Vitamin B12 and Fertility for Both Partners

Folic acid gets the spotlight in preconception advice, and for good reason. Vitamin B12 is its quieter partner. The two work together in the same metabolic pathway that cells use to copy DNA, which is why both matter when the body is making eggs, sperm and, later, a rapidly growing embryo. Unlike folic acid, though, B12 isn't something everyone needs to supplement. It's a "check if you're at risk" nutrient.

Contents

• How much you need
• Who is at risk of running low
• B12 and the female partner: what the studies show
• B12 and sperm: what the studies show
• Getting B12 from food
• If you need a supplement

How much you need

Those are small amounts, and a varied diet with animal foods usually covers them easily. The NIH fact sheet notes that blood B12 levels tend to drop during pregnancy, sometimes into the low range, and usually return to normal after delivery. Starting pregnancy with good stores gives you more room.

Who is at risk of running low

The NIH lists several groups at higher risk of inadequate B12. Several of them are common among people trying to conceive:

Vegans (and some vegetarians)

B12 occurs naturally almost only in animal foods. The NIH notes that vegans who eat no animal products have a higher risk of deficiency, and that exclusively breastfed infants of vegan mothers can develop deficiency early in life. If you're plant-based, B12 should be on your preconception checklist, not an afterthought.

People taking metformin

Metformin is widely used for type 2 diabetes and prediabetes, and often for PCOS. The NIH says metformin might reduce B12 absorption and significantly lower blood B12 levels. That doesn't mean metformin is the wrong choice; it means B12 is worth checking. Never stop or change metformin on your own. If diabetes or prediabetes is part of your picture, howtohaveababy.com's guide to type 2 diabetes and prediabetes before pregnancy covers the bigger plan.

People on long-term acid reducers

Proton pump inhibitors (such as omeprazole and lansoprazole) and H2 blockers can interfere with absorbing B12 from food by reducing stomach acid, according to the NIH. Many people take these for years for reflux without anyone mentioning B12.

People with stomach or bowel conditions or surgery

Gastric bypass and similar procedures reduce absorption, particularly of B12 bound to food. Pernicious anemia, an autoimmune condition affecting the stomach lining, causes deficiency because the body can't absorb B12 normally.

B12 and the female partner: what the studies show

Neural tube defects

The clearest evidence for B12 before pregnancy comes from neural tube defects, the same birth defects folic acid helps prevent. A study led by Trinity College Dublin with NIH scientists, published in Pediatrics in 2009, used stored blood from Irish women collected in early pregnancy in the 1980s, when supplements were rarely used. Accounting for folate:

• Women with B12 below 250 ng/L had about 2.5 to 3 times the risk of an affected pregnancy compared with women with higher levels.
• Women in the deficient range (below 150 ng/L) had about five times the risk.

The authors suggested women have B12 levels above 300 ng/L before becoming pregnant, a target set higher because levels fall in pregnancy, and said the findings needed confirmation in other populations. The NIH fact sheet likewise notes that B12 deficiency in pregnancy might cause neural tube defects.

Fertility treatment outcomes

A 2015 analysis from the EARTH study at Massachusetts General Hospital measured B12 and folate in 100 women going through 154 assisted reproduction cycles. Women in the highest quarter of B12 (above 701 pg/mL) had about twice the probability of live birth of women in the lowest quarter (below 439 pg/mL). Women with both B12 and folate above the median also did better.

Two cautions. It's a small observational study, so it shows an association, not proof that raising B12 improves IVF success. And the women in the lowest group were not necessarily deficient. It's a reason to make sure you're not low, not a reason to push levels as high as possible.

B12 and sperm: what the studies show

For male partners the evidence is thinner. A 2017 review in Biomolecules concluded that most published work points to positive effects of B12 on semen quality, including sperm count, motility and less DNA damage. But much of that support came from animal and laboratory studies. Human results were mixed: some found no clear link between B12 and semen quality, and one main human study of DNA damage used a multivitamin, so B12's own role couldn't be isolated. The author called for more clinical research.

The practical takeaway is the same as for zinc. Correcting a real deficiency makes sense. Adding high doses on top of an adequate intake hasn't been shown to help. Our article on zinc for sperm health walks through the large FAZST trial, which is the best example of why "more is better" often fails for men's fertility nutrients. If DNA damage is the concern, see our guide to sperm DNA fragmentation.

Male partners take metformin and acid reducers and eat plant-based diets too, and their B12 often goes unchecked. If either of those applies, he should ask for a test too. A semen analysis is a sensible baseline before any changes; fertilestart.com reviews at-home sperm tests if a lab visit is hard to arrange.

Getting B12 from food

Clams, oysters, salmon and canned light tuna are all on the FDA/EPA "Best Choices" list for lower mercury, so they fit neatly into preconception eating. Our fish and mercury guide lists the rest. Beef liver tops the NIH table at 70.7 mcg per 3-ounce serving, far more than anyone needs from a single food.

For plant-based eaters, fortified nutritional yeast, fortified plant milks and fortified cereals are the main food sources. Because amounts vary, read labels and don't assume a product is fortified.

If you need a supplement

If a test shows you're low, or you're vegan, your clinician can recommend the right form and dose. A few facts help make sense of the doses you'll see:

• Absorption drops as the dose rises. According to the NIH, about 50% of B12 is absorbed at doses under 1 to 2 mcg, but only about 2% at 500 mcg and 1.3% at 1,000 mcg. That's why supplement doses look far larger than the RDA.
• No upper limit has been set, but that isn't a reason to stack several B12-containing products. Add up what's in your prenatal, any B-complex and any "energy" product.
• If absorption is the problem (pernicious anemia, surgery), oral supplements may not be the right route, which is another reason to test and talk to your clinician first.
• Look for independent quality marks such as USP Verified, which checks that ingredients are present at the declared amounts.

B12 is one of the few nutrients where a simple blood test can turn a vague worry into a clear answer. If you're in a higher-risk group, it's worth asking for one at your preconception visit, for both of you. For more on building the rest of your plate, see our winter fertility grocery list.

Sources

• NIH Office of Dietary Supplements. Vitamin B12: Fact Sheet for Health Professionals. ods.od.nih.gov
• NIH/NHGRI. Low levels of vitamin B12 may increase risk for neural tube defects (on Molloy AM, et al. Pediatrics. 2009). genome.gov
• Gaskins AJ, et al.; EARTH Study Team. Association between serum folate and vitamin B-12 and outcomes of assisted reproductive technologies. Am J Clin Nutr. 2015;102(4):943–950. Penn State
• Banihani SA. Vitamin B12 and semen quality. Biomolecules. 2017;7(2):42. mdpi.com
• CDC. About Folic Acid. cdc.gov
• USP. Consumers choose quality supplements with help from the USP Verified Mark. usp.org