New Delhi, May 11: A micronutrient often prescribed to pregnant women to ensure proper development of the growing foetus may at excessive levels raise the risk of autism, a research study in the US has suggested.

The researchers at the Johns Hopkins University (JHU) have found that if a mother has four times higher than adequate levels of folate - a type of vitamin-B - at the time of birth, her child's risk of developing an autism spectrum disorder doubles.

"We have known long that folate deficiency in pregnant women is detrimental to their child's development. But excessive amounts may also cause harm - we must aim for optimal levels of this nutrient," Margaret Daniele Fallin, professor of the JHU Bloomberg School of Public Health and senior author of the study, said in a media release issued today by the JHU. The study's findings were presented today at the International Meeting on Autism Research in the US.

Folate is found in natural foods such as green leafy vegetables oranges and kidney beans, among other foods, and its synthetic version called folic acid is provided as supplements or fortified food. Women planning to become pregnant need about 400 micrograms folic acid daily, says a document from theUS National Institutes of Health.

The new study has also found that high levels of vitamin B12 in a pregnant woman can triple the risk of her child developing an autism disorder. When both folate and B12 levels are extremely high, the risk of autism shoots up 17 times, the researchers said in their presentation.

Fallin and her colleagues tracked 1,391 mother-child pairs from 1998 to 2013, measuring the mothers' folate levels at the time of delivery and observing their children's development over time. Their analysis revealed the link between excessive levels of folate and vitamin B12 and autism spectrum disorders, a set of conditions marked by difficulty in communicating and repetitive behaviour, among other symptoms.

The researchers say it is unclear why some women had such high levels of folate and vitamin B12 in their blood. They have speculated that two possible explanations or a combination of both may help explain the observations - the women consumed too much folic acid-fortified food or supplements or some women may be genetically predisposed to absorbing large quantities of folate or breaking it down slower, leading to excessive levels.

"I think this is an intriguing, preliminary but plausible finding," said Harsh Pal Singh Sachdev, a senior consultant paediatrician at the Sitaram Bhartia Institute of Science and Research, New Delhi, who was not associated with the study but has in the past cautioned about unbridled market-driven promotion of essential micronutrients.

"These new findings may be a reminder that in the quest to promote fortification and supplementation, the safety aspects of some essential micronutrients may take a back seat," Sachdev told The Telegraph.

Nutrition specialists and sections of paediatricians have long been arguing that India should change its policy of providing periodic doses of vitamin A to all children to ensuring that it reaches only those children who have a deficiency of this vitamin.

Folate is essential for normal cell growth and promotes brain development. The findings suggest more research is required to determine exactly how much of folic acid a woman should consume during pregnancy.