Understanding Nutrition and What Stops Your Body From Absorbing B12?

October 4, 2025 •

4 min read

According to the NIH Office of Dietary Supplements, up to 43% of older adults may have a vitamin B12 deficiency due to malabsorption issues. Understanding what stops your body from absorbing B12 is crucial for maintaining proper neurological function and red blood cell production, especially for those in at-risk groups.

Quick Summary

Several factors can hinder the body's ability to absorb vitamin B12, including autoimmune conditions, gastrointestinal disorders, certain medications, and dietary choices. The complex process of B12 absorption relies on adequate stomach acid and a protein called intrinsic factor, which can be disrupted by various health issues and treatments.

Key Points

Intrinsic Factor Deficiency: Pernicious anemia, an autoimmune disease, is a major cause of B12 malabsorption by preventing the production of intrinsic factor, a protein essential for absorption.
Reduced Stomach Acid: Conditions like atrophic gastritis and long-term use of acid-reducing medications (e.g., PPIs, H2 blockers) can decrease the stomach acid needed to release B12 from food.
Intestinal Damage: Gastrointestinal disorders such as Crohn's disease, celiac disease, and small intestinal bacterial overgrowth (SIBO) can impair absorption in the ileum.
Surgical Alterations: Procedures involving the removal or bypass of parts of the stomach or small intestine (e.g., bariatric surgery) can significantly reduce B12 absorption.
Medication Side Effects: Common drugs like metformin for diabetes and gastric acid inhibitors can interfere with the absorption process over time.
Dietary and Lifestyle Choices: A strict vegan diet without supplementation can lead to deficiency due to inadequate intake, while chronic alcohol abuse can damage the digestive lining.

The Complex Journey of B12 Absorption

To understand what can go wrong, it's essential to first know how the body absorbs vitamin B12 when everything is working correctly. Vitamin B12, or cobalamin, is naturally found in animal products and is bound to protein.

Release in the Stomach: When B12-containing food is ingested, hydrochloric acid and the enzyme pepsin in the stomach release the vitamin from its protein bond.
Binding with Intrinsic Factor: The free B12 then combines with a special protein called intrinsic factor, which is also produced in the stomach.
Absorption in the Small Intestine: The B12-intrinsic factor complex travels to the last part of the small intestine (the ileum), where it is absorbed into the bloodstream.

Any interruption in this intricate, multi-step process can lead to malabsorption and, eventually, a B12 deficiency.

Autoimmune and Gastric Causes

One of the most common reasons for impaired B12 absorption stems from issues within the stomach itself. Autoimmune disorders and conditions affecting stomach acid production are primary culprits.

Pernicious anemia is a significant cause of B12 deficiency, particularly in certain populations, resulting from an autoimmune attack on stomach cells that produce intrinsic factor, a protein vital for B12 absorption. Atrophic gastritis, a chronic inflammation of the stomach lining common in older adults, also reduces the production of both stomach acid and intrinsic factor, hindering the release and binding of B12 from food. Furthermore, chronic H. pylori infection can contribute to atrophic gastritis and subsequent B12 malabsorption.

Medical Conditions Affecting Intestinal Health

Problems beyond the stomach in the small intestine, where B12 is absorbed, can also impede uptake. Inflammatory bowel diseases like Crohn's and celiac disease can damage the ileum, the site of B12 absorption. Small Intestinal Bacterial Overgrowth (SIBO) can lead to bacteria consuming B12 before the body can absorb it. Chronic pancreatitis can disrupt the process of freeing B12 to bind with intrinsic factor. Rare genetic disorders and infections like fish tapeworm can also interfere with B12 absorption.

The Impact of Medications and Surgery

Certain medications and surgical procedures can significantly affect B12 absorption.

Common Drugs That Block B12 Absorption

Medications that reduce stomach acid, such as Proton Pump Inhibitors (PPIs) and H2 blockers, can impair B12 absorption from food. Metformin, a diabetes drug, may also reduce B12 absorption. Other drugs like some anticonvulsants, colchicine, and certain antibiotics have also been associated with lower B12 levels.

