May 10, 2018
A newly published study of mothers and children in Norway has confirmed an association between high phthalate concentrations in maternal urine and increased risk of attention-deficit hyperactivity disorder (ADHD) in children.
The study, led by Stephanie M. Engel, PhD, professor of epidemiology in the University of North Carolina at Chapel Hill’s Gillings School of Global Public Health, investigated growing concerns over whether prenatal phthalate exposure might impact children’s neurodevelopment. Samantha Drover, a doctoral student studying epidemiology in the Gillings School, joined Engel and colleagues from Duke University, The Norwegian Institute of Public Health and Harvard University in this research.
The full study, titled “Prenatal Phthalates, Maternal Thyroid Function, and Risk of Attention-Deficit Hyperactivity Disorder in the Norwegian Mother and Child Cohort,” was published online May 10 by the journal Environmental Health Perspectives.
According to the Centers for Disease Control and Prevention, “Phthalates are a group of chemicals used to make plastics more flexible and harder to break. They are used in hundreds of products, such as storage containers, detergents, automotive plastics, raincoats and personal-care products like soaps, shampoos and nail polishes. People are exposed to phthalates by eating and drinking foods that have been in contact with products containing phthalates. Once phthalates enter a person’s body, they are converted into breakdown products (metabolites) that pass out quickly in urine.”
“Phthalates are ubiquitous in consumer products, and exposure also can occur through the diet,” Engel said. “It is very difficult for individuals to meaningfully change their exposure to phthalates, but where regulations exist, they have been effective at reducing population exposure to these compounds.”
In recent years, research on children’s prenatal exposure to phthalates has linked high exposure levels with externalizing behaviors and executive functioning defects suggestive of ADHD. To learn more about this association, the researchers undertook an investigation into whether prenatal exposure was associated with clinically confirmed ADHD in a population-based, nested case-control study of the Norwegian Mother and Child Cohort (MoBa) between the years 2003 and 2008.
Investigators measured phthalate metabolite levels in urine samples collected mid-pregnancy; they obtained ADHD case information by linking MoBa and the Norwegian National Patient Registry.
Subsequent modeling revealed that increasing quantities of di-2-ethylhexyl phthalate metabolites (DEHP) in maternal urine were associated with a steadily increasing risk of ADHD in children. Children of mothers who were in the highest quintile for DEHP had almost three times the odds of receiving an ADHD diagnosis compared to children whose mother’s DEHP levels were in the lowest quintile. There were no significant variations to this trend based on sex, preterm delivery or maternal thyroid function.
“There is growing concern that phthalates might have neurotoxic effects, with increasing evidence of behavioral and cognitive associations coming from a number of independent studies,” Engel said. “Our study is unique in that we had both a biomarker of prenatal exposure and a clinically diagnosed developmental outcome, which strengthens confidence in these measures. We need more research into the developmental impacts of these and other endocrine-disrupting exposures, but it also may be worthwhile to consider whether more health-protective regulations are warranted.”
Additional research is needed, the authors concluded, to evaluate potential biological mechanisms linking phthalates to ADHD.