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Endocrine Disruptors

2. How do EDCs act?

  • 2.1 What are the mechanisms involved in EDC action?
  • 2.2 What are the major life stages when EDCs could act?
  • 2.3 What are the relationships between the EDC dose and effect?

2.1 What are the mechanisms involved in EDC action?

Research clearly shows that endocrine disrupting chemicals (EDCs) can act in a number of ways in different parts of the body. Most studies have focused on the influence of EDCs on hormone receptors, which are the parts of cells in target tissues that hormones lock on to, in order to trigger an effect. EDCs, by occupying the same receptor sites in the target cells as the natural hormone would do, can mimic the effect of a hormone or block its action.

Other ways in which EDCs can work – such as disrupting hormone production, transport or breakdown – have been shown to be equally important. However, for most studies showing a link between EDC exposure and harmful effects, the mechanisms of ED action are poorly understood.

This makes it difficult to distinguish between direct and indirect effects. So care must be taken when using the results of laboratory studies to predict effects in wildlife populations or in humans in the ‘real’ world. Knowledge of the similarities and differences in the endocrine systems of different animal species and humans is also important when trying to predict effects in one species from observations in another.

A combined body of evidence (epidemiological and laboratory data) will be necessary to understand the situations in which EDC exposure may affect endocrine systems. More...

2.2 What are the major life stages when EDCs could act?

Despite the lack of comprehensive information on how endocrine disrupting chemicals (EDCs) can act, there are particular stages in animal and human lifecycles that are known to be vulnerable to endocrine disruption.

What is known at present is that:

  • exposure to EDCs during the early development of the endocrine system, such as in the womb, or during childhood, may permanently alter its function or its reactions to various signals in the body,
  • exposure during adulthood may not show significant effects as the body is more able to adapt,
  • the same exposure levels may produce different effects during different life stages or even seasons,
  • interactions between different parts of the endocrine system may result in effects which cannot be predicted from existing knowledge, sometimes occurring in unexpected places in the body.

There is considerable knowledge about hormonal responses at the molecular level, but little is known about how these molecular changes might affect health. Until such information becomes available, it will remain difficult and controversial to attribute harmful effects to EDCs. More...

2.3 What are the relationships between the EDC dose and effect?

The question of how much exposure to an endocrine disrupting chemical (EDCs) is necessary to produce an effect is perhaps the most controversial issue in endocrine disruption. One of the reasons for this is because EDCs often act by mimicking or counteracting the actions of hormones produced naturally by the body. These natural hormones are often more powerful than any foreign EDCs and are present in the body at levels already influencing biological function.

Since any EDC presence is an addition to these natural hormones, it can be argued that exposure to low doses of EDCs could have measurable additional effects on biological functions. In other words, EDCs could have no threshold for their activity – that is, any exposure, no matter how small, might trigger some effect.

Reports of EDC effects in laboratory animals at low doses are highly controversial and the subject of intense research. Scientists from different laboratories have not always been able to obtain the same results at low doses. Another issue is whether traditional testing methods are robust enough to pick up low-dose effects.

Similarly, there is controversy over the issue of synergy – that is, whether simultaneous exposure to EDCs with similar actions might produce a greater effect than the sum of their individual activities. Again, scientists from different laboratories have not been able to obtain the same results using combinations of EDCs.

The timing of exposure is also an important factor in how the body responds to different doses - both for humans and wildlife, with respect to effects on development, reproduction, cancer, the immune system and the nervous system.

At present no firm conclusions can be drawn about low-dose effects. Researchers are continuing to look at this possibility. More...

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