Vitamin B12, folate cycle, and fertility

Folate and Vitamin B12: Two Interconnected Nutrients

Folate (vitamin B9) and vitamin B12 work together in the methylation cycle — a series of biochemical reactions required for DNA synthesis, cell division, and the production of SAMe, the body’s primary methyl donor. Deficiency in either nutrient disrupts this cycle, impairing fertility, increasing risk of neural tube defects, and potentially affecting embryo development. Adequate B12 is required to recycle folate into its active forms, and low B12 can produce a functional folate deficiency even when folate intake appears adequate.

MTHFR Variants: What They Mean in Practice

The MTHFR gene encodes an enzyme that converts dietary folate and folic acid into 5-MTHF (the active form used in the methylation cycle). Common variants (C677T, A1298C) reduce this conversion efficiency by 30–65 percent depending on the variant combination. MTHFR variants are common — approximately 10–15 percent of Northern European populations carry the homozygous C677T variant. For those with known MTHFR variants, supplementing with methylfolate (rather than folic acid) bypasses the impaired conversion step. MTHFR results require nuanced interpretation — they are not a fertility sentence and are not the only relevant factor in the methylation cycle.

Vitamin B12: Who Is at Risk of Deficiency

B12 is found almost exclusively in animal products: meat, fish, eggs, and dairy. Strict vegans and long-term vegetarians are at highest risk. Deficiency also occurs with long-term metformin use (impairs absorption), pernicious anaemia, and prior bariatric surgery. Serum B12 below 200 pmol/L indicates deficiency; 200–300 pmol/L may be borderline. Methylmalonic acid (MMA) and homocysteine are more sensitive markers of functional B12 status than serum B12 alone.

Supplementation Forms and Food Sources

Methylcobalamin and adenosylcobalamin are the active, coenzyme forms of B12. Cyanocobalamin is the most common and inexpensive form and is effective for most people. Plant-based B12 sources (nori seaweed, tempeh, nutritional yeast) provide unreliable and variable amounts and should not be relied upon as primary sources in pregnancy or preconception. A dedicated B12 supplement or certified fortified foods are necessary for all who avoid animal products.

Key Takeaways

This article on Vitamin B12, folate cycle, and fertility is designed to give you clear, evidence-informed steps to discuss with your care team. Every fertility journey, pregnancy, and IVF cycle is unique — use this as a starting framework and build your individual plan with your doctor, midwife, or registered dietitian. For safety-critical decisions, current evidence and your clinical team always take precedence over general guidance.

Visual Guide

Keep this one practical: use the first image to understand the context, then apply one actionable step today before moving to the next section.