But researchers say the effect in lower-risk pregnancies remains to be determined.
In the study from Mexico, women who ate daily food bars containing the amino acid L-arginine and antioxidant vitamins during pregnancy had a much lower incidence of preeclampsia than women who ate bars containing the antioxidant vitamins alone or bars containing neither supplement.
Most of the women had a history of preeclampsia and all were considered at high risk for the complication. Preeclampsia affects 2% to 8% of pregnant women overall and a much higher percentage of poor women living in developing countries.
In Latin America and the Caribbean, preeclampsia is the cause of one in four childbirth-related deaths, according to the World Health Organization.
Checking for Preeclampsia
Preeclampsia, the pregnancy-induced high blood pressure occurring with high levels of protein in the urine, can threaten the life of both a pregnant woman and her baby.
The only cure is delivery. Bed rest is generally recommended for milder forms of preeclampsia if the pregnancy is not near full term.
The amino acid L-arginine is produced by the body and has been shown in some animal and human studies to help maintain healthy blood flow. There is also some evidence that preeclampsia may be linked to a deficiency in L-arginine.
In the new study, published May 19 in BMJ Online First, a team of investigators in Mexico and the U.S. recruited close to 700 pregnant women with a previous pregnancy complicated by preeclampsia (95%) or a close relative who had the disorder.
From around the 20th week of pregnancy until the 32nd week the women were asked to eat two specially formulated food bars every day.
About one-third of the women ate bars containing L-arginine and antioxidant vitamins, one-third ate bars containing the vitamins alone, and the rest ate bars with neither the amino acid nor the vitamins. All the bars had the same packaging.
The average woman ended up eating just one of the two recommended bars each day; 125 women stopped eating the bars entirely at some point during the study.
Nevertheless, far fewer of the women in the L-arginine plus vitamin group developed preeclampsia (12.7%) than women in the vitamin-alone (22.5%) group and no-supplement (30.2%) group.
"We saw a very strong protective benefit for supplementation with L-arginine and antioxidant vitamins," study researcher Felipe Vanilla-Ortega, MD, PhD, of Mexico's Universidad Nacional tells WebMD. "Of course our findings need to be confirmed. But preeclampsia is a very dangerous condition, and this appears to be a very inexpensive approach for lowering risk in high-risk women."
Epidemiology professor Liam Smeeth, PhD, of the London School of Hygiene and Tropical Medicine, says many questions remain about the impact of supplementation with L-arginine and antioxidant vitamins.
First among them is whether L-arginine and antioxidant supplementation is of benefit to women living in more economically prosperous countries like the U.S., who are not as likely to be deficient in the nutrients.
In an editorial published with the study, Smeeth writes that calcium supplements have been shown in some studies to reduce preeclampsia risk in Latin America, but not in North America.
"It is possible that higher-risk populations may need different preeclampsia prevention strategies than lower-risk populations," Smeeth tells WebMD.