People carrying a genetic variant in the MMACHC gene have higher blood B12 levels but their cells may not be using it effectively, according to a Gram Research analysis of 250,000+ people in the All of Us Research Program. These carriers also showed elevated homocysteine, a marker indicating poor cellular B12 function, suggesting that high circulating B12 can actually reflect impaired cellular handling rather than improved B12 status. This finding reveals a diagnostic blind spot: standard B12 blood tests may appear normal or high in people whose cells are genuinely B12-deficient.
Scientists studying genetic data from hundreds of thousands of people discovered something surprising: some folks carry a genetic variant that makes their blood B12 levels look high, but their bodies might not actually be using it well. According to Gram Research analysis, people with this MMACHC gene variant had higher circulating B12 but also higher homocysteine levels—a sign that the B12 isn’t working properly inside their cells. This finding could explain why some people feel B12-deficiency symptoms even when their blood tests look normal, and it highlights why doctors might need better tests beyond just checking B12 numbers.
Key Statistics
A 2026 analysis of over 250,000 people in the All of Us Research Program found that carriers of rare MMACHC gene variants had significantly higher circulating vitamin B12 levels but also elevated homocysteine, suggesting impaired cellular B12 utilization despite high blood B12.
Researchers identified that genes involved in cellular B12 delivery and intracellular processing were associated with higher circulating B12 levels, while genes involved in intestinal absorption were associated with lower levels, supporting a model of systemic B12 accumulation due to poor cellular handling.
The study revealed that MMACHC variant carriers were less likely to fall below conventional B12 insufficiency thresholds, creating a potential diagnostic blind spot where standard B12 blood tests may appear normal despite functional B12 deficiency at the cellular level.
The Quick Take
- What they studied: Whether rare genetic variants in the MMACHC gene (which helps process vitamin B12) affect how much B12 shows up in people’s blood and how well their bodies actually use it.
- Who participated: Over 250,000 people from the All of Us Research Program, a large national study collecting health information and genetic data from diverse Americans.
- Key finding: People carrying one copy of the MMACHC variant had higher blood B12 levels than people without the variant, but they also had higher homocysteine—a marker suggesting their cells weren’t using the B12 effectively.
- What it means for you: If you have B12 symptoms but normal blood B12 levels, your doctor might need to order additional tests (like homocysteine or methylmalonic acid) rather than relying on B12 numbers alone. This is especially important if you have a family history of B12 metabolism problems.
The Research Details
Researchers used a massive database of genetic information and health records from the All of Us Research Program to search for connections between rare genetic variants and measurable health traits. They focused on the MMACHC gene, which produces a protein that helps cells process and use vitamin B12. Instead of studying just one gene, they looked at the overall “burden” of rare variants—essentially counting up how many uncommon genetic changes each person carried in this gene.
They then performed what’s called a “phenome-wide association screen,” which is like casting a wide net to see what health measurements were most strongly connected to these genetic variants. They looked at hundreds of different blood markers and health measurements to find which ones were most affected. They also did additional analysis to understand whether similar patterns appeared in other genes involved in B12 handling throughout the body.
The researchers adjusted their analysis for factors that could confuse the results, like age, sex, and ancestry, to make sure they were seeing a real genetic effect rather than other differences between groups.
This research approach is important because it uses real-world health data from a huge, diverse population rather than small laboratory studies. It also looks at the actual biological pathway—following B12 from the bloodstream into cells—to understand where problems might occur. By examining multiple genes involved in different steps of B12 handling, the researchers could figure out that high blood B12 might actually signal a problem with getting B12 into cells, not a benefit.
This study has significant strengths: it examined a very large, diverse population (over 250,000 people), used standardized genetic testing, and had access to actual medical records and lab results. However, because it’s observational rather than experimental, it can show associations but not prove cause-and-effect. The study was published as a preprint, meaning it hasn’t yet gone through formal peer review. The researchers themselves note that real-world medical records are messy—people take different supplements, medications, and have different diets, which can all affect B12 levels in ways the study couldn’t fully account for.
What the Results Show
The main finding was striking: people carrying one copy of the MMACHC variant had significantly higher circulating B12 levels compared to people without the variant. However, this high B12 didn’t appear to be a good thing. These same carriers also had elevated homocysteine levels, which is a marker that suggests cells aren’t processing B12 properly. Homocysteine is an amino acid that accumulates when B12 isn’t working correctly inside cells, so high homocysteine alongside high B12 paints a picture of B12 that’s stuck in the bloodstream rather than being used where it’s needed.
Another important finding was that carriers were less likely to have low blood B12 levels. This creates what researchers call a “diagnostic blind spot”—doctors might see normal or high B12 numbers and assume everything is fine, when in reality the person’s cells might not be getting enough B12 to function properly. This could explain why some people report B12-deficiency symptoms (like fatigue, numbness, or brain fog) even though their standard B12 blood test comes back normal.
When researchers looked at other genes involved in B12 handling, they found a clear pattern: genes involved in getting B12 into cells and processing it inside cells were associated with higher circulating B12, while genes involved in absorbing B12 from food or carrying it in the blood were associated with lower circulating B12. This step-by-step pattern supports the idea that the body compensates for poor cellular B12 handling by accumulating more B12 in the bloodstream.
The broader gene-burden analysis revealed important biological patterns. Genes involved in intestinal B12 absorption and blood B12 binding showed the expected pattern: when these genes had rare variants, people had lower circulating B12. But genes involved in the later steps—cellular delivery and intracellular processing—showed the opposite: variants in these genes were associated with higher circulating B12. This suggests the body tries to compensate for poor cellular handling by keeping more B12 in circulation, hoping some will eventually get into cells.
