Serum and red-blood-cell folate demonstrate differential associations with BMI in pregnant women.

Objective: To examine the association between BMI and folate concentrations in serum and red blood cells (RBC) in pregnant women.
Design: A cross-sectional comparison of folate concentrations in serum and RBC sampled simultaneously from the same individual.
Setting: The Ottawa Hospital and Kingston General Hospital, Ontario, Canada.
Subjects: Pregnant women recruited between 12 and 20 weeks of gestation.
Results: A total of 869 pregnant women recruited from April 2008 to April 2009 were included in the final analysis. Serum folate was inversely associated and RBC folate positively associated with BMI, after adjusting for folic acid supplementation, age, gestational age at blood sample collection, race, maternal education, annual income, smoking and MTHFR 677C→T genotype. In stratified analyses, this differential association was significant in women with the MTHFR CC variant. In women with the CT and TT variants, the differential associations were in the same direction but not significant. Folic acid supplementation during pregnancy did not alter the differential association of BMI with serum and RBC folate concentration. This indicates that the current RBC folate cut-off approach for assessing risk of neural tube defects in obese women may be limited.
Conclusions: BMI is inversely associated with serum folate and positively associated with RBC folate in pregnant women, especially for those with the MTHFR CC variant.