Agricultural Chemicals and Autism: A Growing Link

This past weekend, I finished reading a paper in the journal, Environmental Research, which was an overview of what we know about the relationship of pesticide exposure to the development of autism.[i]  Interesting  that it came out of China, where, “According to University of Melbourne and Zhejiang researcher Baojing Gu, China is the world’s largest consumer of agricultural chemicals, using more than 30 percent of global fertilizers and pesticides on only 9 per cent of the world’s crop land.”[ii]

Highlights of the paper for you:

1. Chronic exposure to pesticides has already been associated with illnesses like cancer, asthma, behavioral abnormalities, hormone disorders and cognitive deficits.  The rapidly developing nervous system of fetuses and babies is particularly susceptible because the blood-brain barrier is not completely developed.  We already know too that even low levels of pesticide exposure can cause central nervous system (CNS) damage to infants and that exposure during pregnancy is associated with neurological disorders in infants.
2. Much research is devoted to looking for genetic causes of autism, “However, recent studies have found a smaller role for genetics and a larger contribution by environmental factors than previously thought.”
3. The increase in autism is terrifying.  A few statistics for you:  a recent study shows that 1 in 132 people globally now has autism.  In the USA, the incidence of autism more than doubled in a decade. A 2016 survey showed that the prevalence of autism in children between 3-17 is now 1 in 40. The paper looks at the incidence of autism in multiple countries around the world including Iceland, Sweden, Spain, Australia, the UK, China and more, pointing out that the increase in numbers is similar throughout the world.
4. Now you might want to sit down before you read this next bullet point:  according to the US Department of Agriculture, the use of the herbicide, glyphosate (i.e. Roundup), increased 6504% between 1991 and 2010.  Multiple studies now confirm that prenatal exposure to glyphosate increases the risk of autism.   There is substantial, and growing, research that links glyphosate exposure to autism through alterations in the bacterial microbiome.  I’ve covered this topic before.  Look here and here.
5. Pyrethroid, a very commonly used insecticide, is known to lead to changes in the brain, and those particular changes are known to be a factor in autism.  However, there is not enough research at this point to definitively confirm a direct correlation.
6. Another pesticide used on crops, chlorpyrifos, has been shown – in animal models – to cause severe brain abnormalities as well as social dysfunction.  In human children exposed to the chemical, abnormalities in the frontal and parietal cortex have been seen. Again, more research is needed to confirm a definitive relationship.
7. Research shows that at least half of children with autism have at least one GI problem: “Gut microbiota disturbances may contribute to the increased risk of ASD via affecting the metabolites, immune system, and neuroactive compounds.” The article points to propionic acid as a likely major factor.  As my regular readers know, I have written about this many, many times.  (See here, here and here, as just three of many more examples.)
8. Mechanisms of action are currently being researched, but disruption to the developing microbiome is one of the main potential culprits: “Gut microbiota disturbances may contribute to the increased risk of ASD via affecting the metabolites, immune system, and neuroactive compounds. “  Glyphosate, as you know from my previous articles, is an herbicide that kills weeds by disruption the shikimate pathway, which is not present in humans…but is present in our gut bacteria.  Pesticides, on the other hand, work by affecting the CNS of insects. While herbicides and fungicides are not designed to be neurotoxins, they have been found to be so in animals.  Their effect on the microbiome is unknown. Thus, more and more research is looking at this link, and looking at how even low dose exposures may be problematic.  Certainly epidemiological evidence points to a strong association between these chemicals and ASD.

The conclusion of these Chinese researchers is that the astronomical increase in the rate of autism makes it “…it is urgent to comprehensively explore the risk factors and potential mechanisms of ASD to provide a scientific basis for ASD prevention.”

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[i] He X, Tu Y, Song Y, Yang G, You M. The relationship between pesticide exposure during critical neurodevelopment and autism spectrum disorder: A narrative review. Environ Res. 2021 Aug 17;203:111902. doi: 10.1016/j.envres.2021.111902. Epub ahead of print. PMID: 34416252.

[ii] https://earth.stanford.edu/news/overuse-fertilizers-and-pesticides-china-linked-farm-size#gs.czg4dh