What are the hormonal disrupters?
The media have not been slow to pick up a news story that the sperm counts of Danish males are worse than their Finnish counterparts. In addition, it seems that genital malformations in boys and testicular cancer in adolescents are also more common in Denmark. The finger of suspicion has been pointed at environmental chemicals, especially those that have structural characteristics to the sex hormones. Such compounds are believed to disrupt the delicate checks and balances involved in the synthesis of the male sex hormones. Other chemical agents act by increasing the amounts of female sex hormones present in males. This phenomenon has been extensively studied by a group of Danish scientists led by Professor Niels Erik Skakkebaek. The explanation proposed by these workers is that Denmark is a more polluted country than Finland.
DDT and birds
This link between environmental pollutants and reproductive difficulties is by no means new. In 1962, Rachel Carson published her best-seller, Silent Spring. The main argument in her treatise was that the widespread use of insecticides was disrupting the reproductive cycles of birds so badly that soon the forest would no longer resound to bird-song in the morning. When Rachel Carson published her book, little was known about how chemicals could interfere with reproduction, in fact the discipline known as reproductive toxicology was still to be conceived. The origin of developmental toxicology can be traced to the thalidomide catastrophe of the 1960s where a seemingly innocuous sedative drug was responsible for around 10,000 children around the world being born with severe deformities in their arms and legs.
Carson described how robins would nest and lay eggs normally but that the chicks would never hatch. The problem was especially prevalent in areas which had been sprayed with DDT and other insecticides (e.g. compounds used to kill the beetles which spread a fungal disease which affects elm trees). A similar phenomenon was subsequently described in seabirds feeding off Baltic Sea fish, including the majestic white tailed eagle whose numbers started to decline dramatically. These birds were especially at risk since they feed off the top of a dietary pyramid as described in the chapter "The Dirty Dozen - not just an old movie?" The most striking effects of DDT are mediated via its metabolite, DDE which functions as an anti-androgen; in other words it counteracts the effects of the natural male sex hormone, testosterone. The effects of the insecticides to evoke oestrogen-like (i.e. female sex hormone-like) effects are less well documented.
Hormonal disrupters do not act exclusively to unbalance the actions of the male and female sex hormones. In fact, the most extensively studied hormonal disrupters disturb the activity of a totally different gland, the thyroid gland and the actions of its natural hormones (e.g. thyroxin). There are many environmental chemicals which possess the ability to alter either the synthesis or the breakdown of thyroxin. One large group of these glucosinolate compounds occurs naturally in a wide variety of vegetables, especially those from the Brassica family (broccoli, brussel sprouts, cauliflower, mustard, turnips). Other plants such as soy, onions and garlic also contain compounds which interfere with thyroid gland activity. In addition to these natural compounds, there are many synthetic chemicals which interfere with the many phases of thyroxin production and release. The most important of these synthetic materials are some herbicides and insecticides, some fungicides as well as the dioxins and polybrominated compounds used as flame retardants.
Thyroxin is a critical hormone since it plays such a central role in balancing energy needs. Its presence is also crucial during development. Thus a lack of thyroxin during the foetal period interferes with the correct development of the central nervous system, which in turn has devastating effects on the viability of the newborn. This has been dramatically demonstrated in amphibian species; if thyroxin is not present in the correct levels at the correct time, then tadpoles fail to metamorphose into mature frogs.
It's the dose that determines that a thing is not a poison
Once again, it is crucial to stop to think for a minute. It is only natural to be concerned about the potential threats posed by hormonal disrupters. But before we are stampeded into a panic reaction, we must consider whether it is theoretically possible to observe effects of synthetic chemicals at the exposure levels which exist today. The rates of synthesis of all hormones are strictly controlled by the body. For example, should the synthesis of thyroxin or the sex hormones be reduced for some reason or other, then the brain and the pituitary gland (which is situated just under the hypothalamic region of the mid-brain) are stimulated into releasing regulatory hormones into the bloodstream and these trigger the glands to accelerate their synthetic activities. In this way it is possible to guarantee that the correct amounts of these hormones are present in the blood. Thus low level disruption by synthetic (or natural compounds in the diet) does not cause any major effects. It is only when the regulatory capacity is overwhelmed that the synthesis of the hormone starts to decline with the inevitable biological consequences.
Rachel Carson estimated that between one to two kilograms of DDT could be sprayed on each tree in a park. This is an incredibly huge amount. These insecticides are rather insoluble in water and therefore the chemicals stay stuck onto the surface of the foliage until the leaves drop off in the autumn. The leaf litter provides the food for earthworms and insects, so these animals accumulate large amounts of the insecticides. The levels may be great enough to kill a bird feeding off these creatures but consumption of lower doses can impair the bird's reproductive capabilities. However, we are faced with a problem - can we really extrapolate from the results of such massive exposures in birds to what may happen in humans exposed to much lower doses.
In the Baltic Sea, reproductive problems are not only encountered in the fish-eating seabirds, but also in one group of mammals – Baltic seals. However, once again caution is needed before we hastily jump to conclusions. Seals consume several kilograms of fish every day. In other words, their exposure levels are at least one hundred times higher than those likely to occur in humans.
To sum up, there is no doubt that our environment does contain chemicals that can be called hormonal disrupters. These chemicals are present in the environment; in fact they are in our diet so that each day we consume small quantities of many of these compounds. Some of these chemicals are synthetic chemicals but others are (and have always been) a natural part of our everyday diet. In fact, we probably understand the effects of the synthetic chemicals better than the normal dietary components. Irrespective of whether the compound is synthetic or natural, we need to be concerned with the safety margin - that's where the research needs to be focussed - the difference between the dose to which an individual is exposed and the dose needed to exert biological effects.
Recently sufficient data has accumulated to indicate that for certain chemicals such as the polybrominated flame retardants, the organotin compounds, even for the insecticides, the safety margins are sufficient in Europe. With respect to dioxins, there are certain specific sub-groups who do consume large amounts of fish (e.g. professional fishermen) and for these individuals, the situation is still unclear and more work needs to be done. Dioxins can influence the activities of a great many enzymes and some of these are involved in the synthesis and/or breakdown of the sex hormones and thyroxin. If dioxins were to be demonstrated to be exerting biological effects on these systems, then it would be logical to encounter the same effects in smokers since smoking causes a similar spectrum of biological effects as exposure to dioxins.
There are illogical, conflicting, even contradictory, beliefs held on this subject. Many plant oestrogens and derivatives of female sex hormones are marketed widely as health supplements. These may well be advertised in the same magazine which is running a “shock exclusive” article full of outrage about the feminization of the males of the species due to the presence of low levels of synthetic hormonal disrupters in our diet.
At present, there are more theories about hormonal disrupters than hard-boiled facts. For most of us, the risk associated with these chemicals is minimal. However, there may be some individuals in the population who do need to be monitored in more detail to determine whether they exhibit any effects from exposure to these hormonal disrupters.
Notes and references
- See the chapter "Why are experts usually so sceptical?"
One level up: Is man defiled by what goes to his mouth?
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