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A dietary supplement given to pregnant women at high risk of pre-eclampsia can reduce the likelihood of the disease occurring, a study says.

Writing in the British Medical Journal, researchers says the presence of an amino acid and antioxidant vitamins in the supplement helps to combat abnormally high blood pressure.

More than 600 women took part in the study in Mexico City.

But experts say more research is needed on the effects of the supplement.

Pre-eclampsia is a serious condition where abnormally high blood pressure and other problems develop during pregnancy.

It affects up to 10% of all first-time pregnancies and is dangerous for both mother and child.

Pre-eclampsia is thought to be linked to a deficiency in L-arginine, an amino acid that helps maintain a healthy blood flow.

So, as part of , researchers in Mexico and the United States gave 228 pregnant women at high risk of pre-eclampsia daily food bars containing L-arginine and antioxidant vitamins.

Another 222 pregnant women received bars containing only vitamins, and another group of 222 got bars containing no L-arginine or vitamins, the placebo group.

The supplements began when women were 20 weeks pregnant and continued until they gave birth.

The research team found that 30.2% of women developed pre-eclampsia in the placebo group, 22.5% in the group given a vitamin bar and 12.7% in the L-arginine and vitamin group.

The study authors said the L-arginine and vitamin group women were significantly less likely to develop pre-eclampsia.

"This relatively simple and low cost intervention may have value in reducing the risk of pre-eclampsia and associated preterm birth," they said.

But they added that further studies are needed to see whether the results can be repeated.

Gail Johnson, midwife teacher at the Royal College of Midwives, cautioned that it was a relatively small study which excluded some high-risk groups.

"It's a starting point for looking at pre-eclampsia, which poses a significant health risk and is an important thing to solve, but there are lots of questions still to be asked about the effects of the supplement."

Andrew Shennan, professor of obstetrics and gynaecology at St Thomas' Hospital, said there were established scientific reasons why L-arginine and antioxidants reduce the occurrence of pre-eclampsia.

"Previous studies, testing each supplement individually, have shown little effect, so it is exciting that in combination they seem to have such a profound effect on preventing pre-eclampsia."

"However, it is unusual that so many people in the control group developed pre-eclampsia - 30% is an unusually high rate of incidence of the disease so I would be interested to see if the effects of the study could be replicated elsewhere."