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(Hypertension. 2004;43:1279.)
© 2004 American Heart Association, Inc.
Scientific Contributions |
From Magee-Womens Research Institute (T.E.P., R.W.P., A.R.D., R.B.N., J.M.R.), Pittsburgh, Pa, and the Department of Obstetrics, Gynecology, and Reproductive Sciences (R.W.P., A.R.D., J.M.R.), School of Nursing (T.E.P.), and Department of Epidemiology (R.B.N.), University of Pittsburgh, Pa.
Correspondence to Dr Thelma Patrick, University of Pittsburgh, School of Nursing, 440 Victoria Building, 3500 Victoria Street, Pittsburgh, PA 15261. E-mail patrickt{at}pitt.edu
| Abstract |
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Key Words: preeclampsia race
| Introduction |
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Homocysteine concentrations are tightly regulated by 2 main enzymatic pathways. Homocysteine can be remethylated to methionine by a pathway requiring folic acid as a methyl donor. In addition to adequate folic acid, the pathway requires vitamin B12 as an important cofactor. Alternatively, homocysteine can be removed by transsulfuration, a pathway dependant on the cofactor vitamin B6. Enzymatic defects in either of these pathway results in increased homocysteine, as does deficiency of folic acid, vitamin B6, or B12. Interestingly, nonpregnant black women are reported to have lower serum folic acid and B6 levels but higher vitamin B12 levels than white women.4
We proposed that the increased frequency of preeclampsia among black women could be secondary to increased serum homocysteine, perhaps because of different dietary intake of vitamins B12 or folic acid. To test this hypothesis, we assessed racial differences in homocysteine, folic acid, and vitamin B12 during normal pregnancy and in pregnancies complicated by preeclampsia.
| Methods |
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Blood Samples
Maternal venous blood samples were collected in the labor suite before delivery. Plasma was prepared with EDTA, and samples were aliquoted and stored at 80°C until assayed.
Homocysteine Determination
Total plasma homocysteine was analyzed according to the procedure of Jacobsen et al.5 The thiol derivatives were detected fluorometrically with excitation at 390 nm and emission at 470 nm. Calibration curves were generated for every assay and were included at the beginning and end of each analytical set. They consisted of normal human plasma spiked with 0, 2.5, 5, 7.5, 10, 15, 20, and 25 µmol/L L-homocystine. The coefficient of variation between assays was 8%.
Folic Acid and B12 Determination
Serum folic acid and B12 concentrations were determined with a radioimmunoassay from Diagnostics Products Corp. The assay procedure was that described by the manufacturer. The detection limit of the assay for folic acid is 0.3 ng/mL and for B12 is 50 pg/mL. The interassay coefficient of variation for folic acid was 9.4% and 6% for B12.
Statistical Methods
Means and standard deviations are reported. Differences in homocysteine, folic acid, and vitamin B12 in the subject groups were analyzed by 2-way ANOVA. Bonferroni/Dunn post-hoc testing was used as appropriate with statistical significance accepted at P<0.01. Correlations were by standard regression analysis with statistical significance accepted at P<0.05.
| Results |
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The mean plasma concentration of total homocysteine was significantly higher in black women compared with white women (P<0.01) and in women with preeclampsia compared with normal pregnancy (P<0.03) (Table 2). Plasma folic acid was significantly lower in the black women when compared with white women (P<0.0001), but did not differ by pregnancy outcome (Table 2). Of the 4 subject groups studied, there was a significant inverse association between homocysteine and folic acid in the black women with preeclampsia (r=0.23, P<0.01) (Figure), but this relationship was not significant in any of the other groups. Vitamin B12 concentrations were significantly increased in black women compared with white women (P<0.0001) (Table 2); however, no differences were observed in B12 concentrations when compared by diagnosis.
