The role of antioxidants to reduce oxidative stress has been supported by smaller clinical trials linking high dose vitamin C and E intake to fewer biomarkers for pre-eclampsia but these findings have not been confirmed by larger studies.
Australian researchers, writing in the Cochrane Database of Systematic Reviews in January 2008, also concluded that no significant benefits were afforded by antioxidant supplementation for reducing the risk of pre-eclampsia, following their review of ten clinical trials involving 6533 women.
The authors of this current study, which was conducted by the US National Institutes of Health research network, said it is the largest to date and one in which treatment was started earliest in pregnancy and among a low-risk cohort.
Dr Alan Guttmacher, acting director of the Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD), one of the study’s funding bodies, said that the results effectively rule out vitamin C and E supplements as a means to prevent hypertensive pregnancy disorders.
Pre-eclampsia, affecting two to three per cent of all pregnancies occur when a mother's blood pressure rises to the hypertensive range, and excretion of protein in the urine becomes too high. It is estimated to be responsible for about 60000 deaths worldwide.
It is not known why some expectant mothers develop pre-eclampsia, although oxidative stress has been proposed to play a part.
The authors explained that the randomised double-blind trial was conducted at 16 sites within the NICHD's Maternal–Fetal Medicine Units (MFMU) Network, and included only women giving birth for the first time.
The researchers said that they enrolled over 10,000 women, starting in the ninth to 16th weeks of pregnancy, and followed them to delivery.
According to the article, the women, in general, were healthy and did not have high blood pressure, protein in the urine - an indication of stress on the kidneys - or gestational diabetes - a form of diabetes that emerges in pregnancy.
They explained that participants were randomly assigned to begin daily supplementation containing 1000mg of Vitamin C and 400 IU of Vitamin E or matching placebo in addition to any pregnancy vitamins they may have been taking.
The researchers found that the combined rates of pregnancy-associated hypertension, seizure, protein in the urine, pre-eclampsia, blood or liver abnormalities, loss of the pregnancy, an underweight baby and preterm delivery did not differ statistically between the two groups.
The combined rate of complications was 6.1 percent among those taking the study vitamins and 5.7 percent of those taking the placebo, they added.
Moreover, the results show that there was little statistical difference in pre-eclampsia between the two groups: 7.2 per cent among women taking vitamins versus 6.7 per cent among those who took the placebo, said the authors.
Lead researcher, Dr Catherine Spong, chief of NICHD's Pregnancy & Perinatology Branch and programme scientist for the MFMU Network, said that the results are welcome for the clarity they provide. “It shows us that what originally appeared to be a promising treatment did not actually offer any benefit clinically."