The pooled evidence in our systematic review showed that supplementation with vitamins C and E during pregnancy does not reduce the risk of preeclampsia in women either at low/moderate or high risk for this disorder. Moreover, we found compelling evidence that vitamins C and E increase the risk of gestational hypertension. In addition, there was some evidence suggesting that vitamin C and E supplementation is associated with a decreased risk of abruptio placentae and an increased risk of PROM and use of any antihypertensive therapy. The reliability and robustness of our results are supported by: 1) the use of the most rigorous methodology for performing a systematic review of randomized controlled trials; 2) the inclusion of all the large planned trials that investigated the efficacy of vitamins C and E during pregnancy for the prevention of preeclampsia in meta-analyses; 3) the high methodological quality of the majority of trials included in the review; 4) the evidence of clinical and statistical homogeneity in the results for most of the maternal outcomes evaluated; 5) the subgroup analyses according to risk status of women at trial entry; 6) the exploration of potential sources of heterogeneity; 7) the symmetrical funnel plots suggesting that there was no evidence of either publication or related biases; and 8) the narrow confidence intervals obtained which made our results more precise.
Our study has some limitations. First, several studies did not report results of some outcome measures considered in our review. Thus, our meta-analysis may be underpowered for such outcomes. It is possible that if these results were reported more consistently, effect sizes might be different. Second, to date, no trials have reported on the long-term consequences of exposure of mothers and their children. Finally, we could not investigate the effect of supplementation with vitamins C and E in women with biochemical evidence of increased oxidative stress due to lack of data. Specific quantitative indices of oxidative stress such as products of lipid peroxidation could be considered as entry criteria in clinical trials of vitamins C and E during pregnancy.
Antioxidants, mainly vitamins C and E, have been proposed as a potential preventive strategy on the basis of data suggesting a role of increased oxidative stress in the pathogenesis of preeclampsia. It is unclear why supplementation with vitamins C and E during pregnancy did not reduce the risk of preeclampsia. First, it is possible that although oxidative stress plays a major role in the pathophysiology of preeclampsia, it is not important in the causal pathway of the disorder. Thereby, it would be unlikely that reversing the oxidative stress would reduce the risk of preeclampsia. On the other hand, oxidative stress could be relevant to the pathogenesis of preeclampsia in only a subgroup of women, with no appreciable benefit of vitamins C and E for the entire population. Nevertheless, in our stratified analyses by risk category at trial entry, supplementation with vitamins C and E did not decrease the risk of preeclampsia in nulliparous women with a singleton pregnancy or women with previous preeclampsia, eclampsia or HELLP syndrome, chronic hypertension, renal disease, pregestational diabetes, BMI ≥30 kg/m2 in the first pregnancy, abnormal uterine artery Doppler velocimetry, antiphospholipid syndrome or multiple pregnancy.
It has been proposed that supplementation with vitamins C and E starting in the early second trimester after placentation has occurred might be too late to affect the pathological process of the condition. However, in the study by Roberts et al,21
there were no significant differences between the vitamin and placebo groups in the frequency of the primary outcome (composite of pregnancy-associated hypertension and serious adverse maternal or perinatal outcomes) among women enrolled before the 13th week of pregnancy. Finally, the beneficial effect of supplementation with vitamins C and E reported initially by Chappell et al14
could have been due to a type I statistical error because such study was not powered for preeclampsia. In addition, this small trial was stopped early after an interim analysis showed a significant decrease in the risk of both the primary outcome (PAI-1/PAI-2 ratio) and the secondary outcome (preeclampsia). Recently, Bassler et al.34
reported that randomized controlled trials that are stopped early for benefit (whether or not as a result of a formal stopping rule) are associated with greater effect sizes than randomized controlled trials that continue to the end. In addition, differences in treatment effect size between truncated and non-truncated randomized controlled trials were greatest in small trials that were stopped early.