Surgical Impact on B12 Absorption

Surgical procedures like gastric bypass and other bariatric surgeries that alter the stomach can remove the source of intrinsic factor and acid essential for absorption. Removal of the ileum (ileal resection) also directly impacts absorption, as this is where the B12-intrinsic factor complex is taken up.

Comparison of B12 Malabsorption Causes


Cause	Mechanism of Action	At-Risk Population	Diagnosis	Management
Pernicious Anemia	Autoimmune destruction of parietal cells, leading to intrinsic factor deficiency.	Individuals, particularly of northern European descent.	Presence of anti-intrinsic factor antibodies.	B12 injections for life.
Low Stomach Acid	Insufficient hydrochloric acid fails to release B12 from food proteins.	Older adults, individuals using acid-reducing drugs.	Gastric analysis, therapeutic trial with high-dose oral B12.	Oral supplements may be effective as they don't require intrinsic factor.
Bacterial Overgrowth (SIBO)	Competing bacteria in the small intestine consume ingested B12.	Individuals with intestinal motility disorders or altered anatomy.	Breath tests, endoscopic sampling.	Antibiotics to treat bacterial overgrowth.
Gastric Surgery	Removal of stomach tissue, which produces intrinsic factor and acid.	Post-bariatric surgery or gastrectomy patients.	Medical history, B12 level monitoring.	B12 injections or high-dose oral supplements.
Crohn's Disease	Inflammation damages the ileum, preventing absorption.	Individuals with this specific inflammatory bowel disease.	Colonoscopy, imaging, blood tests.	Management of the underlying disease, B12 injections.

Dietary and Lifestyle Factors

A strict vegan or vegetarian diet that doesn't include fortified foods or supplements is a risk factor for B12 deficiency due to insufficient intake, as B12 is primarily found in animal products. Chronic, heavy alcohol consumption can also impair B12 absorption by damaging the stomach and intestinal lining.

Conclusion

Numerous factors can disrupt the body's ability to absorb vitamin B12, ranging from autoimmune conditions and gastrointestinal disorders to medications, surgeries, and dietary choices. The absorption process, dependent on stomach acid and intrinsic factor, can be hindered at various stages. Recognizing the symptoms of deficiency, such as fatigue and neurological issues, is important for early diagnosis and treatment. Depending on the cause, management may involve B12 injections, high-dose oral supplements, or addressing the underlying health issue. Consulting a healthcare professional is advisable for anyone concerned about their B12 status.

Frequently Asked Questions

Yes, individuals with certain absorption issues, such as low stomach acid, often absorb B12 more effectively from supplements than from food. High-dose oral supplements or B12 injections are common treatments for severe malabsorption.

Symptoms can develop gradually and include fatigue, weakness, a sore tongue, numbness or tingling in the hands and feet, memory problems, and balance issues. In severe cases, it can lead to anemia and neurological damage.

Yes, as people age, their bodies often produce less stomach acid, which makes it harder to absorb B12 from food. Atrophic gastritis, a common condition in older adults, also reduces intrinsic factor production.

Yes, chronic use of antacids, including proton pump inhibitors (PPIs) and H2 blockers, can suppress stomach acid production. This reduces the body's ability to release B12 from its food protein, leading to malabsorption.

Crohn's disease can cause inflammation and damage to the ileum, the specific part of the small intestine where B12 is absorbed. This direct damage prevents the uptake of the nutrient.

A vegan diet does not cause malabsorption, but it can lead to a B12 deficiency due to inadequate intake, as B12 is naturally found only in animal products. Vegans who don't supplement are at high risk.

Intrinsic factor is a protein secreted by the stomach's parietal cells. It binds to B12 in the digestive tract and is essential for the vitamin's absorption in the ileum. Without intrinsic factor, B12 passes through the intestines unabsorbed.