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"We found that if the mom was taking folic acid during the window around conception, the risk associated with pesticides seemed to be attenuated," said study first author Rebecca Schmidt.
"Mothers should try to avoid pesticides. But if they live near agriculture, where pesticides can blow in, this might be a way to counter those effects," said Schmidt. She is an assistant professor of public health sciences at the University of California, Davis.
It's estimated that one in 68 U.S. children has an autism spectrum disorder, which can range from mild to severe. There is no single cause, but research suggests a combination of genetic and environmental influences plays a role, according to the U.S. National Institutes of Health.
The new study included about 300 children aged 2 to 5 with autism and 220 without the developmental disorder. Children whose mothers took 800 or more micrograms of folic acid (the amount in most prenatal vitamins) had a much lower risk of developing autism, even when their mothers were exposed to household or agricultural pesticides, the researchers said.
Autism risk was higher among children whose mothers were repeatedly exposed to pesticides or whose mothers had low folic acid intake and exposure to agricultural pesticides between three months preconception and three months afterward, the findings showed.
Those two factors combined were associated with higher risk of autism than either low folic acid intake or pesticide exposure alone, Schmidt said in a university news release.
"The mothers who had the highest risk were the ones who were exposed to pesticides regularly," she added.
Folic acid is the synthetic form of vitamin B9, found in supplements and fortified foods. While taking it reduced the associated risk of pesticide-related autism in children, it did not entirely eliminate it, the report noted.
"It would be better for women to avoid chronic pesticide exposure if they can while pregnant," Schmidt said.
Folic acid plays a critical role in DNA repair and synthesis, and in determining which genes are turned on or off, said Schmidt. "These are all really important during periods of rapid growth when there are lots of cells dividing, as in a developing fetus. Adding folic acid might be helping out in a number of these genomic functions," she added.
The study doesn't show a causal link, and there are limitations. For one, participants relied on their memory to report folic acid intake and household pesticide exposure.
The study was published Sept. 8 in the journal Environmental Health Perspectives.
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SOURCE: University of California, Davis, news release, Sept. 8, 2017