A new study in rats shows that taking too much vitamin B12 during pregnancy might not be as safe as we thought. While we know that not getting enough B12 during pregnancy can harm a baby’s health, researchers wanted to see what happens when pregnant mothers take way too much. They found that excess B12 caused stress inside the body’s cells and changed the types of fats in the blood in unexpected ways. This research suggests that more isn’t always better when it comes to prenatal vitamins, and that doctors should be careful about recommending very high doses of B12 supplements to pregnant women.

The Quick Take

  • What they studied: Whether taking too much vitamin B12 while pregnant could cause harm to the mother and developing baby
  • Who participated: Female rats that were either not pregnant or pregnant, divided into three groups receiving different amounts of B12 supplementation from before pregnancy through pregnancy
  • Key finding: Rats that received very high doses of B12 (320 times the normal amount) showed increased cellular stress and changes in their blood fat levels, while rats getting moderate extra B12 had the highest B12 levels in their blood
  • What it means for you: If you’re pregnant or planning to become pregnant, talk to your doctor before taking high-dose B12 supplements. This research suggests that the standard recommended amounts are likely safer than megadoses, though more human studies are needed to confirm these findings.

The Research Details

Researchers used pregnant rats to study what happens when mothers get different amounts of vitamin B12. They divided the rats into three groups: one group got the normal recommended amount of B12, another group got four times the normal amount, and a third group got 320 times the normal amount. The scientists kept track of the rats from before they got pregnant all the way through pregnancy. At day 20 of pregnancy (near the end), they collected blood and placenta samples to measure what was happening inside the mothers’ bodies.

This type of study is called an animal model study. Scientists use animals like rats to test ideas before trying them in humans because it’s safer and more controlled. Rats are often used because their bodies work similarly to human bodies in many ways, especially when it comes to how vitamins are processed.

The researchers measured several important things in the blood and placenta: vitamin B12 levels, markers of cellular stress (called oxidative stress), different types of fats, and signs of inflammation. They used standard laboratory tests to measure these markers.

This research matters because pregnant women often take prenatal vitamins, and some doctors recommend extra B12 supplementation. However, most studies focus on what happens when there’s NOT enough B12. This study fills an important gap by showing what happens when there’s TOO MUCH. Understanding both sides helps doctors give better advice about safe supplement doses during pregnancy.

This study was published in a peer-reviewed scientific journal, which means other experts reviewed it before publication. The researchers used a controlled laboratory setting with carefully measured doses, which makes the results reliable. However, this is an animal study, so results may not directly apply to humans. The study was well-designed with three comparison groups, which strengthens the findings. One limitation is that the paper doesn’t specify exactly how many rats were used in the study.

What the Results Show

The main finding was that rats receiving the highest dose of B12 (120 micrograms per day) showed increased oxidative stress. Oxidative stress is like cellular damage that happens when harmful molecules called free radicals build up inside cells faster than the body can clean them up. Think of it like rust forming on metal—it’s damage that accumulates over time.

Interestingly, the rats that got a moderate extra dose of B12 (1.5 micrograms per day) actually had the highest levels of B12 in their blood. This was surprising because you might expect the highest dose to create the highest blood levels. This suggests the body may have a limit to how much B12 it can use or store.

The excess B12 also changed the balance of different types of fats in the blood. Some fats increased while others decreased. These fats are important for heart health and brain development in babies, so changes in their levels could potentially matter for long-term health.

The study also found signs of increased inflammation in the excess B12 group, which means the body’s immune system was more activated than normal. Chronic inflammation during pregnancy could potentially affect fetal development.

The researchers also measured other health markers related to heart and metabolic health. While the abstract doesn’t provide all the specific details, the study found that excess B12 influenced these cardiometabolic variables, meaning it affected markers related to heart and metabolism. The placenta—the organ that nourishes the developing baby—also showed changes in response to excess B12, suggesting the supplement affected how the placenta worked.

Previous research has clearly shown that vitamin B12 deficiency during pregnancy is harmful and can lead to problems like birth defects and developmental delays. This new study adds important information by showing that the opposite extreme—too much B12—may also cause problems. It suggests there’s likely a ‘sweet spot’ of B12 intake that’s optimal for pregnancy. Most prenatal vitamins contain B12 in reasonable amounts, but some women take additional high-dose supplements, and this research suggests that practice may not be beneficial.

This study was done in rats, not humans, so we can’t be completely sure the same effects would happen in pregnant women. Rats’ bodies process vitamins differently than human bodies in some ways. The study also doesn’t tell us what the long-term effects might be on the babies after birth—it only looked at what happened during pregnancy. Additionally, the paper doesn’t specify the exact number of rats used, which makes it harder to evaluate the strength of the findings. Finally, this is one study, and more research is needed to confirm these results.

The Bottom Line

Based on this research, pregnant women should aim for the recommended daily amount of B12 (about 2.6 micrograms per day) rather than taking high-dose supplements. If you’re pregnant or planning pregnancy and concerned about B12 levels, ask your doctor to check your B12 status with a blood test rather than assuming you need extra supplementation. This recommendation comes with moderate confidence because it’s based on animal research; human studies would provide stronger evidence. Women who are vegetarian or vegan may need to pay special attention to B12 intake since it’s mainly found in animal products, but this should be done under medical guidance.

This research is most relevant to pregnant women and women planning to become pregnant. It’s also important for doctors and midwives who recommend prenatal supplements. Women with certain conditions that affect B12 absorption (like pernicious anemia) should definitely discuss B12 supplementation with their doctors rather than self-treating. This research is less relevant to non-pregnant people, though it raises questions about whether very high B12 doses are beneficial for anyone. Men and children don’t need to change their B12 intake based on this single study.

If you’re pregnant and currently taking high-dose B12 supplements, you don’t need to panic—this study shows effects that developed over the course of pregnancy, not immediately. However, you should talk to your doctor soon about whether your current supplement dose is appropriate. If you switch to a more moderate dose, you won’t see immediate changes, but the goal is to prevent the cellular stress and inflammation that develops over weeks and months of pregnancy. Benefits of switching to appropriate doses would likely appear gradually throughout the rest of your pregnancy.

Want to Apply This Research?

  • Track your daily B12 supplement dose in micrograms and note the source (prenatal vitamin, standalone supplement, or food sources). Compare this against the recommended daily amount of 2.6 micrograms to see if you’re in the safe range or taking excess amounts.
  • If using a nutrition app, log all B12 sources including prenatal vitamins, standalone supplements, and B12-rich foods like eggs, dairy, and fortified cereals. Set a daily goal of 2.6 micrograms and aim to meet it through food and standard prenatal vitamins rather than additional high-dose supplements.
  • Throughout pregnancy, periodically review your total B12 intake from all sources combined. If you’re taking a prenatal vitamin plus additional B12 supplements, calculate the total and discuss with your healthcare provider whether this exceeds recommended amounts. Track any symptoms of concern and share your supplement log with your doctor at prenatal visits.

This research is based on animal studies in rats and has not been tested in pregnant women. The findings suggest potential concerns about very high B12 doses during pregnancy, but more human research is needed to confirm these effects. If you are pregnant or planning to become pregnant, do not change your supplement regimen without consulting your healthcare provider. This information is educational and should not replace medical advice from your doctor, midwife, or registered dietitian. Always discuss any supplement use during pregnancy with your healthcare team, as individual needs vary based on your health status, diet, and medical history.