Vitamin B12 deficiency gets framed almost entirely as a dietary issue — something vegetarians and vegans need to worry about. That framing misses a substantial group of people who eat B12-rich food regularly and still become deficient, because the cause has nothing to do with their diet.
Why absorption, not just intake, is the real story for many people
B12 absorption is a genuinely complex, multi-step process: it needs to be released from food by stomach acid, bind to a protein called intrinsic factor produced in the stomach, and then be absorbed in a specific segment of the small intestine. A problem at any one of these steps can cause deficiency regardless of how much B12 is being eaten.
Common medications that quietly interfere with absorption
Long-term use of proton pump inhibitors and other acid-suppressing medications reduces the stomach acid needed to release B12 from food, and this effect compounds with duration of use — a real concern for anyone on these medications for years for reflux management. Metformin, one of the most widely prescribed diabetes medications, is separately associated with reduced B12 absorption with long-term use, and this connection is not as widely known or screened for as it should be.
Conditions that directly impair absorption
Pernicious anemia is an autoimmune condition that destroys the stomach cells producing intrinsic factor, directly blocking B12 absorption regardless of intake — this is a genuinely distinct mechanism from dietary deficiency and requires different, typically injectable, treatment since oral B12 cannot be reliably absorbed without intrinsic factor. Celiac disease and Crohn's disease, particularly when they affect the segment of small intestine where B12 is absorbed, cause deficiency through direct intestinal damage. Prior gastric or intestinal surgery, including some weight-loss surgeries, can remove or bypass the sites needed for normal absorption.
Why the symptoms are often subtle and easy to misattribute
Fatigue, difficulty concentrating, mood changes, and mild memory issues are common early symptoms and are easy to attribute to stress, ageing, or an unrelated cause. More specific but later-appearing signs include numbness or tingling in the hands and feet, difficulty with balance, and a sore or smooth-appearing tongue — these neurological symptoms are a signal not to delay further, since some nerve damage from prolonged, untreated deficiency can become permanent.
Why testing B12 alone sometimes misses the picture
A borderline-normal B12 level does not always rule out functional deficiency at the cellular level. When there is a strong clinical suspicion despite a borderline B12 result, additional markers — methylmalonic acid and homocysteine — can reveal a functional deficiency the initial B12 level alone did not clearly show.
What this means practically
If you are on long-term acid-suppressing medication or metformin, have an inflammatory bowel condition or celiac disease, have had relevant gastric or intestinal surgery, or have unexplained fatigue or neurological symptoms, B12 deficiency is worth actively testing for — not assumed absent simply because your diet includes meat, eggs, or dairy.
This article is educational and does not replace an individual consultation. If you are on long-term acid-suppressing medication or metformin, ask your doctor whether periodic B12 checking is appropriate for you.