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| Discussion |
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Our findings are consistent with a study in nonpregnant women. Premenopausal black women had higher plasma total homocysteine, lower plasma folic acid, and higher vitamin B12 concentrations than white women.4 When these data were analyzed adjusting for multivitamin use and intake of ready-to-eat cereal, reported to be more prevalent in white women, plasma total homocysteine concentrations did not differ significantly by race, but plasma folic acid remained significantly lower, and vitamin B12 significantly higher in the black women. These data suggest a nutritional intervention may be of value to increase folic acid and lower homocysteine concentrations in our black population. Although data regarding nutritional intake and compliance with prenatal vitamins were not obtained, in general, the means and distribution of plasma folic acid and vitamin B12 were similar by race regardless of diagnosis.
A growing body of literature indicates the importance of ethnic and racial factors to considerations of B12 metabolism and its disorders. Blacks have significantly higher B12 levels than whites.68 Because serum B12 levels are often influenced by factors unrelated to B12 intake, stores, or deficiency, it is unclear whether the differences in concentrations reflect B12 status. The ethnic differences in B12 concentration, which are present in cord blood, childhood, and pregnancy, probably arise from combinations of hereditary and acquired causes.9 From our data and reported studies,10 continued analysis by race is necessary when addressing homocysteine, folic acid, and B12 in relation to health and disease.
Before initiating a nutritional study, other possible sources of increased homocysteine must be ruled out. Although genetic mutations were not explored in this study, one of the most common genetic polymorphisms associated with mild hyperhomocystinemia is a point mutation in the 5,10-methylenetetrahydrofolate reductase (MTHFR) gene, a C -to-T substitution at nucleotide 677 (C677T). This mutation has a very low incidence among black populations.1115 Several studies have investigated the incidence of this polymorphism among women with preeclampsia.1619 The majority of these studies report no significant increase in the prevalence of this polymorphism among women with preeclampsia compared with women with a normal pregnancy outcome.
We did not assess the vitamin B6, the cofactor in the transsulfuration pathway. The association of low levels of vitamin B6 and cardiovascular disease has been reported to be independent of homocysteine when studied in general populations2022 and in the assessment of racial differences between Asian Americans and whites.10 Further, the addition of B vitamins to folic acid supplementation achieves little additive effect in lowering homocysteine.23
In general, homocysteine concentration increases with age and decreases as pregnancy advances. Walker et al report a concentration of 5.5 µmol/L (95% CI: 3.3 to 7.5) at 36 to 42 weeks of gestation, compared with 7.9 µmol/L (95% CI: 6.2 to 9.6) in nonpregnant women.24 The homocysteine concentrations in this study were higher in the younger black group and, despite sampling at an earlier gestational age, were higher in the presence of a diagnosis of preeclampsia.
Finally, it may be beneficial in future studies to assess erythrocyte folic acid concentration to differentiate between short-term and long-term nutritional deficits of folic acid. Inadequate folic acid intake first leads to a decrease in serum folic acid concentration, then to a decrease in erythrocyte folic acid concentration, a rise in homocysteine concentration, and eventually to megaloblastic changes in the bone marrow and other tissues with rapidly dividing cells.25 Serum folic acid concentration of <3 ng/mL indicates a negative folic acid balance at the time that a blood sample is drawn. Erythrocyte folic acid concentration does not reflect recent or transient changes in dietary folic acid intake.9 In all experimental studies subjecting volunteers to folic acid deprivation, a decrease in folic acid concentration occurred within 1 to 3 weeks, followed by a period of weeks or months when folic acid concentrations are low but there was no other evidence of deficiency;9 however, the mean value of folic acid exceeded this level for all subject groups in this study.
Perspectives
Homocysteine, a risk factor for atherosclerosis, is higher in black pregnant women and higher still in black women with preeclampsia compared with white pregnant women, and these differences are partially related to folic acid. This finding has implications for the higher rates of preeclampsia in blacks and may have long-term implications for future cardiovascular risk. Lastly, racial differences in atherosclerotic risk factors merit further exploration for their significance to the higher incidence of preeclampsia in black women.
| Acknowledgments |
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Received October 14, 2003; first decision October 24, 2003; accepted March 15, 2004.
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