Vitamin B12 Deficiency and Your Genes: FUT2 and TCN2 Explained

Here's a question I get a lot: if I eat enough B12, why would my level ever be low? Your B12 status isn't just about how much meat, eggs, or fortified cereal you eat. Two genes, FUT2 and TCN2, help determine how efficiently your gut absorbs B12 and how well your blood carries it to your cells. Carry certain variants in either one, and you can eat a totally normal diet and still run low. That's the short version. Here's the longer one, because I think the details actually matter here.

B12 absorption is a multi-step process, and genes touch several steps

B12 doesn't just get absorbed the way sugar or salt does. It needs stomach acid to release it from food, a protein called intrinsic factor to shuttle it through your small intestine, and then a carrier protein to move it into your bloodstream once it's in. I went and read a 2023 BMJ review by Bruce Wolffenbuttel and colleagues on this, and it lays out the standard treatment response for deficiency: oral cyanocobalamin at 20 to 50 mcg daily, or 50 to 150 mcg daily, is what's most commonly recommended to prevent it. Notice that range. It's wide. People don't respond uniformly, and genetics is one reason why.

There's a 2020 Frontiers in Nutrition review I keep coming back to that catalogued the genetic variants known to shape blood levels of vitamins, B12 among them, across some genuinely large population studies. Diet explains part of the variation. Genetics explains another part. Neither one explains all of it, and I don't think anyone claims otherwise.

What FUT2 secretor status has to do with B12

FUT2 sorts people into two groups: secretors, who show certain blood-type antigens in their saliva, gut lining, and other secretions, and non-secretors, who don't. You've probably heard it mentioned for gut microbiome composition or norovirus susceptibility more than for vitamins. But secretor status also gets studied alongside circulating B12, because the same gut environment that FUT2 helps build affects how nutrients get processed on their way into your blood.

And here's the honest caveat, the one I want you to actually sit with. FUT2's link to B12 shows up in population genetics research on nutrient biomarkers. It is not a "if you have this variant, you will be deficient" rule. Full stop. Genetic variants like this nudge a population average. They don't diagnose you. I'd tell anyone: don't read a FUT2 result and assume you're deficient, and don't read it and assume you're fine, either. Get the blood test.

TCN2: the protein that carries B12 through your blood

TCN2 is more direct, and honestly it's the gene I find more interesting. It codes for transcobalamin II, the protein that binds B12 in your bloodstream and delivers it into your cells. Without functional transcobalamin, B12 can be sitting right there in your blood and still never reach the tissues that need it. Strange, when you think about it. Plenty of B12, wrong destination.

The 2018 Biomarkers of Nutrition for Development (BOND) review of vitamin B12, in The Journal of Nutrition, puts it plainly: "Genetic deficiency of transcobalamin is associated with severe B12 deficiency." That's an extreme case, a rare inherited transcobalamin deficiency, not the common TCN2 variant most of us carry. But it's the clearest illustration I've found of why the transport step matters just as much as the absorption step. A more common TCN2 variant is a milder version of the same idea: it can shift how efficiently B12 gets delivered, without causing a rare disease.

Who should watch their B12 more closely

Genetics is one layer. It sits on top of several other well-documented reasons B12 runs low. If more than one of these applies to you, I'd pay attention.

People on metformin. I'd flag this one first, because the evidence is strong and the drug is everywhere. A 2023 World Journal of Diabetes review found that B12 deficiency is repeatedly linked to malabsorption of B12 among people with type 2 diabetes treated with metformin. This is one of the best-documented drug-nutrient interactions in primary care. It stacks right on top of any genetic tendency toward lower absorption.

Older adults. B12 absorption declines with age on its own, largely because stomach acid production drops. A 2019 Mayo Clinic Proceedings review, also by Wolffenbuttel and colleagues, found that "even a dose of 10 μg/d can increase vitamin B12 levels to more than 200 pmol/L in elderly individuals (>65 years)." Small daily doses go a long way once you actually know you're low. Knowing matters more than guessing. It always does with this vitamin.

People planning a pregnancy. The 2021 PRIYA trial, published in Frontiers in Pediatrics, found that pre-conceptional vitamin B12 supplementation improved maternal B12 status and offspring neurodevelopment at two years of age. Worth sitting with that one for a second: two years out, and the effect was still measurable. A related 2018 review in the Annals of the New York Academy of Sciences noted that intervention studies have tested daily B12 doses ranging from 2.5 to 1000 μg and their effect on blood status, which gives you a sense of just how wide the effective range actually is.

People tracking mood or homocysteine. A 2025 systematic review in Frontiers in Nutrition described how deficiencies in B9 and B12 are associated with decreased neurotransmitter biosynthesis and higher homocysteine levels. Homocysteine is the marker most clinicians actually check when B12 status is in question. It's worth asking your doctor about if you fall into a higher-risk group.

None of this replaces a blood test. I'll say that twice if I have to. Genetics tells you where to look harder, not what your number actually is.

Where genetic testing fits in

A DNA panel adds context here, not a diagnosis, and I want to be blunt about that distinction because it gets blurred constantly in this industry. Biome's DNA panel reports on Vitamin B12 using the TCN2 rs1801198 and FUT2 rs602662 markers, flagged at a moderate evidence tier. That tier matters. It means researchers have found a real association between these variants and B12 levels in the population, not that a lab can read your DNA and hand you a deficiency verdict. A moderate-evidence marker is a reason to get your B12 checked periodically, particularly if you're also on metformin, over 65, or planning a pregnancy. It isn't a substitute for the check itself, and I'd be lying to you if I said otherwise.

If you're curious what your own results show, ask whatever testing company you use how it defines and labels evidence strength. And don't take a "moderate" finding as more certain than it is. It isn't.

FAQ

Can a genetic test tell me if I have a B12 deficiency? No. Genetic variants in FUT2 and TCN2 are associated with B12 levels at a population level, but a blood test is the only way to know your actual current status.

Is FUT2 secretor status the same thing as B12 deficiency risk? Not the same thing. Secretor status affects gut biology broadly, including microbiome composition, and shows up in nutrient-biomarker research, but it's one input among several, not a standalone risk score.

Does everyone with a TCN2 variant need extra B12? No. Most people who carry a common TCN2 variant absorb and transport B12 normally most of the time. The gene matters most when it combines with another risk factor, like metformin use or older age.

If I'm on metformin, how often should I check B12? That's really a conversation for your prescribing doctor, since the right interval depends on your dose, duration, and other labs. What the research supports is that metformin and B12 malabsorption are linked closely enough that periodic checking is standard practice for many clinicians.

What B12 dose actually raises blood levels? Trials cited in the research above have tested doses from as low as 2.5 μg up to 1000 μg daily, with even a 10 μg daily dose shown to meaningfully raise levels in adults over 65. The right dose for you depends on how deficient you are. So work it out with a blood test in hand, not a guess.