What Causes Low Vitamin B12?

Low vitamin B12 is rarely a single-cause problem. Most often, it develops slowly over months or years when your body cannot absorb the vitamin from food, when your diet does not provide enough of it, or when a medical condition destroys the cells that carry it. The most common cause is a stomach condition called pernicious anemia, which blocks absorption entirely. Other frequent causes include long-term use of acid-reducing medications, weight loss surgery, and digestive disorders like Crohn’s disease.

How Does the Body Normally Absorb Vitamin B12?

Understanding absorption helps explain why B12 deficiency happens. You cannot simply eat B12 and have it work. The process requires several steps.

First, stomach acid releases B12 from the protein in food. Then a protein made in the stomach, called intrinsic factor, binds to the free B12. The bound pair travels to the lower part of the small intestine, where it is absorbed into the bloodstream. Any break in this chain can cause deficiency.

This is why B12 problems are often absorption problems, not diet problems. A healthy diet cannot fix a broken absorption process.

What Causes Low Vitamin B12? The Main Medical Reasons

The most common cause is pernicious anemia. This is an autoimmune condition where the immune system attacks the cells in the stomach that produce intrinsic factor. Without intrinsic factor, B12 cannot be absorbed no matter how much you eat. Pernicious anemia is more common in older adults and in people with other autoimmune conditions like thyroid disease or type 1 diabetes.

Chronic inflammation of the stomach lining, called atrophic gastritis, also reduces B12 absorption. This condition thins the stomach lining and reduces both acid and intrinsic factor production. It is common in older adults and often goes unnoticed for years.

Digestive diseases that damage the small intestine can also cause deficiency. Crohn’s disease and celiac disease both inflame the intestinal lining where B12 is absorbed. Even if the stomach produces enough intrinsic factor, the damaged intestine cannot take up the vitamin.

Can Medications Cause Low Vitamin B12?

Yes. Several common medications interfere with B12 absorption or use.

Proton pump inhibitors (PPIs) like omeprazole, esomeprazole, and lansoprazole reduce stomach acid. Since acid is needed to release B12 from food, long-term use can lead to deficiency. The risk rises with duration of use, especially beyond two years.

Metformin, the most widely prescribed medication for type 2 diabetes, also lowers B12 levels. Studies consistently show that long-term metformin use is linked to lower B12 status. The effect is more pronounced with higher doses and longer treatment duration.

Some people taking these medications develop deficiency while others do not. Routine testing is reasonable for people on long-term therapy, especially if they have symptoms or other risk factors.

Can Surgery Cause Low Vitamin B12?

Surgery that removes or bypasses parts of the stomach or small intestine can cause B12 deficiency.

Weight loss surgery, particularly gastric bypass, is a well-documented cause. The surgery reroutes food past the stomach and upper small intestine, which are the primary sites of B12 absorption. Most people who have gastric bypass need lifelong B12 supplementation.

Surgical removal of part or all of the stomach, done for ulcers or cancer, also eliminates the cells that make intrinsic factor. This creates the same absorption problem as pernicious anemia.

People who have had ileal resection, surgery to remove part of the lower small intestine, can also develop deficiency because that is where B12 absorption actually occurs.

What Role Does Diet Play in B12 Deficiency?

Dietary deficiency is less common but still real. It develops when a person consumes virtually no animal products over a long period.

Vitamin B12 is found naturally only in animal foods: meat, fish, poultry, eggs, and dairy. Strict vegans who do not take supplements or eat fortified foods are at genuine risk. Vegetarians who eat eggs and dairy usually get enough, though some research suggests they may have lower stores than meat eaters.

Older adults can also develop dietary deficiency even when eating meat. As stomach acid production declines with age, the ability to extract B12 from food weakens. The body may still absorb B12 from supplements or fortified foods because these forms do not require acid for release.

What Are the Symptoms of Low Vitamin B12?

Symptoms develop gradually, which makes them easy to miss or attribute to aging.

Nerve damage is the most serious concern. It can cause numbness or tingling in the hands and feet, balance problems, and memory difficulties. These neurological symptoms can become permanent if the deficiency is not treated early.

Other common symptoms include fatigue, weakness, pale skin, shortness of breath, and a sore or smooth tongue. Some people experience mood changes or irritability. Anemia on a blood test is often the first clue.

Not everyone with low B12 has symptoms. Some people have laboratory deficiency without any noticeable problems.

How Is Low Vitamin B12 Diagnosed?

Diagnosis starts with a blood test. Doctors typically measure serum B12, but this test alone can miss some cases. A more sensitive marker called methylmalonic acid (MMA) rises when B12 is low at the tissue level. Homocysteine is another marker that can be elevated.

If the cause is unclear, doctors may test for antibodies against intrinsic factor or parietal cells. A positive test confirms pernicious anemia.

Treatment depends on the cause. For absorption problems, injections or high-dose oral supplements are standard. For dietary deficiency, oral supplements or dietary changes usually suffice. High-dose oral B12 works because a small percentage of the vitamin is absorbed passively without intrinsic factor, enough to correct deficiency when given in large amounts.

Who Is at Highest Risk for B12 Deficiency?

Certain groups carry higher risk and should consider testing even without symptoms.

Adults over 60 are at increased risk because stomach acid production declines with age. People with autoimmune diseases have higher rates of pernicious anemia. Those with digestive disorders, a history of stomach surgery, or long-term use of PPIs or metformin should also be monitored.

Strict vegans and vegetarians who do not supplement are at risk regardless of age. Pregnant women with low B12 may pass the deficiency to their infants, which is why prenatal screening matters for women with dietary restrictions or absorption issues.

Can Low B12 Be Prevented?

Prevention depends on the underlying cause. For dietary deficiency, eating animal foods or taking supplements prevents the problem. For absorption issues, prevention requires identifying the cause and supplementing accordingly.

People taking PPIs or metformin long-term should discuss B12 monitoring with their doctor. Those with known absorption problems need ongoing supplementation, often for life.

There is no evidence that high-dose B12 supplements prevent disease in healthy people with normal levels. The body excretes excess B12 in urine, so megadoses offer no benefit beyond correcting deficiency.