Feb 16 2012

Mercury Still Not Correlated with Autism

Another study, published recently in PLoS One, fails to show a correlation between mercury and autism. This was a study of mercury excretion in the urine, comparing subjects with autism to their siblings as well as controls without autism spectrum disorder (ASD), both in mainstream and special schools. They found no significant difference among the groups, even controlling for kidney function (creatinine clearance), age, gender, and amalgam fillings.

To put this study into context – there are those who claim that mercury toxicity is what causes ASD, and in fact ASD is simply misdiagnosed mercury toxicity. There is no question that mercury is indeed a neurotoxin, but toxicity is all about dose, so the question is are children being exposed to mercury in high enough dose to cause toxicity. Further, it is difficult to extrapolate from preclinical studies (in test tubes and petri dishes) to living organisms. We need to further know what happens to the toxin in the body, and how the body handles it.

With regard to the forms of mercury found in some vaccines (although much less than in previous years) and tuna fish, the body seems to rid itself of the mercury sufficiently quickly to prevent build up to toxic levels. Of course, this remains a hot topic because of the persistent claims by the anti-vaccine movement that vaccine cause ASD, and some who cling to the discredited notion that it is mercury in vaccines that is the culprit. There are also the so-called “mercury militia” who blame environmental mercury (from vaccine and elsewhere) on all human illness, not just autism.

As further background, it’s helpful to note the chain of argument that has occurred with respect to the role of mercury in autism. Studies have consistently found no correlation between mercury exposure and the risk of ASD. Proponents of the mercury hypothesis have therefore argued that there is a subpopulation of vulnerable children who metabolize and excrete mercury differently than the general population, and it is within this subpopulation that mercury causes ASD.

Logically this may be true, but the argument is little more than special pleading, although a common one. Scientists are familiar with the usual list of special pleading arguments made to dismiss negative evidence. These include: that the dose studied was too low, the treatment duration was too short, the placebo or comparison treatment was also effective, or the looked-for effect only exists in a subpopulation. Each one of these arguments is logically consistent – if true they would explain the negative results without meaning that the phenomenon is not true. They may even be true in specific cases. What makes them special pleading is when they are invoked ad hoc to explain negative evidence without good justification.

With respect to mercury and autism, when studies failed to show a correlation proponents argued that only a subpopulation are vulnerable to mercury toxicity because (they speculated) they have impaired mercury excretion. That is the context for this study – if this particular instance of special pleading were true, then children with ASD would be more likely to have decreased excretion of mercury in their urine. Alternatively, if they are being exposed to more mercury as the cause they would have increased excretion of mercury in their urine. The study found no difference – no increase or decrease in mercury in the urine of children with ASD vs controls.

The authors are appropriately cautious in their conclusions. They say simply that their study does not provide support for the mercury hypothesis (not that it proves it wrong), and further they acknowledge that their study is modest in size (56 children with ASD and 197 total controls). A larger follow up study would be more definitive. But when we add this study to the existing body of research showing a lack of correlation between mercury and ASD it does add to our confidence in this lack of correlation.

Still, in my opinion, the most damning evidence for the mercury hypothesis is the fact that after the removal of thimerosal (which is mercury-based) from the routine childhood vaccine schedule in the US in 2001-2002, there was no subsequent drop in the rise in ASD incidence. Proponents predicted (correctly) that there should be if their claim was correct. Autism diagnoses continue to rise – now a full decade after the removal of thimerosal from the routine vaccine schedule. The supply of special pleading, however, is endless.

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