Recent research continues to support the idea that both a genetic and environmental component may be potentially associated with the development of autism spectrum disorder (ASD). Heritability is high in ASD as can be seen from family and twin studies. It is most certainly polygenic (multiple genes may contribute to ASD risk rather than there being just “one ASD gene”), and may be associated with both conserved mutations (inherited from parents) but also de novo mutations.  The genetic risks related to ASD are highly complex and further research continues to be carried out to better understand this multifaceted process. Studies have shown that environmental factors associated with ASD, are similarly complex and carry a long list of potential culprits including heavy metals, persistent organic pollutants (DDT, PCBs for example) and chemicals such as phthalates and BPA. 
The impact of heavy metals on the risk of ASD development is of particular interest due to the various routes of exposure including vaccines, food, water and inhalation. Currently, research indicates the compounds gaining the most attention are mercury, arsenic, aluminum, and lead all of which have been implicated in an increased risk of ASD, however, there are challenges in understanding the complexity between the type of exposure and the timing and quantity of the exposure. 2, ,  Recent investigations have revealed there may be certain developmental time frames during which exposure to specific types of heavy metals may lead to increased ASD risk; these time frames are likely during early infancy and childhood but may also exist during prenatal periods. Additionally, it has been hypothesized that when certain essential elements are deficient during critical developmental stages, ASD risk may also rise. As an example, zinc is required for the regulation of many metal transport mechanisms, most importantly for the mobilization and excretion of heavy metal toxins. If deficient in this element, the ability to eliminate these metals may increase ASD development risk. 
To learn more about the critical time frames relating to heavy metal exposure and ASD development, researchers from multiple centers internationally explored these areas by estimating pre- and post-natal exposure profiles of essential and toxic elements using validated tooth-matrix biomarkers. Researchers compared 32 twin pairs (17 monozygotic twins and 15 dizygotic twins) discordant for ASD to determine whether fetal and postnatal metal dysregulation had an impact on ASD risk.
The results from this study indicated critical developmental windows in which dysregulation of particular elements may increase risk of ASD; findings included:
- Higher lead levels were observed over the prenatal period and first 5 months postnatally
- Zinc levels were lower in ASD cases during the third trimester
- Manganese levels were consistently lower in ASD cases both pre- and postnatally, with the highest deficiency measured at 4 months after birth
Why is this Clinically Relevant?
- Lifestyle factors, particularly optimizing nutrition and reducing stress, may support healthy levels of metal regulating elements such as zinc and manganese in both mother and fetus
- Reducing lead exposure in mother and fetus prenatally and in the child during the first months of life is extremely important in reducing risk factors associated with ASD development
- Optimizing mother’s nutrition during breastfeeding to ensure adequate intake of essential elements such as zinc and manganese may reduce risk of essential element deficiency in the newborn
Arora M, Reichenberg A, Willfors C, Austin C, Gennings C, Berggren S, Lichtenstein P, Anckarsater H, Tammimies K, Bolte S. Fetal and postnatal metal dysregulation in autism. Nature Communications 8, Article number: 15493 (2017). doi:10.1038/ncomms15493
 Torre-Ubieta L, Won H, Stein J, Geschwind D. Advancing the understanding of autism disease mechanisms through genetics. Nat Med. 2016 Apr; 22(4): 345–361. doi: 10.1038/nm.4071
 Sealey L.A. et al. Environmental factors in the development of autism spectrum disorders. Environmental International. Volume 88, March 2016, Pages 288-298.
 Li H, et al. Blood Mercury, Arsenic, Cadmium, and Lead in Children with Autism Spectrum Disorder. Biol Trace Elem Res. 2017 May 8. doi: 10.1007/s12011-017-1002-6.
 Ye B, et al. The association of environmental toxicants and autism spectrum disorders in children. Environmental Pollution. 227:234-242 · August 2017. DOI: 10.1016/j.envpol.2017.04.039
 Arora M, Reichenberg A, Willfors C, Austin C, Gennings C, Berggren S, Lichtenstein P, Anckarsater H, Tammimies K, Bolte S. Fetal and postnatal metal dysregulation in autism. Nature Communications 8, Article number: 15493 (2017). doi:10.1038/ncomms15493