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Researchers found that of nearly 93,000 Norwegian children they followed, those born to obese dads had double the risk of developing autism. But the odds were still small: just under 0.3 percent were diagnosed with autism, versus 0.14 percent of kids with normal-weight fathers.
The findings, published online April 7 in Pediatrics, are the first to link fathers' obesity to autism risk. And experts stressed that it's not clear whether dads' extra pounds, per se, cause the increase.
One possibility is that there's an "indirect" association, said lead researcher Dr. Pal Suren, of the Norwegian Institute of Public Health in Oslo.
Certain gene variations, for example, could be linked to heightened risks of both obesity and autism, Suren explained. Or, he said, obese men might be more likely to have certain environmental exposures that contribute to autism risk.
On the other hand, Suren said, it's possible that fathers' obesity has some direct effect -- by altering sperm quality, for instance. But for now, that's all speculation.
What was surprising, the researchers said, is that mothers' obesity was not tied to a heightened autism risk. Some past research has pointed to such a connection.
But in this study, any link between moms' weight and risk for the developmental disorder disappeared once fathers' weight was taken into account.
This offers hints that fathers' weight might actually be more important, although it's not clear why, Suren said. Much more research is needed to understand what's going on, he said.
"It would definitely be beneficial to replicate our analyses in population studies from other countries, to see whether the association is generalizable to other populations," Suren said.
An autism expert who reviewed the study agreed. And if the link is confirmed, "then you need to understand why this association exists," said Dr. Andrew Adesman, chief of developmental and behavioral pediatrics at Steven and Alexandra Cohen Children's Medical Center of New York.
For now, Adesman stressed that the risk linked to fathers' obesity was small.
Of the almost 93,000 children who were followed until age 7 on average, 419 were diagnosed with an autism spectrum disorder. That included 25 children with autistic disorder born to obese fathers.
"So most of the autism cases were not related to paternal obesity," Adesman said. Plus, he added, "over 99.5 percent of kids born to obese men did not have autism. That's reassuring."
Still, Adesman said, even if fathers' obesity had only a small effect on autism risk, that would still be concerning on the broad population level since obesity has become so common across the globe.
According to the latest figures from the U.S. Centers for Disease Control and Prevention, about one in 68 U.S. children has been diagnosed with an autism spectrum disorder. The term refers to a group of developmental disorders that, to varying degrees, impair people's ability to communicate and socialize with others.
No one knows the exact causes of autism, but experts generally agree that it's a complex mix of genetic vulnerability and environmental exposures.
Researchers have managed to find a few hundred genes linked to autism risk. There are no definite environmental culprits yet, but studies have suggested that certain factors during pregnancy might be important, including mothers' exposure to air pollution, low intake of the B vitamin folate and viral infections.
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