Several groups face a significantly higher risk of vitamin B12 deficiency, including older adults, people who follow plant-based diets, individuals on certain long-term medications, and those with digestive conditions that impair absorption. The daily recommended intake for adults is 2.4 mcg, a small amount that most omnivores get easily from meat, fish, eggs, and dairy. But getting enough B12 into your bloodstream depends on a surprisingly complex chain of steps, and a breakdown at any point can leave you deficient.
Vegans and Vegetarians
People who eat little or no animal products are among the most consistently affected groups. B12 occurs naturally almost exclusively in animal-derived foods, so without fortified foods or supplements, plant-based eaters gradually deplete their stores. Among vegans, reported deficiency rates range from 5% to 52% depending on the population studied and how deficiency is measured. One UK study found that 52% of male vegans had deficient blood levels. Vegetarians fare somewhat better, with deficiency rates between 6% and 14%, likely because eggs and dairy provide some B12.
The wide range in those numbers reflects differences in supplement use. Vegans who consistently take a B12 supplement or eat fortified foods (like nutritional yeast, plant milks, or fortified cereals) can maintain normal levels without difficulty. The risk is highest among those who rely on whole-food plant diets without any supplementation strategy, and it climbs the longer someone follows the diet without addressing B12 intake.
Adults Over 60
Aging itself changes how your body handles B12. The stomach lining gradually thins in a condition called chronic atrophic gastritis, which becomes increasingly common after age 60. This matters because your stomach needs to produce both acid and a protein called intrinsic factor to extract B12 from food and absorb it in the small intestine. When the stomach lining deteriorates, production of both drops, and food-bound B12 passes through without being absorbed.
Roughly 2% of people over 60 to 70 develop pernicious anemia, an autoimmune form of this process where the body’s own immune system attacks the cells that produce intrinsic factor. It’s more common in people of European ancestry and in women, though it occurs across all populations. Pernicious anemia causes a particularly severe and progressive deficiency because B12 absorption is almost completely blocked. Younger people with this condition tend to show up with iron-deficiency anemia first, while those over 60 more typically present with the classic B12 deficiency picture: a specific type of anemia with enlarged red blood cells, along with neurological symptoms.
People on Long-Term Medications
Two widely prescribed drug categories interfere with B12 absorption over time: acid-reducing medications and metformin.
Proton pump inhibitors (PPIs) and H2 blockers, commonly taken for acid reflux and ulcers, work by suppressing stomach acid. That same acid is essential for releasing B12 from the proteins in food. Long-term use of either type of acid reducer lowers B12 absorption regardless of the specific drug. If you’ve taken one of these medications for more than a year or two, your B12 status is worth monitoring.
Metformin, one of the most commonly prescribed medications for type 2 diabetes, carries its own B12 risk. People who have taken metformin for more than three years are roughly 2.4 times more likely to be B12 deficient compared to those who have used it for a shorter period. The exact mechanism isn’t fully understood, but the association is strong enough that periodic B12 screening is a reasonable step for anyone on long-term metformin therapy.
People With Digestive Disorders
B12 absorption depends on a healthy terminal ileum, the very last stretch of the small intestine. Crohn’s disease frequently targets this exact segment of gut. Inflammation, bacterial overgrowth, and mucosal damage can all impair B12 uptake, but surgical removal of diseased portions of the terminal ileum is the single most important cause of B12 malabsorption in Crohn’s patients. Once that tissue is gone, oral B12 supplements may not be enough, and injections are often necessary.
Celiac disease, small intestinal bacterial overgrowth (SIBO), and other conditions that damage or inflame the gut lining can also reduce B12 absorption, though typically less dramatically than ileal resection.
People Who Have Had Weight Loss Surgery
Bariatric procedures, particularly gastric bypass and sleeve gastrectomy, alter the anatomy of the digestive tract in ways that reduce B12 absorption. After gastric bypass, the portion of the stomach that produces intrinsic factor is largely bypassed, and food takes a different route through the intestines. Research tracking patients after both types of surgery found that B12 status began declining within two months, with more pronounced drops by six months. B12 absorption capacity measurably decreased following both procedures. Lifelong supplementation is standard practice after bariatric surgery for this reason.
Breastfed Infants of Deficient Mothers
Infants are entirely dependent on their mother’s B12 stores during pregnancy and breastfeeding. When a mother is deficient, her breast milk delivers inadequate B12 to the baby, and symptoms in the infant typically appear between 2 and 12 months of age. These can include vomiting, lethargy, failure to thrive, low muscle tone, and a halt or reversal of developmental milestones. About half of affected infants develop abnormal movements like tremors, twitching, or involuntary jerking.
The consequences can be severe. B12 deficiency in infants disrupts the formation of myelin, the protective insulation around nerve fibers. Brain imaging in deficient infants has shown cortical atrophy, thinning of key brain structures, and delayed myelination. This risk is highest among infants of mothers who follow strict vegan diets without supplementation, but it also applies to any mother whose own B12 levels are low, whether from diet, malabsorption, or undiagnosed pernicious anemia. Pregnant and breastfeeding women need 2.6 mcg of B12 daily, slightly more than the standard adult recommendation.
How Deficiency Is Detected
Standard blood tests measure serum B12 levels, but the results can be misleading. A level below 200 pg/mL is generally considered deficient, but subclinical deficiency, where stores are low enough to cause problems without flagging as overtly abnormal, can occur at levels below 300 pg/mL. In one study, only 1.6% of patients were deficient using the stricter cutoff, but 8.9% qualified as deficient using the higher threshold.
A more sensitive approach is testing for methylmalonic acid (MMA), a compound that builds up in the blood when B12 is insufficient. MMA levels above 260 nmol/L suggest functional deficiency even when serum B12 looks borderline normal. In the same study, 10.8% of patients had elevated MMA, catching cases that serum B12 alone would have missed. If you fall into any of the higher-risk groups above and a standard B12 test comes back in the low-normal range, asking about MMA testing can provide a clearer picture.