No Evidence for Autism, Mercury Link

by Brendon Nafziger, DOTmed News Associate Editor | October 30, 2009
No difference in mercury levels
between autistic and healthy children
A study looking at hundreds of children failed to find a link between mercury exposure and autism, according to a paper published in Environmental Health Perspectives.

Researchers from the MIND Institute at the University of California, Davis analyzed blood samples from around 460 children between two and five years old. They found, after controlling for factors such as diet, no difference in mercury levels between autistic children, normal controls and those with development disorders such as Down syndrome.

These findings, while tentative, could help throw cold water on fears over a link between childhood vaccination and autism. Although since 2001 mercury is no longer present in most vaccines, except for some influenza shots, the presence of mercury -- in the form of thimerosal -- in vaccines has popularly been blamed for the spike in autism cases over the last 20 years.

The few recently vaccinated children in the study did not show unusual blood mercury levels.

Yet, another popular fear did receive some confirmation: although the rise was small, children with mercury-based dental fillings who also chewed gum did have slightly elevated levels of mercury in their blood.

Nevertheless, this increase was dwarfed by the factor with the single greatest impact on mercury blood levels: the amount of fish in the child's diet. In fact, healthy children actually had higher mercury levels than autistic ones unless diet was controlled for, as autistic children - many of whom are picky eaters - tend to avoid fish, according to the researchers.

Only around one percent of children in both autistic and normal groups had medically significant levels of mercury, in line with national estimates.

Be cautious with results

Still, in an interview with DOTmed News, Irva Hertz-Picciotto, Ph.D., a professor of environmental and occupational health at UC Davis, and lead author of the study, urges caution in interpreting the results. The study only looked at correlations, not causes, and the effects of mercury can't be totally ruled out as much of the mercury found in the body is not in the blood.

"One of the reasons we are cautious is that 5 percent of mercury is in the blood, the other 95 percent is in soft tissue or excreted in urine or hair," she says.

To address this problem, Dr. Hertz-Picciotto says she and her team are analyzing mercury levels in hair follicles from a baby's first haircut. This could both give them a more complete picture of mercury levels and let them see if high levels go on to precede an autism diagnosis. Dr. Hertz-Picciotto says she expects to submit results from the study next month.