Can BPA Plastics Cause Autism? What the Study Found

Recent research has attracted substantial media coverage over whether plastics could have a role in the development of autism.

More specifically, it examined exposure in the womb to bisphenol A (BPA), a substance used in hard plastics, and whether this is linked to boys developing the neurodevelopmental disorder.

Crucially, the research does not demonstrate that BPA-containing plastics cause autism.

It does, however, indicate that BPA may influence oestrogen levels in babies and school-aged boys, potentially affecting the likelihood of an autism diagnosis.

Here is what the study actually found.

What is BPA?

BPA is a chemical component of hard plastics and has been in use for several decades. As it occurs in plastics used for some food and drinks containers, many people encounter small amounts of BPA each day.

Concerns about its possible health effects have existed for some time, because BPA can weakly imitate the effects of oestrogen, a hormone produced in the body.

Although this hormonal action is weak, lifelong low-level exposure has raised health concerns. As a precaution, some nations have prohibited BPA in baby bottles; in Australia, it is being voluntarily phased out of baby bottles.

Autism: what is it and what causes it?

Autism is a neurodevelopmental disorder diagnosed through differences in social communication and repetitive and/or restricted behaviours.

Autistic people can also experience other difficulties, including seizures, changes in motor function - such as problems with fine motor coordination when holding a pencil or turning a key in a door - anxiety, sensory differences, sleep problems and digestive upset.

The severity of these characteristics varies widely, meaning autistic people can have very different experiences of everyday life.

To date, most research has described autistic people who can engage very successfully with their communities and may have exceptional abilities in particular areas. However, there remains a major gap in understanding the many profoundly autistic people who need round-the-clock care.

Genetics has a strong influence on autism, with more than 1,000 genes associated with it. Yet the cause of autism is unknown in most cases, for several reasons.

Detailed gene sequencing is not routinely carried out for children with autism. While particular genes clearly account for some forms of autism, it is more commonly likely to arise through complex interactions between numerous genes. This is extremely difficult to identify, including in large-scale research.

Environmental influences may also play a part in the development of autism. For instance, certain anti-seizure medicines are no longer prescribed during pregnancy because they increase the risk of children developing neurodevelopmental disorders such as autism.

This new research explored another potential environmental influence: BPA exposure in the womb. It involved several strands of investigation, including research in people and mice.

What did the human research show?

Researchers examined a cohort of 1,074 Australian children, around half of whom were boys. By the age of seven to 11 years, 43 children - 29 boys and 14 girls - had received an autism diagnosis. Their average age was nine years.

Urine samples were collected from 847 mothers late in pregnancy, then tested for BPA concentrations. The researchers concentrated their analysis on the samples with the greatest BPA levels.

They also assessed gene changes using umbilical-cord blood taken at birth. The aim was to examine activity of the aromatase enzyme, which is linked with oestrogen levels. Children whose gene changes suggested lower oestrogen levels were categorised as having "low aromatase activity".

The researchers identified an association between high BPA levels in mothers and a higher autism risk among boys with low aromatase activity.

For the final analysis, there were too few girls who had both an autism diagnosis and low aromatase levels to include. The findings were therefore restricted to boys.

What did the mouse study find?

The researchers also investigated what happened when mice were exposed to BPA in the womb.

Mice exposed in this manner showed more grooming behaviour, considered an indication of repetitive behaviour, and less social-approach behaviour, considered an indication of reduced social interaction.

Following BPA treatment, the team also detected changes in the amygdala, a brain area that has an important role in processing social interactions.

They concluded that high BPA levels may suppress the aromatase enzyme, changing oestrogen production and altering the development of neurons in mouse brains.

There are, however, several reasons to interpret the mouse findings cautiously:

• Mouse behaviour cannot be assumed to correspond directly with human behaviour.
• The BPA was not administered to every mouse in the same way: some received injections beneath the skin, while others consumed BPA in a sweet jelly. This could affect the amount of BPA the mice actually received or the way they metabolised it.
• The daily dose used, 50 micrograms per kilogram, exceeded the levels to which people in Australia are exposed and was far higher than the concentrations detected in the mothers' urine samples.

What is the key message on BPA and autism?

An association between two factors - here, BPA exposure in the womb and autism - does not establish that one factor causes the other.

Nevertheless, drawing on the mouse research, the team proposes a possible mechanism: high BPA concentrations could suppress the aromatase enzyme, alter oestrogen production and change how neurons develop in mouse brains.

Does this study identify the cause of autism? On its own, no. Autism did not develop in every baby whose mother had BPA in her urine, so exposure to these plastics by itself is not enough to cause autism. A range of influences, including genetics, is likely to contribute.

The study does suggest a possible gene-environment interaction, in which babies with certain genetic variations may be more vulnerable to BPA's effects and at greater risk of autism. Further research would be needed to establish this.

It is important to recognise that many other potential contributors to autism are supported by similar levels of evidence. Ultimately, the cause of autism remains uncertain for most people.

Elisa Hill-Yardin, Professor and Head, Gut-Brain Axis Laboratory, RMIT University

This article is republished from The Conversation under a Creative Commons licence. Read the original article.