Autistic children are invariably heavier than others. Researchers at the US National Institutes of Health, the Centers for Disease Control and Prevention and the Cincinnati Children's Hospital say that extraordinary growth of hormones could be contributing to such a phenomenon.
Other studies have already shown that children with autism have very rapid head growth in early life.
However, this study is confined to boys. Researchers were not able to rope in enough volunteers from girls for the project. Anyway autism itself tends to afflict boys more than girls.
"The study authors have uncovered a promising new lead in the quest to understand autism," said Dr. Duane Alexander, Director of the National Institute of Child Health and Human Development.
"Future research will determine whether the higher hormone levels the researchers observed are related to abnormal head growth as well as to other features of autism," Alexander said in a statement.
Writing in the journal Clinical Endocrinology, Dr. James Mills of the NICHD and colleagues said they compared the height, weight, head circumference and levels of growth-related hormones to growth and maturation in 71 boys with autism to a group of 59 healthy boys.
The boys with autism had higher levels of two hormones that directly regulate growth -- insulin-like growth factor-1 and IGF-2. The boys also had higher levels of hormones that indirectly affect growth.
The researchers did not measure the boys' levels of human growth hormone, which for technical reasons is difficult to evaluate.
The boys with autism and those with autism spectrum disorders had a greater head circumference on average, weighed more and had a higher body mass index than the other boys, although there was no difference in height between the two groups of boys.
No one knows what causes autism, a complex developmental disorder that includes problems with social interaction and communication.
Symptoms range from mild awkwardness seen in Asperger's syndrome, to severe disability and mental retardation. A recent CDC survey found that 1 in every 150 U.S. children has autism or an autism spectrum disorder, a less severe condition related to autism, such as Asperger's.
Several genes have been linked with autism, but environmental factors may also play a role, experts say.