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FRIDAY, June 2, 2017 (HealthDay News) -- An analysis of chemicals in baby teeth suggests that exposure to lead in pregnancy or shortly after birth might make infants more vulnerable to developing autism.
At the same time, the investigators found that babies who don't get enough zinc and manganese during the same time frame may also face an increased risk for the developmental disorder.
Zinc and manganese are typically found in certain foods.
The teeth used in the study came from a pool of twins who were studied at various points in their development. The scientists determined lead, zinc and manganese levels by using lasers to map the growth rings in baby teeth.
Environmental exposures can change constantly, explained study author Dr. Manish Arora, vice chairman and division chief of environmental health at the Icahn School of Medicine at Mount Sinai in New York City.
That means that "we need to consider not only how much of a chemical someone is exposed to, but also when that exposure happens," he noted.
"We found that the environmental neuro-toxicant lead was higher in autistic children than their unaffected twin siblings, and this higher exposure started before birth. We also found that the essential elements zinc and manganese were lower in children affected by autism spectrum disorder," Arora said.
Teeth are considered a reliable "record of environmental exposure," given that they continuously develop throughout pregnancy and after birth. Tooth layers, therefore, serve as time-stamped "imprints" of chemical exposure, much like tree rings indicate growth patterns, the researchers said.
For the study, tooth analyses were conducted among 32 Swedish twin pairs, born in 1992 or later. Of these, 17 were identical twins and 15 were fraternal twins. Pairs variously included two autistic siblings, one sibling with autism, or two healthy siblings. Tooth samples were also taken from 12 children whose twin siblings did not participate.
Beyond finding a link between autism risk and varying exposure to lead and/or zinc and manganese, the team also found evidence that metal exposure in early infancy might affect autism severity when it develops.
For example, higher lead exposure when an infant reached the 3-month mark was linked to a higher risk for greater autism severity. Lower manganese at the same point was also linked to worse autism severity. No such link was observed with respect to zinc exposure, however.
That said, Arora cautioned that the exact mechanism by which metal exposure affects autism risk and severity remains unclear. And the study did not prove that metal exposure causes autism risk to rise or fall.
For example, Arora noted that lower exposure to zinc and manganese may not actually "cause" autism risk to go up. Rather, it could be "that autistic children are unable to process some of these essential nutrients" to which they are, in fact, adequately exposed.
In that scenario, the proactive taking of supplements to minimize risk "would not be very useful," he said.
Harvard research scientist Andrea Roberts agreed, and added caveats of her own.
"Although the study is interesting, it is very small [with] only 19 persons with autism," she noted.
And because "the mother's diet intake of zinc and manganese would have been the same for both children with and without autism, it is the differences in how the children metabolized the metals, rather than differences in the mother's behavior, that distinguished kids with and without autism," Roberts said.
But Thomas Frazier, chief science officer for Autism Speaks, characterized the findings as "very important."
"The major implications I see are that this may suggest either a direct effect of lead or lack of minerals like zinc/manganese on early life brain development," he said, "increasing risk for autism or a gene-environment interaction.
At the same time, he agreed with Arora and Roberts "that the low levels may reflect issues with regulating and processing elements rather than just a simple issue of low levels that need to be remediated."
Arora agreed, saying, "It is too early to make any such [supplementation] recommendation. Additional work is needed, and we have now expanded this study to other countries to dig deeper into the mechanisms underlying our finding."
The findings were published June 1 in the journal Nature Communications.
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