Maternal nutrition and genetics are determinants of breast-milk nutrient composition and, as such, are determinants of the nutritional exposure of breastfed infants.


The aim of this study was to determine whether common maternal single nucleotide polymorphisms (SNPs) in folate-dependent enzymes are associated with breast-milk folate content in a cohort of mothers enrolled in the Maternal–Infant Research on Environmental Chemicals (MIREC) study.


The MIREC study is a Canadian prospective pregnancy cohort study that recruited 2001 participants between 2008 and 2011. Five folate-related SNPs—MTHFR 677C>T (rs1801133), MTHFR 1298A>C (rs1801131), MTHFR 1793G>A (rs2274976), MTR 2756A>G (rs1805087), and MTRR 66A>G (rs1801394)—were genotyped. Breast milk was sampled ∼1 mo postpartum, and tetrahydrofolate (THF), 5-methyl-THF, 5-formyl-THF, 5,10-methenyl-THF, and unmetabolized folic acid (UMFA) were measured using liquid chromatography–tandem mass spectrometry in a subset of participants (n = 551). Associations were assessed using Wald’s test. Associations were considered significant if P ≤ 0.01 (Bonferroni correction for multiple testing).


None of the SNPs were associated with total breast-milk folate. However, the MTHFR 677C>T SNP was associated with breast-milk UMFA (R= 0.01; unadjusted P = 0.004), explaining a small portion of total variance; this association remained significant when adjusted for other covariates, including supplemental folic acid consumption. The MTHFR 1793G>A and MTRR 66A>G SNPs tended to be associated with 5-methyl-THF (R= 0.008, P = 0.04) and reduced folates (THF + 5-methyl-THF + 5-formyl-THF + 5,10-methenyl-THF; R= 0.01, P = 0.02), respectively.


We found that total breast-milk folate content was not associated with any of the folate-related SNPs examined. The association between the MTHFR 677C>T SNP and breast-milk UMFA, albeit modest, highlights the need to better understand the determinants of breast-milk folate and the impact they might have on milk folate bioavailability.