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What's not clear from the new study, however, is whether there's a common cause behind these conditions.
"More research is needed to determine whether there is a causal relation between allergies and autism, or something else causes both conditions," said senior study author Dr. Wei Bao, an assistant professor at the University of Iowa.
Thomas Frazier, chief science officer for the advocacy group Autism Speaks, said, "Parents and clinical providers should be aware of the increased prevalence and ensure that individuals receive appropriate evaluation for allergies with subsequent treatment. This is particularly true for very young children and nonverbal or minimally verbal children who may not be able to express to parents or providers the effects of allergies."
Frazier, who was not involved with the study, added that the findings suggest that allergies might be a contributing factor to challenging behaviors, such as irritability and mood shifts, in people with autism.
Autism spectrum disorders (ASDs) are neurodevelopmental disorders that have been increasing steadily over the past few decades. They currently affect about about 1 in 59 American children, according to federal estimates. People with ASDs typically have difficulty with social interactions, language and communication, and may engage in repetitive behaviors.
The new study included a nationally representative group of almost 200,000 U.S. children between the ages of 3 and 17. It used data collected in a survey from 1997 to 2016. The autism and allergy information was based on answers from a parent or guardian.
The researchers found that compared to those without ASD, children with autism were more likely to have a:
- Food allergy -- 11 percent versus 4 percent,
- Respiratory allergy (sneezing, itchy, watery eyes) -- 19 percent versus 12 percent,
- Skin allergy (rashes, eczema) -- 17 percent versus 10 percent.
After the researchers controlled the data to account for other factors that might link these conditions, such as education, income and location, the odds that someone with an ASD would have a food allergy were more than twice that of someone without an ASD. For respiratory allergies, the odds were 28 percent higher, and for skin allergies, they were about 50 percent higher.
The study was only designed to find an association between these conditions, not a cause-and-effect relationship. But Bao suggested that a problem in the immune system might underlie these conditions.
Dr. Punita Ponda, associate division chief of allergy and immunology at Northwell Health in Great Neck, N.Y., said she's definitely seen allergies and autism occur together. But, she added, there are only theories so far on why that would be.
One theory is that the gut microbiome -- the natural bacteria found in your digestive system -- might be altered somehow, and may trigger inflammation that plays a role in these conditions.
Another, as Bao noted, is a possible common problem somewhere in the immune system, said Ponda, who wasn't involved with the study.
Frazier added, "The honest answer is we just don't know yet, and it is possible to have several different mechanisms that link autism and allergies."
And, treatment -- especially for food allergies -- can be tough. "If a child with autism has had a piece of pizza every day and now they can't have it like they did before, they may not understand the reason why the food has been removed. Some children may refuse to eat when favorite foods are taken away," Ponda explained.
No matter what the cause, autism spectrum disorders can complicate the diagnosis and treatment of allergies, according to Ponda. Children with autism may not be able to effectively communicate how they're feeling and what symptoms they're experiencing. Physical exams may be harder to do, and allergy testing can be a challenge when communication skills are limited, she said.
The study was published June 8 in JAMA Network Open.
In an editorial in the same issue of the journal, Dr. Christopher McDougle, from the Lurie Center for Autism at Massachusetts General Hospital, said doctors should check for food allergies before initiating treatments designed to reduce behavior problems.
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