"Glutamate is important in protein building and as an energy source," he noted.

Autistic children are known to have a different composition of gut flora, the microbes inhabiting our intestines, with around eight species of Clostridia bacteria not found in most children, Nicholson said.

Clostridal bacteria include some "nasty ones," he added, such as C.difficile, a hard-to-kill bug responsible for many hospital-acquired infections, as well as the germ that produces botulinum, a powerful neurotoxin. But there's no evidence for a causal relationship with autism: it's just as likely some autistic children have behaviors, diets or changes to the immune system that result in unusual populations of gut microbes.

The compounds showing the greatest (and only reliable statistical) difference between autistic subjects and controls were with succinate, involved in amino acid metabolism, and byproducts of metabolizing nicotinic acid. High excretion of one of these products, NMND, is also implicated in Parkinson's disease. A structurally similar byproduct found in badly made street heroin causes a Parkinson's-like condition in street addicts, Nicholson said.

Also, large amounts of taurine were shed in the urine of some, but not all, autistic subjects. Taurine, a sulfur-containing, non-protein-building amino acid, helps create the bile acids involved in breaking down fat in the gut.

"We know in liver diseases you get taurine in the urine," Nicholson said. Earlier research has pointed to sulfur metabolism problems in autistic children.

But the study in many ways raises more questions than it answers, and it's still too early to tell if a urine test really could reliably help diagnose autistic children, or at least a subset of them.

"We need a more detailed study," Nicholson said.

But if it works it could help push back the age at diagnosis, which is important, because earlier treatment is usually the most successful, Nicholson said. Right now, most autistic children are diagnosed between 18 months and two years of age, he said.

About 30 percent of autistic children in the study had gastrointestinal disorders, Nicholson said. At a Pediatric Academic Societies meeting in Vancouver earlier this month, researchers reported around 45 percent of autistic patients had GI troubles, although the evidence for GI disorders in those with autism has been controversial.

The children with GI disorders did not form a separate cluster in the statistical analysis, Nicholson said, meaning they didn't seem more likely to shed unusual quantities of chemicals in their urine.

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