Why is my body not absorbing B12? A guide to malabsorption

According to the National Institutes of Health, up to 20% of adults over 60 in the U.S. may have vitamin B12 deficiency. This often happens when your body is not absorbing B12 properly, even with adequate intake from food or supplements.

Quick Summary

This guide details the complex reasons behind vitamin B12 malabsorption, including autoimmune disorders, stomach acid deficiencies, and gastrointestinal diseases. It also covers the role of specific medications and surgical interventions that can impair B12 uptake.

Key Points

• Pernicious Anemia, an autoimmune condition, is a primary cause, preventing intrinsic factor production vital for B12 absorption.

• Gut health issues like Crohn's disease, celiac disease, and bacterial overgrowth can impair the small intestine's ability to absorb B12.

• Aging often leads to decreased stomach acid, hindering the release of B12 from food.

• Medications such as metformin and acid-reflux drugs (PPIs, H2 blockers) can interfere with B12 absorption.

• High-dose oral B12 can be absorbed via passive diffusion, offering an alternative to injections when intrinsic factor is lacking.

• Treatment depends on the underlying cause, potentially involving injections or high-dose oral supplements.

The Journey of B12: What Needs to Happen for Absorption?

To understand why your body is not absorbing B12, it's crucial to first grasp the complex journey this vital nutrient takes through your digestive system.

1. Release in the Stomach: The process begins when you eat animal-based foods containing B12, such as meat, fish, eggs, and dairy. In your stomach, hydrochloric acid releases B12 from its binding proteins.
2. Binding with Intrinsic Factor: The now-free B12 then binds to a special protein called intrinsic factor, which is produced by the parietal cells in your stomach lining.
3. Absorption in the Small Intestine: This B12-intrinsic factor complex travels to the final section of the small intestine, called the ileum. There, the complex binds to receptors, allowing the B12 to be absorbed into your bloodstream.
4. Transport in the Bloodstream: Finally, B12 binds to a protein called transcobalamin II for transport to your body's tissues.

Root Causes of B12 Malabsorption

When this delicate process is disrupted at any stage, B12 malabsorption occurs. Factors like pernicious anemia, atrophic gastritis, gastrointestinal disorders (such as Crohn's disease and celiac disease), surgical procedures affecting the stomach or intestine, and certain medications like PPIs, H2 blockers, and metformin can interfere with B12 absorption. Aging and chronic alcohol use can also impact absorption.

B12 Absorption Pathways: Food vs. Supplements

Understanding how B12 from different sources is absorbed is key, especially with malabsorption issues.

Feature	B12 Absorption from Food	B12 Absorption from Supplements (High-Dose Oral)
Mechanism	Requires release from protein by stomach acid, followed by binding to intrinsic factor for absorption in the ileum.	Relies on passive diffusion, which bypasses the need for intrinsic factor. High doses are needed to overcome malabsorption via this alternate pathway.
Stomach Acid Dependency	High: Depends on sufficient hydrochloric acid to free B12 from its binding proteins.	Low: Not dependent on stomach acid to release B12 as the supplement form is already 'free'.
Intrinsic Factor Dependency	High: Requires intrinsic factor to bind and transport B12.	Low: Bypasses the intrinsic factor mechanism.
Absorption Site	Primarily in the terminal ileum.	Throughout the digestive tract via passive diffusion.
Efficacy in Malabsorption	Poor: The primary absorption pathway is blocked.	Effective: High doses can force absorption through an alternative, less efficient pathway, making oral supplements viable for many patients.

Diagnosis and Treatment Approaches

If malabsorption is suspected, a healthcare provider can diagnose it with blood tests measuring B12, MMA, and homocysteine levels.

Treatment is based on the underlying cause:

• B12 Injections: Standard for severe deficiencies and conditions like pernicious anemia, bypassing the digestive system.
• High-Dose Oral Supplements: Effective for some cases, utilizing passive diffusion for absorption.
• Treating Underlying Conditions: Addressing issues like Crohn's, SIBO, or medication effects is crucial.

Conclusion

Understanding why your body isn't absorbing B12 is vital for managing deficiency. Malabsorption can stem from various causes, including autoimmune diseases, digestive disorders, medications, and aging. Identifying the specific reason allows for appropriate treatment, such as injections or high-dose oral supplements, to correct the deficiency and prevent serious health problems. For more information, consult resources like the {Link: MSD Manuals https://www.msdmanuals.com/home/disorders-of-nutrition/vitamins/vitamin-b12-deficiency} or the NIH Office of Dietary Supplements.

NIH Office of Dietary Supplements: Vitamin B12 Fact Sheet

Frequently Asked Questions

What is the difference between dietary B12 deficiency and malabsorption?

Dietary B12 deficiency occurs when there is insufficient intake of the vitamin. Malabsorption happens when you consume enough B12, but your body cannot properly absorb and utilize it due to underlying medical issues.

Can I have a B12 deficiency even if my blood tests are normal?

Yes, it is possible to have a 'functional' B12 deficiency where blood tests are normal but the body has difficulty processing or utilizing the vitamin, potentially causing neurological symptoms.

Is pernicious anemia hereditary?

Pernicious anemia can run in families, suggesting a genetic predisposition. It's an autoimmune condition where the immune system attacks cells producing intrinsic factor.

How does metformin affect B12 absorption?

Metformin can interfere with B12 absorption. Long-term use may reduce serum B12 levels, requiring monitoring for potential deficiency.

How is B12 malabsorption treated?

Treatment depends on the cause. Injections are often necessary for most malabsorption cases to bypass faulty absorption. High-dose oral supplements can also be effective by using an alternative absorption method.

Can B12 deficiency cause neurological problems?

Yes, severe or long-term B12 deficiency can cause significant neurological problems, including nerve damage, memory loss, and confusion, which may become permanent if untreated.

Does gastric surgery impact B12 absorption?

Yes, surgeries like gastric bypass can significantly impair the ability to produce intrinsic factor and absorb B12. Lifelong supplementation is often required.