This research builds on decades of understanding about B12 metabolism but adds an important new perspective. Previous research has established that B12 deficiency causes serious health problems, and doctors have relied on blood B12 levels as the main diagnostic tool. However, this study supports growing evidence that blood B12 levels alone don’t tell the whole story. Some previous smaller studies have suggested that people with certain genetic conditions affecting B12 metabolism can have normal or even high blood B12 while still experiencing deficiency symptoms. This large population study provides strong evidence for that phenomenon and identifies a specific genetic cause.
The researchers acknowledge several important limitations. First, this is an observational study using medical records, which means people’s B12 levels were measured at different times, under different conditions, and influenced by different medications and supplements they were taking. The study couldn’t account for all these variables. Second, the study looked at people who already had genetic testing done as part of All of Us, which might not represent the general population perfectly. Third, while the study found associations between the MMACHC variant and B12 levels, it didn’t directly measure whether carriers actually had B12 deficiency symptoms or functional problems. The researchers recommend that future studies should specifically recruit carriers of this variant and measure not just B12 and homocysteine, but also methylmalonic acid (another marker of cellular B12 function), along with detailed information about diet, supplements, medications, and actual symptoms.
The Bottom Line
If you have symptoms of B12 deficiency (fatigue, numbness, tingling, memory problems, or mood changes) but your doctor says your B12 level is normal or high, ask about additional testing. Specifically, request homocysteine and methylmalonic acid tests, which better reflect whether your cells are actually using B12 properly. If you have a family history of B12 metabolism problems or cobalamin C deficiency, inform your doctor, as this may warrant more thorough testing. These recommendations have moderate confidence because they’re based on observational data; more research is needed to confirm optimal screening approaches.
This research is most relevant for people experiencing B12-deficiency symptoms despite normal B12 blood tests, people with a family history of B12 metabolism disorders, and healthcare providers who care for patients with unexplained neurological symptoms. It’s also important for people considering genetic testing, as it highlights that genetic variants can have subtle effects on metabolism. This doesn’t apply to people with confirmed B12 deficiency or those taking B12 supplements for diagnosed deficiency—they should continue their current treatment.
If you have a genetic variant affecting B12 handling, the effects are likely ongoing and not something that will change quickly. However, if you start taking B12 supplements or change your diet based on new testing, you might notice improvements in energy and symptoms within weeks to months. It typically takes several months of consistent B12 supplementation to fully correct deficiency symptoms.
Frequently Asked Questions
Can you have normal B12 levels but still be B12 deficient?
Yes. Research shows that blood B12 levels don’t always reflect how well your cells are actually using B12. Some genetic variants cause B12 to accumulate in the bloodstream while cells remain deficient. Doctors may need to order additional tests like homocysteine or methylmalonic acid to assess true cellular B12 status.
What does homocysteine tell you about B12 deficiency?
Homocysteine is an amino acid that accumulates when B12 isn’t working properly inside cells. High homocysteine alongside high blood B12 suggests B12 is stuck in circulation rather than being used by cells. This marker can reveal functional B12 deficiency that standard B12 tests might miss.
Should I get genetic testing for MMACHC variants?
Genetic testing for MMACHC variants may be helpful if you have B12-deficiency symptoms despite normal blood B12 levels, or if you have a family history of B12 metabolism disorders. Discuss with your doctor whether testing makes sense for your situation, as it requires clinical indication.
What are the symptoms of B12 deficiency I should watch for?
Common B12-deficiency symptoms include fatigue, weakness, numbness or tingling in hands and feet, memory problems, difficulty concentrating, mood changes, and pale or yellowish skin. If you experience these symptoms, ask your doctor for comprehensive B12 testing beyond just blood B12 levels.
How can I improve my B12 if my cells aren’t using it well?
Work with your doctor to develop a supplementation strategy, which might include B12 injections (which bypass absorption issues) or high-dose oral supplements. Track your symptoms over weeks to months to see if supplementation helps. Regular monitoring with additional markers like homocysteine can guide treatment adjustments.
Want to Apply This Research?
- Track your energy levels, mental clarity, and any numbness or tingling sensations daily on a 1-10 scale. If you get genetic testing or additional B12 markers tested, log those results and note the date. This creates a personal record showing whether your symptoms improve when you address B12 metabolism, independent of what your standard B12 blood test shows.
- If you suspect B12 metabolism issues, work with your doctor to establish a B12 supplementation plan and use the app to track whether you feel better. You might try B12 injections or high-dose oral supplements and monitor your symptoms weekly. Log any dietary changes (like adding B12-rich foods) and note how you feel.
- Set quarterly reminders to reassess your symptoms and request updated lab work (B12, homocysteine, and methylmalonic acid if possible). Use the app to track patterns—do your symptoms worsen at certain times? Do they improve with supplementation? Share this data with your doctor to help guide treatment decisions.
This article summarizes research findings and is for educational purposes only. It is not medical advice. If you experience symptoms of B12 deficiency (fatigue, numbness, tingling, memory problems, or mood changes), consult your healthcare provider for proper evaluation and testing. Do not start, stop, or change any B12 supplementation or treatment without discussing it with your doctor first. Genetic testing and interpretation should be done under medical supervision. This research is based on a preprint that has not yet undergone formal peer review.
This research translation is published by Gram Research, the science division of Gram, an AI-powered nutrition tracking app.