Supplementation with vitamins C and E was clearly associated with a small but significant increase in the risk of gestational hypertension. This finding was consistent with the higher use in both antihypertensive therapy and magnesium sulfate, and a marginally significant increase in antenatal hospitalization due to hypertension. However, it is possible that these results reflect a reporting bias because only two studies described the use of antihypertensive therapy and only one study reported the use of magnesium sulfate and antenatal hospitalization for hypertension. Vitamin C and E supplementation during pregnancy also appeared to be associated with a significant increased risk for PROM and a non-significant increased risk for PPROM. Nevertheless, a sensitivity analysis excluding two trials responsible for statistical heterogeneity showed that women supplemented with vitamins C and E had a 67% increased risk of PPROM. The direction of the treatment effect was consistent in the two trials reporting PROM and in four of six trials reporting PPROM. These findings stand in contrast to the emerging evidence suggesting that oxidative stress caused by increased reactive oxygen species formation and/or antioxidant depletion may disrupt collagen and cause premature membrane rupture.35,36
The explanation for why supplementation with vitamins C and E increases the risk of gestational hypertension and PROM is unknown. Banerjee et al.27
have hypothesized that non-antioxidant effects of exogenous vitamin E could have detrimental effects on human pregnancy. Vitamin E therapy could prevent an immunologic switch from T-helper cell 1 to T-helper cell 2 that is vital for early-to-late transition in normal pregnancies. Moreover, vitamin E could be a potential interferon-gamma mimetic that might facilitate persistent proinflammatory reactions at the fetal-maternal interface. Regardless of what causes the increase in gestational hypertension and PROM, these findings raise concern about the use of vitamins C and E during pregnancy at the doses used in the trials included in our review.
Unexpectedly, we found that supplementation with vitamins C and E during pregnancy was associated with a significant reduction (37%) in the risk of abruptio placentae. All five studies reporting on this secondary outcome showed a similar trend. Recruitment of a sufficient cohort of women to a randomized controlled trial to confirm this finding would be very difficult. Notwithstanding, this association could be of interest for the investigation of etiology and pathophysiology of abruptio placentae. In 1957, Martin et al37
reported that nine out of ten cases of abruptio placentae occurred in women with low serum ascorbic acid levels during pregnancy. Moreover, Clemetson and Cafaro38
reported in 1981 that women with low serum ascorbic acid levels during pregnancy (<0.4 mg/dL) had a significantly higher risk of developing abruptio placentae than women with normal levels (unadjusted RR 9.8, 95% CI, 3.5 to 27.4). Two studies by Sharma et al39,40
showed that serum levels of vitamins A, C and E were lower in women with abruptio placentae than in women with normal pregnancies. Finally, Ejima et al41
have found evidence that oxidative stress is involved in placental dysfunction and abruptio placentae in a model of placental dysfunction associated with inflammation in pregnant mice. Thus, the role of vitamins C and E in the etiology and pathogenesis of abruptio placentae deserves further research.
It has been suggested that supplementation with vitamins C and E during pregnancy could reduce the risk of preeclampsia in women with a low baseline antioxidant status. In the study by McCance et al,22
women with a low antioxidant status at baseline (plasma ascorbate <10 μmol/L or serum α-tocopherol ≤5 μmol/mmol cholesterol) assigned to receive vitamins C and E had a reduced risk of preeclampsia compared with similar women assigned to receive placebo, although the numbers were small and the differences did not achieve statistical significance. No other trials reported their results according to baseline antioxidant status. In the study by Villar et al,19
vitamin C and E supplementation did not prevent preeclampsia in high-risk women presumed to have low antioxidant status on the basis of data from previous studies in the participating centers. In a small randomized controlled trial, supplementation with antioxidants (vitamins A, B6, B12, C, and E, folic acid, N-acetylcysteine, copper, zinc, manganese, iron, calcium, and selenium) was associated with a reduction in the rate of preeclampsia from 29% to 7% (RR 0.24, 95% CI 0.06 to 1.01) in 60 women with low antioxidant status (superoxidedismutase <1102 U/g Hb or <164 U/mL) at 8 to 12 weeks of gestation.42
A completed but not yet published randomized controlled trial involving 360 women with low antioxidant status at 10–12 weeks gestation assessed whether supplementation with vitamins C and E is beneficial in such women.43
Three previous meta-analyses examined the effect of vitamin C and E supplementation during pregnancy on the risk of preeclampsia.44–46
The authors of these reviews concluded that supplementation with vitamins C and E does not prevent the development of preeclampsia in agreement with results of our meta-analysis. However, the last 4 trials published during 2009 and 2010,19–22
with a total of 14,781 women, were not included in any of these previous reviews. In addition, one of these meta-analyses included quasi-randomized and non randomized trials.46
In conclusion, the results of this review do not support routine supplementation with vitamins C and E during pregnancy to prevent preeclampsia or other adverse maternal or perinatal outcomes in women at both low/moderate and high risk for such disorder. Further research is required to determine the long term effects of supplementation with vitamins C and E during pregnancy for both women and children.