"Actually, I don't think that many parents will be disappointed," Amaral said. "On the contrary, it is very difficult to maintain children on the [GFCF] diet. Some parents who may have failed in maintaining the diet may feel relieved to hear that it might not be a benefit to their child even if they had persisted."
Many parents of autistic children look to alternative therapies, including special diets. According to Dr. Daniel Coury, medical director of the Autism Treatment Network, about 20 percent of parents within the the network use complementary methods to treat their autistic children, and more than half of them rely to some extent on diets.
In particular, parents of autistic children often report improvements with the popular but scientifically unproven gluten-free, casein-free diet.
This diet has been promoted by celebrities such as Jenny McCarthy, whose best-selling book, "Louder Than Words," detailed her use of diets as one way of treating her autistic son. Many autism communities have also touted the purported benefits of specialized diets for their children.
While many parents of children with autism report gastrointestinal pain in their children, the question remains whether the association between autism and bowel disease exists.
Not all parents have had success with the diets. Diane Marshall, 42, of Montclair, N.J., put her son David, 13, on a gluten-free, casein-free diet for a year when he was 4 years old. Marshall said she'd read success stories from some mothers who tried the diet for their autistic children. At the time, David had severe eczema and runny bowel movements, she said.
"We definitely thought the diet would help the autism," said Marshall.
Although the gastrointestinal issues subsided, Marshall said she did not attribute the end of her son's stomach problems to the diet. And she said the diet did not help her son overcome autism.
"There are a lot of things out there that are based on evidence, like teaching methods, that will help our kids a lot, but not diets," said Marshall.
"There has not been any research to substantiate the GFCF diet for children with autism who do not have celiac disease or wheat/milk allergies," said Dr. Stefani Hines, a development-behavioral pediatrician at William Beaumont children's hospital in Royal Oak, Mich.
But this does not mean that parents are necessarily empty-handed when it comes to scientific approaches to improving autism symptoms, Hines said.
"So far the research that has been the most promising has been aimed at applied behavioral analysis," she said. "We still have much to learn regarding best treatment practices for children with autism."
Study author Hyman said more research needs to be done in this area with similar rigor but with larger numbers, so that subtle effects can be detected. It's important, she said, for families to get the best information they can in deciding which interventions to pursue.
She also said families who elect to make dietary changes need to pay careful attention to general nutrition. "When you eliminate dairy as an entire category, you need to pay attention to vitamin D, calcium and protein," she said.