(ENS) -- In a paper published this week, researchers at the University of Arkansas identified a marker in autistic children that suggests these children are more susceptible to the harmful effects of mercury and other toxic chemicals than or than other children or adults.
Some vaccines given to infants and children are preserved with thimerosal, which is half mercury by weight, and parents worry that these vaccines, including flu shots, may trigger autism in their children.
Now new evidence that strengthens that theory has been found.
Lead author on the new study is Dr. Jill James of the University of Arkansas for Medical Sciences. James served for 14 years as a senior research scientist with the Food and Drug Administration and is currently professor of pediatrics at the University of Arkansas for Medical Sciences.
Published in the December issue of the "The American Journal of Clinical Nutrition," the purpose of the study was to evaluate the concentrations of eight metabolites in the plasma of children with autism.
Twenthy children with autism and 33 control children participated in the study. Researchers measured methionine, S-adenosylmethionine (SAM), S-adenosylhomocysteine (SAH), adenosine, homocysteine, cystathionine, cysteine, and oxidized and reduced glutathione.
Relative to the control children, the children with autism had statistically significantly lower baseline plasma concentrations of methionine, SAM, homocysteine, cystathionine, cysteine, and total glutathione.
They had significantly higher concentrations of SAH, adenosine, and oxidized glutathione.
James and her team concluded that this metabolic profile indicates an increased vulnerability to oxidative stress -- destruction caused by free radical molecules -- and this pattern "may contribute to the development and clinical manifestation of autism."
In addition, the James team found that the plasma profile of the 20 autistic children studied shows a decreased capacity for methylation -- a process essential for the normal development and functioning of organisms.
The Environmental Working Group of Washington, DC, which has been researching this issue as well, says James' findings show that autistic children have a common weakened ability to protect themselves from the effects of small amounts of toxic metals in their bodies.
"The autism epidemic is alerting us to the importance of individual susceptibility to environmental pollutants," said Richard Wiles, senior vice president of the Environmental Working Group.
"Environmental safeguards that protect a theoretical average person still leave thousands at risk," said Wiles. "Increased understanding of susceptibility will provide the basis for stronger health policies that truly protect the most vulnerable."
A review published earlier this year by the National Academy of Sciences Institute of Medicine concluded that the available science showed no mercury-autism link. But the review left the possibility open that vaccines preserved with mercury might trigger autism in a small subset of susceptible children. James and her team examine the issue of susceptibility in a small subpopulation, with the conclusion that the blood balance of this small population made it more susceptible to mercury in vaccines than other children are.
"The findings raise serious concerns about autistic children's overall exposure to environmental contaminants," said the EWG. "Mercury is of particular significance because of its proven toxicity to the developing brain and nervous system, and its documented high exposures from a variety of sources such as canned tuna, dental fillings and vaccines preserved with mercury based thimerosal."
In October 1998, the U.S. Food and Drug Administration (FDA) banned the use of thimerosal in over the counter medications. Since July, 1999, the FDA has encouraged manufacturers to remove thimerosal from vaccines. Not all manufacturers have complied.
Today, most vaccines that are being manufactured do not contain thimerosal, or they only contain a trace. But old thimerosal products are still on the shelf and may be used until stocks are gone.
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