1 Department of Obstetrics and Gynecology, Rabin Medical Center, Beilinson Campus, Petah Tiqva, and Sackler Faculty of Medicine, Tel Aviv University, Tel Aviv, and 2 Institute of Chemical Pathology, Sheba Medical Center, Tel Hashomer, Israel
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Abstract |
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Key words: homocysteine/methylmalonic acid/pregnancy/vitamin B12
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Introduction |
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Vitamin B12 and folic acid catalyse the remethylation of homocysteine (Hcy) to methionine. Hyperhomocysteinaemia may be caused by a deficiency of cobalamin or folate metabolites. Nutrition also plays a significant role: a diet rich in fresh vegetables and fruits as well as vitamin supplementation has been associated with lower Hcy concentrations (Miner et al., 1996). Furthermore, studies have shown that methylmalonyl-CoA, a metabolite of various amino acids, accumulates when its mutase is blocked by deficiencies of its vitamin B12 co-factors or in the presence of congenital abnormalities of the mutase (Cox and White, 1962
).
The measurement of serum concentrations of methylmalonic acid (MMA) and Hcy metabolites related to vitamin B12 deficiency was introduced in 1990 to establish vitamin B12 deficiency at the biochemical level (Allen et al., 1990). Using a combination of serum metabolite concentrations, serum vitamin concentrations, clinical characteristics, and the response to therapy, it has been demonstrated that every vitamin B12-deficient patient who will respond to therapy has an elevation of MMA and/or Hcy, whereas findings of normal concentrations of both metabolites rule out clinical vitamin B12 deficiency (Stabler et al., 1996
). In line with these findings, Metz et al. suggested that the drop in vitamin B12 serum concentration in pregnancy does not reflect B12 deficiency at the biochemical concentration, and that to establish true B12 deficiency in pregnancy, the concentration of serum Hcy (but not MMA) should be used (Metz et al., 1995
).
The aim of the present study was to assess vitamin B12 concentrations in normal pregnant women and to examine whether low vitamin B12 concentrations are associated with changes in the biochemical indices of vitamin B12 deficiency, erythrocyte count or erythrocyte morphological indices, as reflected by erythrocyte mean corpuscular volume (MCV).
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Materials and methods |
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All study participants underwent a complete blood count and serum B12 determination. Those with low B12 concentrations (100 pmol/l) were designated the study group and compared for plasma Hcy and urine MMA concentrations with an equal number of subjects with normal serum B12 concentrations (>100 pmol/l) matched for age, parity and gestational age (control group).
Hcy was measured in plasma samples that were separated from erythrocytes shortly after blood was drawn. Plasma was reduced by sodium borohydride, followed by treatment with the fluorescent probe monobromobimane. Separation of Hcy by HPLC column and fluorescent detection was performed as previously described (Ueland et al., 1993). Urine analysis for MMA was performed by thin-layer chromatography on silica gel G plate, followed by chromatography in amyl acetateglacial acetic acid H2O and Fast Blue B reagent. Further assessment of MMA concentrations was achieved by capillary gas chromatography-mass spectrometry (Stabler et al., 1986
).
Normal values for plasma Hcy and urine MMA concentrations are 411 µmol/l and <9 mg/g creatinine respectively. Values are presented as mean ±SD. The study groups were compared with Student's t-test; P-values less than 0.05 were considered significant.
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Results |
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Discussion |
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Vitamin B12 deficiency may be associated with various pregnancy complications. A derangement of methionine-Hcy metabolism has been noted in ~20% of cases of neural tube defects and recurrent spontaneous abortion (Eskes et al., 1996). However, in a later study (Sutterlin et al., 1997
), no alteration could be demonstrated in serum folic acid and vitamin B12 concentrations in women with unexplained recurrent spontaneous abortions. Pre-eclampsia may also be related to high plasma Hcy concentrations (Laivuori et al., 1999
), with a direct relationship between the increase in Hcy and the severity of the pre-eclampsia. In the same study, though vitamin B12 concentrations were found to be reduced in pre-eclampsia, there was no correlation between plasma Hcy and vitamin B12. Finally, vitamin B12 apparently plays an important role in the metabolism of the nervous system (Healton et al., 1995
). Using cranial magnetic resonance imaging, a retardation of myelination in a child's nervous system due to maternal dietary vitamin B12 deficiency has been demonstrated (Lovblad et al., 1997
).
The usual means for delineating vitamin B12 malabsorption secondary to intrinsic factor deficiency versus other malabsorption factors (Schilling test) is contraindicated in pregnancy because of the risk of radiation exposure. Therefore, therapy is usually instituted empirically in pregnant patients with low serum vitamin B12, and diagnostic testing is delayed until the postpartum period (Campbell, 1995). This practice, however, may lead to overtreatment of pregnant patients without true vitamin B12 deficiency. Our results, together with others (Metz et al., 1995
), suggest that serum Hcy and urine MMA be measured in every pregnant patient with low serum vitamin B12 before treatment with vitamin B12 injections is instituted.
In conclusion, routine serum B12 testing is unnecessary in pregnancy. However, if low serum vitamin B12 is demonstrated, for example, in cases of anaemia refractory to iron and folic acid treatment, it should be followed by MMA and Hcy evaluation.
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Acknowledgments |
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Notes |
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References |
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Submitted on June 15, 1999; accepted on September 17, 1999.