foods are needed to prevent deficiency. Other B vitamins like folate are more available in plants, but unbalanced diets can still lead to low levels.

Alcohol Abuse

Heavy alcohol use significantly raises the risk of B vitamin deficiencies, especially thiamine (B1) and folate (B9). Alcohol damages the digestive system, hindering nutrient absorption. It also increases B vitamin metabolism and loss, potentially causing neurological issues like Wernicke-Korsakoff syndrome from severe thiamine deficiency.

Medications and Increased Requirements

Certain medications and life stages can deplete B vitamin stores by interfering with absorption or increasing needs.

Medications

Metformin: Can lower B12 by interfering with absorption.
PPIs and H2-receptor antagonists: Reduce stomach acid, hindering B12 release from food.
Anticonvulsants and methotrexate: Can interfere with folate metabolism.

Increased Nutritional Needs

Increased physiological demands can lead to deficiency if intake isn't adjusted.

Pregnancy and breastfeeding: Require higher folate and B12.
Age: Reduced stomach acid and intrinsic factor production can lead to poorer B12 absorption in older adults.

Comparison: Malabsorption vs. Dietary Deficiency


Feature	Malabsorption-related Deficiency	Dietary Deficiency
Underlying Cause	Impaired absorption due to medical conditions, such as pernicious anemia, gut disorders, or gastric surgery.	Inadequate intake of B-vitamin rich foods, often due to a restrictive diet like veganism or alcoholism.
Onset	Often gradual, as existing vitamin stores (e.g., B12) are depleted over months or years.	Can be relatively quick, especially for water-soluble B vitamins not stored in large amounts.
Key Example	Pernicious anemia affecting B12 absorption due to lack of intrinsic factor.	A strict vegan diet without supplementation leading to B12 deficiency.
Absorption Issue?	Yes, the core problem is a failure to absorb nutrients, not necessarily a lack of intake.	No, the body can absorb the vitamin, but insufficient amounts are consumed.
Prevalence	More common in specific populations, such as older adults and those with autoimmune or digestive disorders.	More common in individuals with very restrictive dietary patterns or chronic alcohol use.
Treatment Focus	Addressing the underlying medical condition, often with injections or high-dose supplements.	Improving dietary intake and, if necessary, oral supplementation.

Conclusion

While poor diet causes vitamin B deficiency, particularly in specific groups, malabsorption is the predominant and more complex main cause. Conditions like pernicious anemia and gut disorders interfere with the body's ability to process and absorb B vitamins, even with adequate intake. Distinguishing these causes is vital for proper diagnosis and treatment, as dietary changes alone won't fix malabsorption. If concerned, consult a healthcare provider for testing and guidance.

For more information on the various types of deficiencies and their implications, you can visit the NHS inform website.

Frequently Asked Questions

The most common cause of vitamin B12 deficiency is malabsorption, often due to conditions that prevent the body from absorbing the vitamin from food. For many older adults, this is linked to decreased stomach acid, while for others, pernicious anemia is the primary culprit.

A vegan diet is a risk factor for vitamin B12 deficiency since B12 is primarily found in animal products. However, with fortified foods and proper supplementation, deficiency can be avoided. A well-planned diet can provide other B vitamins like folate.

Yes, chronic alcohol abuse can lead to severe B vitamin deficiencies, especially thiamine (B1) and folate (B9). Alcohol impairs the absorption and utilization of B vitamins and increases their elimination from the body.

Medical conditions causing vitamin B deficiencies include pernicious anemia, Crohn's disease, celiac disease, and gastritis. Surgeries like gastric bypass can also impair absorption. These conditions often prevent proper absorption in the digestive tract.

Certain medications can interfere with B vitamin levels. Examples include metformin, proton pump inhibitors (PPIs), H2-receptor blockers, and some anticonvulsants, which can reduce absorption over time.

As individuals age, they may produce less stomach acid, which is necessary to free vitamin B12 from food proteins for absorption. This age-related decline makes older adults more susceptible to developing deficiencies.

A dietary deficiency results from a lack of intake of B vitamins from food, while a malabsorption-related deficiency occurs when the body cannot properly absorb the vitamins it consumes. Malabsorption is often tied to underlying medical issues, whereas a dietary deficiency can be corrected with dietary changes and oral supplements.