In a new study of California moms, women whose children had autism recalled getting less folic acid through food and supplements early in their pregnancies than those whose kids didn't develop the disorder.
Meeting recommendations for folic acid -- at least 600 micrograms per day -- in the first month of pregnancy was tied to a 38 percent lower chance of having a kid with autism or Asperger's, researchers reported last week in the American Journal of Clinical Nutrition.
Folic acid -- the synthetic form of the B-vitamin folate -- has been added to breakfast cereals and other grains in the United States since 1998 because of evidence showing deficiencies in pregnant women made it more likely their babies would have brain and spine birth defects.
Questions have remained about whether lack of the vitamin, or difficulty processing it, might increase the risk of mental retardation and certain developmental disorders as well.
Folate "becomes very critical in the early stages of life… as well as the first year of life, when basically the brain is establishing connections and functions," said Edward Quadros from SUNY Downstate Medical Center in Brooklyn, New York.
"If there is a folate deficiency, this disrupts a lot of functioning with the brain," Quadros, who has studied autism and folic acid but wasn't involved in the new research, told Reuters Health.
The possible link between folic acid and autism remains controversial, researchers noted.
Some scientists have thought extra folic acid during pregnancy might actually be tied to a higher chance of autism.
"There were a lot of hypotheses on how perhaps the folic acid fortification in the U.S. was responsible for the increased prevalence of autism spectrum disorders, so that was also a concern," said Rebecca Schmidt, the lead author of the new study from the University of California, Davis.
"When we starting looking at this, I thought it could go either way," she said.
Even with the new findings, there's no proof that had some moms in the study gotten more folic acid in their pregnancy diets, their children wouldn't have developed an autism spectrum disorder.
One limitation is women had to remember their month-to-month diets and supplement use from a few years ago by the time they were surveyed, which makes their reports less reliable.
First month most important
Schmidt and her colleagues surveyed the mothers of 429 preschoolers with an autism spectrum disorder and 278 with normal development about their diet and supplement use before and during pregnancy. Using that information, they calculated how much daily folic acid women were getting each month.
Throughout their pregnancies, mothers of kids without autism got more folic acid through fortified foods and vitamins than those who ended up having an autistic child.
That difference was greatest in the first month of pregnancy, when mothers of normally-developing babies remembered getting an average 779 micrograms of folic acid daily and 69 percent of them at least met the daily guidelines.
That compared to an average 655 micrograms in moms of autistic kids, 54 percent of whom got the recommended 600 micrograms or more per day.
One serving of fortified breakfast cereal, or three-quarters of a cup, has 400 micrograms of folic acid. Lentils and spinach, two natural sources of folate, both have between 100 and 200 micrograms per half-cup.
The link between folic acid and autism remained in the new study when Schmidt's team took into account mothers' age and race as well as whether they smoked or drank alcohol during their pregnancy.
Not all researchers in the field are on board with implicating low folic acid in autism and Asperger's.
"It is very difficult to convince the scientific community more than anybody else," Quadros said.
"I would be very careful," said Dr. Fernando Scaglia, who has also studied autism and folate deficiency at Baylor College of Medicine in Houston, Texas.
"I think that more studies need to be done to see if this can be replicated."
Then, he told Reuters Health, researchers would have to understand exactly what's going on in the brain that would tie less folate during development to autism.
"It seems to be good for neural development overall, but I think we do need to figure out how it's working," Schmidt told Reuters Health.
For now, when it comes to folic acid during pregnancy, she said, "The recommendations that are out there already are pretty good to follow."