Scientists discovered that taking too much vitamin B12 might cause problems with how your body makes blood cells, especially if you have certain genetic changes. The research shows that excess B12 changes the helpful bacteria in your gut, which can lead to inflammation and increase the risk of blood cell disorders. The good news? The study found that a natural substance called butyrate (found in some foods) might reverse these harmful effects. This research was done in mice, so more studies are needed before we know if it applies to people.

The Quick Take

  • What they studied: Whether taking too much vitamin B12 could cause problems with blood cell growth and gut health, especially in people with certain genetic mutations
  • Who participated: Laboratory mice, some with a genetic change (TET2 mutation) that affects blood cell production, and some without this change
  • Key finding: High doses of B12 changed the gut bacteria in ways that increased inflammation and boosted abnormal blood cell growth in mice with the genetic mutation
  • What it means for you: If you take B12 supplements, this research suggests you should avoid excessive doses and talk to your doctor about the right amount for you. However, this study was done in mice, so we need human studies to confirm these findings apply to people.

The Research Details

Researchers used laboratory mice to study how vitamin B12 affects blood cell growth and gut bacteria. Some mice had a genetic change (TET2 mutation) that makes them more likely to develop blood cell disorders, while others were normal. The scientists gave some mice extra B12 and tracked how it changed their gut bacteria, inflammation levels, and blood cell production over time.

They measured several things: the types of bacteria living in the gut, levels of helpful substances produced by gut bacteria (called short-chain fatty acids), how much inflammation was happening in the body, and how many abnormal blood cells were being made. They also tested whether giving mice a substance called butyrate could fix the problems caused by too much B12.

This type of study is important because it allows scientists to carefully control conditions and measure specific effects that would be hard to study in people. However, results in mice don’t always translate directly to humans.

Understanding how B12 affects gut bacteria and blood cell growth is important because some people take B12 supplements without knowing if high doses are safe. This research suggests that the connection between B12 and blood cell problems might work through the gut microbiome, which opens up new ways to prevent or treat these conditions.

This study was published on a preprint server, meaning it hasn’t been reviewed by other scientists yet (peer review). The research was done carefully in controlled laboratory conditions, but it only involved mice, not humans. The findings are interesting but need confirmation in human studies before we can be confident they apply to people. The reversibility of effects with butyrate is a positive sign that the mechanism is real.

What the Results Show

When mice received high doses of B12, their gut bacteria changed significantly. Specifically, the bacteria that produce helpful short-chain fatty acids (especially butyrate) decreased. These short-chain fatty acids are important for keeping the gut lining healthy and reducing inflammation.

In mice with the TET2 genetic mutation, high B12 made the problem worse. These mice showed increased inflammation throughout their body and more abnormal blood cell production. Their immune cells became more reactive to bacterial signals, which means their bodies were responding too strongly to normal gut bacteria.

Interestingly, when researchers gave the mice butyrate (the helpful substance that was missing due to the B12-caused changes), the harmful effects of excess B12 went away. The inflammation decreased, and the abnormal blood cell growth slowed down. This suggests the problem wasn’t B12 itself, but rather how B12 changed the gut bacteria and reduced butyrate production.

The study found that high B12 affected both normal mice and mice with the genetic mutation, but the effects were much more serious in mice with the TET2 mutation. This suggests that people with certain genetic changes might be more vulnerable to B12-related problems. The research also showed that the inflammatory response in the body was directly connected to the changes in gut bacteria, supporting the idea that the gut microbiome is a key player in this process.

Earlier research had noticed that people with high B12 levels were more likely to develop blood cell disorders, but scientists didn’t understand why. This study provides a possible explanation: B12 might work through the gut microbiome. The findings fit with other research showing that gut bacteria play important roles in blood cell development and immune system function. However, most previous studies in humans were observational (just watching what happens), while this study actually tested the mechanism in a controlled way.

This research was done only in mice, so we don’t know for certain if the same thing happens in humans. The study didn’t test different doses of B12 to find a safe level. It also didn’t look at how long it takes for problems to develop or how common these problems are. The research focused on one specific genetic mutation (TET2), so we don’t know if other genetic changes would respond the same way. Finally, this is a preprint that hasn’t been reviewed by other scientists yet.

The Bottom Line

Based on this research, the recommendation is moderate confidence: avoid taking B12 supplements in doses higher than recommended by your doctor. If you take B12 supplements, discuss with your healthcare provider whether you actually need them and what dose is appropriate for you. If you have a family history of blood cell disorders, this conversation is especially important. This research also suggests that foods containing butyrate (like certain fermented foods) or fiber that helps your gut bacteria produce butyrate might be protective.

This research is most relevant to people who take B12 supplements without a medical reason, people with a family history of blood cell disorders or leukemia, and people with genetic mutations affecting blood cell production. People with documented B12 deficiency who need supplementation should continue taking it as prescribed by their doctor, but should discuss appropriate dosing. Healthy people who eat a balanced diet with B12-containing foods (meat, dairy, eggs) don’t need to worry about this research.

In the mice studied, changes in gut bacteria happened relatively quickly (within weeks), but the development of blood cell problems took longer. If these findings apply to humans, it might take months or years of excessive B12 supplementation before serious problems develop. The good news is that the effects appeared reversible when butyrate was added, suggesting that stopping excess B12 and improving gut health might help prevent progression.

Want to Apply This Research?

  • Track weekly B12 supplement doses (if taking them) and note any symptoms like unusual fatigue, bruising, or infections. Also track fiber intake and fermented food consumption, as these support healthy gut bacteria.
  • If you’re taking B12 supplements, set a reminder to discuss your dosage with your doctor. In the meantime, increase fiber intake and add fermented foods (like yogurt, sauerkraut, or kimchi) to support healthy gut bacteria that produce butyrate.
  • Monthly check-ins to review B12 supplement usage and adjust based on doctor recommendations. Track energy levels and any new symptoms. If you have risk factors for blood cell disorders, work with your doctor to establish baseline blood work and schedule regular monitoring.

This research was conducted in mice and has not yet been peer-reviewed. The findings do not currently apply to human recommendations. If you take B12 supplements or have concerns about blood cell health, consult with your healthcare provider before making any changes. People with documented B12 deficiency should continue prescribed supplementation. This article is for educational purposes and should not replace professional medical advice.