The rise in noncommunicable diseases (NCDs) affecting children worries researchers. Some believe that this increase could be caused by the many manufactured chemicals on the market. This is what 25 specialists from the Consortium for Children’s Environmental Health explain in a review article published on January 8 in the prestigious medical journal New England Journal of Medicine (NEJM). The authors point out that some 350,000 manufactured chemicals, chemical mixtures and plastics are listed in the world. A figure 50 times higher than in 1950 and which could triple by 2050. However, their manufacturing, their pre-marketing evaluation as well as the post-marketing surveillance of long-term harmful effects on health would be, according to the authors of the publication, subject to too few legal or political constraints: “Less than 20% of them have been tested for their toxicity, and even fewer for toxic effects in infants and children”.
A problem that they compare with the increase in health alerts over the last 50 years in the United States. Thus, “the incidence of childhood cancers increased by 35%, congenital malformations linked to male reproduction doubled, the prevalence of childhood asthma tripled and that of pediatric obesity almost quadrupled,” the researchers continue. . They add that neurodevelopmental disorders now affect one in six children and that autism spectrum disorders are diagnosed in one in 36 children. At the same time, deaths from non-communicable diseases such as strokes and many cancers have decreased. in adults.
Children more fragile than adults
To explain this difference with adults, researchers point out that children are less resistant to certain chemicals. “Although the maxim: ‘the dose makes the poison’ is still relevant, it is clear that the timing of exposure during early human development is at least as important,” they warn, adding that studies Scientific research in recent years shows that even brief, low-level exposures to certain toxic chemicals during periods of early vulnerability, such as during pregnancy or a child’s early years, are associated with increased risk of disease and disability that can persist throughout life. “A fetus or a child is more fragile at the same doses of exposure to chemical substances (drugs, toxic products, etc.) because the latter will interact with the mechanisms of development and maturation of metabolic pathways, particularly at the cerebral level, which are still in progress, but completed in adults”, explains Professor Stéphane Marret, head of the neonatal pediatrics and neuropediatrics department at Rouen University Hospital, who did not participate in the study.
The authors of the article NEJM cite for example the health scandals linked to the cases of 10,000 babies born with a congenital malformation of the limbs after their mothers ingested thalidomide (an anti-nausea and sedative used in the 1950s and 1960s) at the start of pregnancy. , or that of infants born with profound neurological deficiencies due to their mothers’ involuntary consumption of mercury-polluted fish. They recall, again, the results of a study in which functional MRIs were carried out on children whose mothers lived next to a highway when they were pregnant and which showed greater inflammation of the brain and a drop in IQ scores.
Although they are cautious in recalling that chemicals are probably not the only cause of this increase in non-communicable diseases and that it is “probable that there are other as yet unknown links”, the researchers nevertheless affirm that Protecting children from the dangers of chemicals “requires a fundamental review of current legislation and a restructuring of the chemical industry to take into account children’s health” and multiple exposures.
Risk factors still poorly identified
The study itself and its coverage in the media sparked contrasting reactions. It has sometimes been criticized for a supposed alarmist nature or a lack of scientific data supporting the real or supposed risks of manufactured chemicals, but also for the links made between these products and neurodevelopmental disorders (disorders of the spectrum of the autism, intellectual development, language development and academic learning, development of attention with hyperactivity or even motor development disorders). In reality, although the authors cite these disorders in their article, they do not specifically detail their links with manufactured chemicals. On the other hand, their comments were sometimes distorted and amplified in the press. “The article from NEJM is factual, very well argued and measured”, notes Professor Marret.
What do we really know about the links between the increase in neurodevelopmental and autism spectrum disorders and manufactured chemicals? The undeniable explosion in the number of diagnoses of these disorders in recent years can, at least in part, be explained by the broadening of the criteria (people who were not identified yesterday are today) and better detection by professionals. “The increase in neurodevelopmental disorders in children, which I see in my research and clinically, is not only linked to the fact that they are better detected, but because there are other contributing factors. risks, particularly environmental, which come into play”, defends Professor Marret. But while certain chemicals are studied, such as phthalates and other endocrine disruptors, there is not always strong evidence demonstrating their direct roleparticularly for autism.
Low levels of evidence should not preclude caution
Neurodevelopmental disorders are multifactorial, explains this specialist. They are the consequence of the action of several factors: genetic, environmental, epigenetic, etc. “Several risk factors must add up so that the child’s development trajectory is impacted, to the point of creating difficulties,” explains Professor Marret. In the same way, it is accepted that autism spectrum disorders are partly genetic, since nearly a thousand gene variants that act on the development and maturation of skills have been identified. But, with rare exceptions, just one of these mutations is not enough to trigger an autism spectrum disorder. Each can, however, combine with other genetic variants, or other environmental factors, some of which are probably not yet known.
To illustrate the problem, Professor Marret cites the cases of exposure to alcohol of fetuses throughout pregnancy. In the event of proven exposure, some women will be able to give birth to a child with one or even several malformations and/or neurodevelopmental disorders, others a child who has no malformation, but who will later develop a brain deficit disorder. attention with hyperactivity (ADHD), even a child without complications. For what ? Because alcohol consumption during pregnancy, harmful in itself, may or may not be the trigger for other risk factors, genetic or epigenetic. “When we know this, we better understand the limitations of certain epidemiological studies. We need to have studies with a large number of subjects to have sufficient statistical power and highlight a risk factor,” continues the researcher. This is why research still encounters great difficulties in determining that this or that risk factor is associated with certain diseases (asthma, diabetes or obesity) or certain neurodevelopmental disorders.
“The link between manufactured chemicals and the development of autism spectrum disorder is very difficult to study in epidemiology, confirms Claire Philippat, researcher at Inserm and specialist in the links between chemical pollutants and child development. Existing cohorts with biological samples during pregnancy and early childhood, which are essential to properly measure exposure to compounds such as phthalates, are generally too small to study this disorder.
“And not only does detecting a single risk factor require studies with a considerable number of participants, but a factor is generally not sufficient on its own to explain a pathology,” continues Professor Marret. Thus, obtaining strong levels of evidence making it possible to directly link a pollutant to a disease or disorder can be an obstacle course and requires conducting several epidemiological studies with thousands of participants, then carrying out toxicity studies in experimental models. . This is why current work is most often content to highlight a single risk factor, with low levels of evidence, while calling for continued research in order to confirm – or not – a potential danger. It is this same problem that explains the controversies – sometimes violent – around many chemical products, notably pesticides and herbicides such as glyphosate.
Thus, Professor Marret, like many other specialists, remains cautious but is concerned about the potential harmful effects as yet unidentified and potentially underestimated and also calls for continued research on these subjects. “Part of the hope could rest on national and international initiatives aimed at carrying out large-scale studies in the general population with very early recruitment – from pregnancy – in order to collect biological samples and measure chemicals, to accurately estimate exposures”, estimates Claire Philippat. Another hope could lie in the development of artificial intelligence tools which could help to better detect and quantify the different risk factors.
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