To characterize changes in vitamin B-12 (B-12) and folate status biomarkers across pregnancy and to assess the influence of high plasma folate on B-12 status.
Prospective cohort comprising 299 women followed during the three trimesters of pregnancy in a primary health center in Rio de Janeiro, Brazil. Plasma concentrations of total B-12, total homocysteine (tHcy), methylmalonic acid (MMA) and folate were measured. The combined B-12 status indicator (cB-12) was calculated from B-12/tHcy*MMA corrected for age and folate. Longitudinal linear mixed-effect regression models were used to assess changes across time.
In the 1st trimester, 3.1% and 19.6% presented low (B12<148 pmol/L) and marginal B-12 (148-221 pmol/L) while 2.2% and 13.3% had low (<10 nmol/L) and high (>45 nmol/L) plasma folate. Mean (SD) plasma B-12 concentrations (pmol/L) decreased from 317.3 (116.7) in the 1st to 259.1 (134.6) in the 2nd to 239.1 (83.5) in the 3rd trimester (ß=-3.82, p<0.001). tHcy (µmol/L) presented a U-shaped curve [6.3 (1.7), 5.1 (1.2) and 5.6 (1.5). MMA (nmol/L) increased [176.5 (104.1), 197.0 (117.0) and 249.0 (153.6) (ß=2.60, p=0.02)]. Overall B-12 status (cB-12) declined across pregnancy (ß=-0.008, p<0.001). Plasma folate (nmol/L) also decreased [27.2 (10.4), 24.7 (9.6) and 23.9 (9.6) (ß=-0.13, p=0.001)]. Women in the lowest tertile of cB-12 and upper tertile (mean=39.2 nmol/L) of plasma folate in the 1st trimester had worse B-12 status across pregnancy (p<0.001) compared with women in the lowest tertile of both cB-12 and plasma folate (mean=16.3 nmol/L) such that the prevalence of B-12 <221 pmol/L in 1st, 2nd and 3rd trimester= 58%, 79% and 67%, respectively in the group with first-trimester low B-12 and high folate status and 39%, 63% and 58%, respectively in the group with first-trimester low B-12 and normal folate status.
B-12 and folate status in apparently healthy pregnant women is characterized by a physiological decrease across trimesters which is accentuated in women with low B-12 status and high plasma folate in early pregnancy.