Why Is My B12 Low If I'm Not Vegetarian or Vegan?

What this actually measures

A serum or plasma B12 test measures the vitamin circulating in blood at the moment of the draw, used as a stand-in for whole-body B12 status. It doesn't directly measure how much is stored in the liver or how well the last meal's B12 was actually absorbed, which is why a borderline number is often followed by a more specific functional marker, methylmalonic acid, rather than a repeat of the same test.

The assumption that gets in the way

Search “low B12” and the results assume a plant-based diet before they assume anything else. That assumption is reasonable often enough to have become the default, but it doesn’t hold for a reader who already eats meat, fish, eggs or dairy and still got a low number back. For that reader the diet story is usually wrong, and the more useful question is a physical one: where between the stomach and the small intestine did the vitamin actually stop getting through.

Eating it and absorbing it are two different jobs

B12 doesn’t move from plate to bloodstream in one step. Food-bound B12 first has to be split away from the protein it’s attached to, which needs stomach acid and an enzyme called pepsin. The freed B12 then has to bind a protein made by the stomach’s own cells, intrinsic factor, and that pairing is what actually gets picked up — not by the stomach, but much further along, in the final stretch of the small intestine called the distal ileum.

Three separate points of failure, none of which has anything to do with how much B12 was on the plate. That’s the structural reason a diet-first explanation misses so often in someone who isn’t restricting animal foods: the chain has three links that have nothing to do with intake, and any one of them breaking produces the same low number on the same test.

Why “not vegetarian” doesn’t rule this out

It’s worth being specific about how small the intake side of this actually is. NIH’s data put the median daily B12 intake for the US population at 3.4 micrograms, comfortably above the 2.4-microgram RDA for adults, and most people eating any meat, fish, dairy or eggs clear that easily — a few ounces of salmon or a glass of milk covers the day several times over. When intake is already generous, a low result stops pointing at the plate and starts pointing at everything downstream of it.

NIH’s own fact sheet frames the main causes of B12 deficiency as vitamin B12 malabsorption from food, pernicious anaemia, and postsurgical malabsorption, with straightforward dietary deficiency listed as a separate and, for a non-vegetarian, less likely cause. It also states plainly that in many cases the specific cause is never identified — which is a more honest place to land than forcing every low result into a single tidy explanation.

Four things that explain it far more often than diet does

Getting older changes your stomach chemistry. Atrophic gastritis — a gradual decline in stomach acid production — is common with age, affecting an estimated 10 to 30 percent of older adults, and it directly reduces how much B12 gets separated from food protein in the first place. This is specific enough that health guidance recommends adults over 50 get a meaningful share of their B12 from supplements or fortified food, because the synthetic form in those products is already free and doesn’t need stomach acid to release it.

An autoimmune reaction against your own stomach. Pernicious anaemia is an autoimmune condition, affecting roughly 1 to 2 percent of older adults, in which the immune system attacks the stomach cells that make intrinsic factor. Without intrinsic factor, B12 largely can’t be absorbed regardless of how much is eaten, which is why pernicious anaemia is usually treated with injected B12 rather than an oral dose — the oral route depends on the exact absorption step the condition disables.

Gut conditions and past surgery. Coeliac disease and Crohn’s disease can damage or bypass the section of intestine that does the actual absorbing, and gastric or bariatric surgery removes or alters the stomach cells that make acid and intrinsic factor in the first place. Surgery that removes the distal ileum has the same effect from the other end of the chain — the receptors that pick up B12 are simply no longer there.

A medication you might already be taking. Metformin is the one that surprises people, because it’s prescribed for blood sugar, not digestion. NIH’s fact sheet reports that 10 to 30 percent of people on metformin show reduced B12 absorption in small studies, and one randomised, placebo-controlled trial found that 4.3 years of metformin use lowered B12 levels by 19 percent and raised the risk of deficiency by 7.2 percent compared with placebo. Long-term proton-pump-inhibitor use is also on the list, though NIH is upfront that the evidence on whether PPIs actually change B12 status is conflicting rather than settled — which is a more honest position than treating it as proven either way, and the practical response is monitoring rather than alarm.

What the number on the report is actually telling you

Reference ranges for B12 vary more than most bloodwork, and two respectable sources drawing the line in different places is itself informative rather than a contradiction. MedlinePlus lists a typical US lab band of roughly 299 to 1,054 pg/mL, with anything under about 299 pg/mL flagged as a possible sign of deficiency. NIH’s Office of Dietary Supplements uses a lower, firmer threshold — below approximately 170 to 250 pg/mL — to call deficiency outright.

A result sitting between those two bands — low-normal on one scale, not yet deficient on the other — is a genuinely ambiguous number, not a rounding error. That’s the zone where methylmalonic acid earns its place: NIH’s fact sheet describes it as a more specific indicator of true B12 deficiency than homocysteine, because homocysteine also climbs with low vitamin B6 or folate and can’t tell those apart from B12 on its own.

There’s a second reason the plain number can be misleading in either direction: NIH’s guidance also notes that folate can correct the megaloblastic anaemia B12 deficiency causes without correcting the nerve damage that comes with the same deficiency, which is part of why a normal-looking blood count and a borderline B12 result together don’t fully close the question the way a single reassuring number might suggest.

Why this is graded moderate, not strong

The mechanism at the centre of this — that absorption, not intake, is the more common failure point once someone is already eating animal products — comes directly from NIH’s own framing of the main causes of B12 deficiency, and that part is about as solid as this kind of claim gets.

It’s graded moderate rather than strong for two honest reasons. First, NIH itself states that in many individual cases the specific cause of a deficiency is never identified, so “which of these four explains your result” is a question this guide can narrow, not answer. Second, the evidence on at least one commonly cited contributor — long-term PPI use — is explicitly described by NIH as conflicting, and repeating it as settled fact would overstate what’s actually known. Where the metformin figures come from a specific randomised trial, they’re reported here as that trial’s finding, not as a universal rule for every person on the medication.

When to stop reading and get it checked

A mildly low B12 with no symptoms, in someone who isn’t in any of the higher-risk groups above, is usually a repeat-test-and-investigate situation rather than an emergency — worth raising with whoever ordered the test, not worth panicking over on its own.

That changes if numbness or tingling in the hands or feet, unsteady balance, or new confusion shows up alongside the number. Those are neurological signs of B12 deficiency, and NIH’s own guidance is specific that they can appear before — or without — any change on a standard blood count, and that the nerve damage they signal can become permanent if the underlying deficiency isn’t found and corrected. That combination is a reason to see a clinician promptly rather than start a supplement on your own schedule, particularly because self-treating before the right blood test can make the result harder to interpret afterward.

The practical version

• Ask which reference range your lab used, and whether the result is closer to the NIH deficiency band or just under a more generous “normal” cutoff — those are different situations with the same word attached to them.
• If the number is borderline and the symptom picture doesn’t match a comfortable result, ask whether methylmalonic acid is worth adding, since it’s built specifically to resolve that ambiguity.
• Mention metformin or any long-term acid-reducing medicine to whoever is reviewing the result — both are recognised, testable contributors that have nothing to do with what’s on your plate.
• Treat neurological symptoms — numbness, balance changes, new confusion — as a reason to move faster than a routine repeat test would suggest.