Vitamin B12: Who's at Risk of Deficiency and How to Tell

Vitamin B12 is unusual among nutrients in how narrowly it occurs in nature. Outside of foods derived from animals — meat, fish, eggs, and dairy — and a handful of fortified products, it’s genuinely hard to come by in a normal diet. That single fact does most of the work in explaining who ends up at risk, and why deficiency can develop slowly and quietly, sometimes for years, before it’s caught.

Why B12 is so easy to fall short on

B12 (cobalamin) is made by certain bacteria and archaea, not by plants or animals themselves. Animals accumulate it by eating microbially contaminated feed or, in the case of ruminants, hosting B12-producing bacteria in their gut. The upshot is that plant foods, in their natural state, contain little to no reliably usable B12 — which is different from most other vitamins, where at least some plant source exists even if it’s not the richest one.

The body also stores several years’ worth of B12 in the liver under normal circumstances, and it recycles a portion of what it uses through a process called enterohepatic circulation. That’s part of why deficiency, when it happens from low intake alone, tends to build gradually rather than showing up quickly the way some other nutrient shortfalls might.

Who’s actually at risk

Deficiency risk generally comes from one of two directions: not taking in enough B12, or not absorbing it well even when intake looks adequate. A few groups show up repeatedly in discussions of who’s most affected:

• Older adults. Absorbing B12 from food requires adequate stomach acid and a protein called intrinsic factor. Both can decline with age, and a condition called atrophic gastritis — which becomes more common later in life — can reduce absorption even when someone eats plenty of B12-containing food.
• People eating vegan or largely plant-based diets. Without fortified foods or supplements, a diet with little to no animal products may not supply meaningful B12 over time. This applies to strict vegans most directly, though some vegetarians who eat little dairy or egg may also be at some risk.
• People taking metformin long-term. Metformin is one of the most commonly prescribed medications for type 2 diabetes, and long-term use has been associated with lower B12 levels in some people, possibly related to effects on absorption in the small intestine.
• People on long-term acid-reducing medications, including proton pump inhibitors (PPIs) and H2 blockers. Because stomach acid helps release B12 from food, reduced acid production over extended periods may lower absorption for some users.
• People with certain digestive conditions or a history of gut or stomach surgery, such as Crohn’s disease, celiac disease, weight-loss (bariatric) surgery, or other procedures that alter the stomach or small intestine where B12 absorption happens.
• People with pernicious anemia, an autoimmune condition where the body doesn’t make enough intrinsic factor, making B12 from food essentially unabsorbable regardless of intake.
• People who drink alcohol heavily or regularly, which may affect both intake and absorption over time.

Falling into one of these groups doesn’t mean deficiency is guaranteed — many people in these categories maintain adequate B12 status, especially with attention to diet or supplementation. It does mean B12 may be worth a conversation with a healthcare provider, particularly if any of the symptoms below are also present.

The symptoms that are easy to miss

This is arguably the trickiest part of B12 deficiency: it can affect the nervous system in ways that show up before, or even without, the changes in blood cells that many people associate with the condition.

Signs sometimes linked to low B12 include:

• Persistent fatigue or low energy that doesn’t have an obvious explanation
• Numbness or tingling in the hands or feet
• Balance problems or unsteadiness
• Difficulty concentrating or a sense of mental fogginess
• Memory changes
• Mood changes, including low mood or irritability
• A sore or unusually smooth-feeling tongue, or mouth sensitivity

None of these are specific to B12 — they overlap with dozens of far more common conditions, from thyroid issues to sleep deprivation to ordinary stress. That’s exactly why they’re easy to dismiss or attribute to something else, and why B12 deficiency can go unrecognized for a long stretch.

A related wrinkle is that a routine complete blood count can look reassuringly normal in some people with genuinely low B12, particularly if folate intake (from diet or supplements) is high enough to mask the anemia that would otherwise be a red flag. Because neurological symptoms don’t always track neatly with blood count changes, some clinicians will check B12 levels directly — rather than relying on a blood count alone — when deficiency is a reasonable concern based on someone’s risk factors or symptoms.

Food sources and where fortification fits in

For people without absorption issues, dietary sources of B12 include meat, poultry, fish and shellfish, eggs, and dairy products. Organ meats, particularly liver, tend to be especially concentrated sources. Fortified foods — some plant milks, nutritional yeasts, and breakfast cereals, for example — are formulated to supply B12 for people who don’t eat animal products, and checking a label is the only reliable way to know whether a given product actually contains it.

Because absorption, not just intake, matters so much for this particular nutrient, food sources alone may not be enough for everyone at elevated risk. Supplemental B12 — as an oral tablet, sublingual form, or in some cases an injection — is commonly used and, for people with absorption problems, may bypass some of the steps that intrinsic-factor-dependent absorption requires. What form and dose makes sense varies by person and underlying cause, which is a conversation for a clinician rather than a guess.

Getting tested

If you fall into one or more risk groups, or you’re noticing symptoms like the ones above, it’s reasonable to ask a healthcare provider about B12 testing rather than trying to interpret symptoms on your own. A clinician can weigh your history, medications, and diet, decide whether direct B12 testing (or additional markers some labs use) is appropriate, and figure out whether the issue is intake, absorption, or something unrelated to B12 entirely.

Related reading

• The importance of consuming vitamin D and vitamin K2 on a regular basis
• Signs of magnesium deficiency and why they are easy to miss
• Nose-to-tail eating: nutrients hiding in the parts we usually skip

This article is for general education only and is not a substitute for personalized medical advice — if you’re concerned about vitamin B12 deficiency, please talk with a qualified healthcare professional.