The impact of endocrine disruptors on intelligence

The human endocrine system is composed of several glands and other tissues that secrete many hormones. All of these hormones are important at different stages of life. The endocrine system works using a negative feedback mechanism. Endocrine glands continuously measure hormone levels circulating in the blood and respond to the body’s needs by normally secreting just the right amount of hormone required at the right time.

According to the World Health Organization (WHO), endocrine disruptors are chemicals of natural or artificial origin that, being foreign to the body, can interfere with endocrine system functioning and cause adverse effects on the body of an individual or his or her descendants.[5] Endocrine disruptors can act in different ways: they can mimic or interfere with the action or regulation of a natural hormone. In all cases, they alter hormone levels either downwards, often during critical phases of growth, or upwards, creating toxic effects.

There are hundreds of suspected or proven endocrine disruptors. A list published by the United Nations Environment Committee in 2017 already contained 45 that were well established by rigorous studies.[9] The list includes commonly used products such as the following[10]:

A bit of history

It was not until the early 1990s that the term “endocrine disruptor” first appeared in scientific literature.[7] However, the discovery of the adverse health effects of industrial contaminants such as mercury (Minamata disease in Japan) and the effects of organochlorine pesticides on wildlife reproduction dates back to the 1950s. Over the years, hundreds of scientific studies have led to the restriction of some of these products (e.g., glyphosates, organochlorine pesticides, bisphenol A, etc.) or even a complete ban.

There are thousands of chemicals used in industry, particularly in the food and cosmetic industries. What’s more, new products appear on the market every year. Therefore, identifying the most dangerous for human health is a never-ending task. Unless there is a major disaster, such as the mercury contamination of Minamata Bay in Japan in the 1950s, endocrine disruptors are identified one molecule at a time from often limited observations. For obvious ethical reasons, toxicity studies can only be carried out on animals, usually small rodents such as rats or mice. It is then easy for industries using these products to challenge the extrapolation of data from these small animals to humans. The situation is even more complex when dealing with a product that is toxic for a short period of time (e.g., the first few years of life or during pregnancy) or whose effects are not manifested until the next generation.

An interesting case

Diethylstilbestrol (DES) was used from the 1930s to the 1970s as synthetic estrogen to prevent miscarriages, premature births and certain complications at the time of delivery. Pre-market toxicity studies showed that it was not toxic to pregnant women. However, it was found to be toxic to their offspring, especially girls. These effects only appeared in adulthood as fertility problems or vaginal cancers. Alarmingly, what first attracted the interest of industry in synthetic estrogen was bisphenol A, a chemical found in the plastic of some baby bottles, among other products. That goes to show the power of this substance as an endocrine disruptor![9]

The thyroid gland is often cited as an example to explain the link between endocrine disruptors and intelligence. The influence of the mother’s thyroid hormone levels on the brain development of the fetus is well known. Thyroid hormones contain iodine. In many countries, the soil is low in iodine, so locally produced fruits and vegetables are not sufficient to maintain an acceptable level of iodine in pregnant women. When this results in a significant deficiency of thyroid hormones which goes untreated, the newborn is at risk of developing a combination of physical deformities and mental retardation called “cretinism.” This problem was more common a hundred years ago (endemic goiter) and was virtually eliminated by the mandatory addition of iodine to salt between the 1920s and 1930s.

Despite this progress, several substances suspected of being endocrine disruptors, particularly those containing chlorine, fluorine or bromine, have a known effect on thyroid function.[6] In pregnant women, they are believed to block the uptake of iodine by the thyroid gland, which then stops producing thyroid hormones. Without provoking obvious cases of cretinism, these substances or mixtures of substances (cocktails) may act on the brain development of children, creating a possible link between endocrine disruptors and a decline in intelligence.

Nevertheless, the development of intelligence is likely due to many factors (environment, genetics, educational system, etc.). It may be premature to lay the blame on endocrine disruptors as the sole cause of the decline in intelligence observed in several countries, particularly on the European continent.[11]

Not all endocrine disruptors can be eliminated from our immediate environment. However, we can limit our exposure by developing certain protective reflexes:

• Avoid foods that may contain endocrine disruptors: read the labels, avoid overly processed products and eat organic!
• Ventilate your home, and avoid laminates that contain a lot of PVC or furniture treated with flame retardants.
• Choose simple household cleaning products.
• Be especially vigilant during pregnancy and the growth of young children (bottles, pacifiers, toys, etc.).

It takes decades for a chemical product to obtain the status of a “proven endocrine disruptor” and for countries to begin regulating its use. However, we are not exposed to a single endocrine disruptor but to a whole cocktail, each contributing modestly to the creation of a much larger toxic effect. A laboratory demonstration of this type of synergistic effect is probably impossible to achieve with current means. So, does this mean we are doomed to see the IQ of future generations constantly declining? Perhaps not, but it is still important to guard against the effects of endocrine disruptors, especially during pregnancy and early childhood.