Low vitamin B12 comes down to two basic problems: you’re not getting enough from your diet, or your body can’t absorb what you’re eating. Most people assume it’s the first, but malabsorption is actually the more common culprit, especially after age 50. Understanding which category you fall into matters because the fix is completely different for each one.
Your Diet May Not Supply Enough
Vitamin B12 is found naturally only in animal products: meat, fish, eggs, and dairy. If you follow a vegan diet and don’t take a supplement or eat fortified foods, your levels will eventually drop. Vegetarians who eat eggs and dairy are less at risk but can still fall short if those foods aren’t regular staples. The timeline can be surprisingly slow. Your liver stores several years’ worth of B12, so someone who stops eating animal products today might not show deficiency symptoms for two to five years. That delay often makes the connection hard to spot.
Your Stomach May Not Be Doing Its Job
Before your body can use the B12 in food, your stomach has to do two things: release acid to separate B12 from the proteins it’s bound to, then produce a protein called intrinsic factor that escorts B12 to the part of your small intestine where it gets absorbed. If either step fails, the B12 in your meal passes right through you.
Gastritis, or inflammation of the stomach lining, is one of the most common causes of B12 deficiency. It reduces the acid your stomach produces, which means B12 stays locked to food proteins and never becomes available for absorption. This is especially relevant for older adults. The stomach lining naturally thins with age, a condition called atrophic gastritis, and the resulting drop in acid output directly limits how much B12 you can pull from food.
Heavy alcohol use causes a similar problem. Chronic drinking damages the digestive lining over time, impairing your body’s ability to absorb B12 and several other nutrients.
Pernicious Anemia: An Autoimmune Cause
Pernicious anemia is the most common cause of B12 deficiency in the UK and a major cause worldwide. It’s an autoimmune condition where your immune system attacks the cells in your stomach lining that produce intrinsic factor. Without intrinsic factor, B12 can’t bind to the receptor in your small intestine that pulls it into your bloodstream. It doesn’t matter how much B12-rich food you eat; the delivery system is broken.
Because the problem is absorption rather than intake, people with pernicious anemia typically need B12 injections rather than oral supplements. Doctors can test for specific antibodies against intrinsic factor and stomach lining cells to confirm the diagnosis.
Intestinal Conditions That Block Absorption
B12 is absorbed in the last section of your small intestine, called the distal ileum, through a specialized receptor. Any condition that damages or removes this stretch of intestine can cause permanent absorption problems. Crohn’s disease is a leading example, particularly when it affects the lower small intestine. Celiac disease, which damages the intestinal lining in response to gluten, can also interfere.
Surgical changes to your digestive tract carry the same risk. Gastric bypass and other bariatric surgeries reroute or remove portions of the stomach and small intestine, reducing both acid production and the absorptive surface area for B12. A gastrectomy, where part or all of the stomach is removed (often for cancer treatment), has the same effect. People who’ve had these procedures typically need lifelong B12 supplementation.
Medications That Lower B12
Several widely prescribed drugs interfere with B12 absorption, sometimes without patients or even their doctors connecting the dots.
- Metformin, the most commonly prescribed diabetes medication, lowers B12 through multiple pathways: it alters gut motility, promotes bacterial overgrowth, and reduces B12 uptake in the small intestine. Updated product information now lists B12 deficiency as a common side effect, potentially affecting up to 1 in 10 people taking the drug.
- Proton pump inhibitors (PPIs), used for acid reflux and heartburn, suppress stomach acid. Since acid is essential for releasing B12 from food, long-term PPI use can gradually deplete your stores.
- H2 blockers, another class of acid-reducing drugs, work through a similar mechanism.
- Oral birth control pills have also been associated with lower B12 levels, though the mechanism is less well understood.
If you’ve been on any of these medications for more than a year or two, it’s worth having your B12 checked.
How B12 Deficiency Is Diagnosed
A standard blood test measures serum B12 levels. Most labs define deficiency as levels below 200 to 250 pg/mL, but there’s a gray zone. If your results fall between 150 and 399 pg/mL, your doctor may order additional tests to see whether your cells are actually starved for B12 even if blood levels look borderline.
The most useful follow-up marker is methylmalonic acid (MMA), which builds up in your blood when B12 is too low to support normal cell chemistry. Homocysteine, another amino acid, also rises during B12 deficiency and has a sensitivity greater than 95% for identifying a functional shortage. If both are elevated alongside a borderline serum B12, the diagnosis is fairly clear. Some people also have what’s called a functional B12 deficiency, where blood levels appear adequate but the proteins that transport B12 between cells aren’t working properly, leading to neurological symptoms despite seemingly normal lab results.
Symptoms That Develop Over Time
B12 deficiency tends to creep up gradually, which is part of what makes it easy to dismiss. Early signs are often vague: fatigue, weakness, and general fogginess that could be blamed on stress or poor sleep.
As the deficiency deepens, two categories of problems emerge. On the blood side, your body can’t produce healthy red blood cells, leading to anemia. This shows up as pale skin, shortness of breath, and a rapid heartbeat as your cardiovascular system works harder to deliver oxygen. Severe anemia can strain the heart enough to cause heart failure.
The neurological effects are often more troubling and can occur even without anemia. Pins and needles in the hands and feet are a classic early sign, caused by damage to peripheral nerves. Left untreated, this can progress to difficulty walking, problems with balance and coordination, vision changes, and memory loss. Nerve damage from prolonged B12 deficiency can become permanent, which is why catching it early matters so much, particularly if you fall into any of the higher-risk groups: over 50, vegan or vegetarian, taking metformin or PPIs, or living with an autoimmune or digestive condition.