Endocrine Disruption
Endocrine Disruption
In 1971 doctors at the Massachusetts General Hospital reported high rates of unusual cancers of the vagina in teenage girls. Researchers traced the problem to a medicine their mothers were given during pregnancy intended to help prevent miscarriage—a synthetic estrogen called diethylstilbestrol (DES). DES has also been linked to other health problems, ranging from vaginal and uterine malformations and immune problems in girls, to undescended testicles, sperm abnormalities, and possibly testicular cancer in boys exposed to DES before birth. DES is an example of an endocrine disruptor.
Endocrine disruptors are chemicals in our environment that interfere with hormones —natural chemical messengers that travel in the bloodstream and regulate many important physiological activities. Endocrine disruptors may be natural phytoestrogens (estrogenlike chemicals that are made by plants) or synthetic chemicals used in medications, dietary supplements, cosmetics, and household products. They may also show up in pollution. Some examples of endocrine disruptors are listed in the table "Examples of Endocrine Disrupting Chemicals."
The endocrine system, which regulates growth and development and controls important functions in humans and animals, includes hormones and hormone receptors distributed throughout the body. The hormone estrogen helps to regulate bone development, blood clotting, female puberty, the menstrual cycle, many of the changes that occur during pregnancy, and fetal development. The hormone testosterone helps male fetuses develop, regulates male puberty, and promotes sperm production. Thyroid hormones affect energy level, appetite, heart rate, and metabolism in adults. In fetuses and infants, thyroid hormones are essential for normal growth and the development of the brain. There are many other hormones that are less well known, but equally important, including hormones secreted by the hypothalamus in the brain, the pineal gland, the pituitary gland, the parathyroid glands, and the adrenal glands (see figure).
Just as a woman's menstrual cycle returns to normal after she stops taking birth control pills, adult endocrine systems are often able to recover after exposure to artificial hormones. In the very young, however, short-term exposure can have permanent effects. Hormones regulate the normal development of organs such as the brain and reproductive system. Most scientists have therefore focused on the threat endocrine disruptors may pose to fetuses and infants.
Examples of Research on Endocrine Disruption
In 1987 researchers at Tufts Medical Center in Boston were studying breast cancer cells growing in plastic dishes in the laboratory, when they noticed that the cells began to grow rapidly as if exposed to estrogen, even when no estrogen was added to the dishes. They traced the problem to nonylphenol, a chemical leaching from the plastic laboratory dishes. Now researchers use breast cancer cells to test chemicals for estrogenic effects.
In the early 1990s, scientists in Florida studying alligators living in a lake contaminated with DDT and related pesticides noticed that the male alligators had tiny penises and the female alligators had abnormal-looking ovaries.
| chemical | use | mechanism | health effect |
| diethylstilbesterol (des) | medication | mimics estrogen | in humans – female – vaginal cancer, reproductive tract abnormalities; male – abnormalities of the penis and testicles, semen abnormalities |
| genistein | naturally occurring in soybeans | mimics estrogen, blocks testosterone | in adult humans – lowers cholesterol, may decrease breast cancer risk. in animals – infertility. |
| bisphenol a | resin in dental sealants, lining of food cans, and polycarbonate plastics | mimics estrogen | in male mice – alters prostate size, decreases sperm production, affects behavior |
| vinclozolin | pesticide/fungicide | inhibits testosterone | in male rodents – feminization, nipple development, abnormal penis development |
| polychlorinated biphenyls (pcbs) | no longer made; still found as a pollutant | inhibit thyroid hormones | in humans – delayed neurological development; iq deficits |
| dioxin | by-product of industrial processes including incineration | decreases estrogen; decreases testosterone; alters thyroid hormone | in female rodents – delayed puberty, increased mammary cancers. in male rodents – decreased testosterone, penis and testicular abnormalities, feminized sexual behavior. in humans – decreased thyroid hormone levels; decreased testosterone; cancers |
Most of the alligator eggs in the lake did not hatch that year and this phenomenon has persisted into the twenty-first century. These effects may have resulted from pesticide residues in the alligators' food sources. Research has confirmed that DDT acts like estrogen when it enters the body, and that its breakdown product , DDE, blocks male hormones such as testosterone. Other pesticides in the lake have also been shown to mimic estrogen.
Polychlorinated biphenyls (PCBs) are environmentally persistent chemicals that interfere with thyroid hormones. The Great Lakes have been heavily contaminated with PCBs from industrial sources, leaking electrical equipment, and landfills. Researchers have observed that salmon in the Great Lakes have goiter (enlargement of the thyroid gland) and have trouble reproducing. Studies of children born to mothers who ate fish from the Great Lakes at least two times per month found that PCB levels in these mothers were higher than in women who did not eat Great Lakes fish. Their infants were born with abnormally small heads and showed signs of abnormal neurological development. Even when they were eleven years old, the exposed children had significantly lower IQ scores and were twice as likely to be two years behind their peers in their level of reading comprehension.
The Endocrine Disruptor Controversy
The theory that chemicals in the environment may be disrupting hormones and causing health problems in wildlife and humans was first published in 1992. Since that time, the general concept of endocrine disruption has gone from a radical theory to an accepted fact. Scientists agree that some chemicals mimic or block hormonal effects, that wildlife populations in some contaminated areas have been affected by the endocrine-disrupting effects of chemicals in the environment, and that some humans have been affected in unusual circumstances.
Scientific debates focus on whether there is a risk to the general population of humans and animals from the low levels of endocrine disrupting chemicals they are exposed to on a daily basis. Some scientists point out that even low levels of endocrine disruptors may have subtle effects on development of the fetus. These scientists point to trends such as apparent increases in rates of birth defects of the penis in infant boys, declining sperm counts in adult men (see the graph), and increasing rates of hormone-associated cancers such as breast, testicular, and prostate cancer. Other scientists point out that most endocrine disruptors are far weaker than our natural hormones, and that most people and animals are exposed to such low levels of the chemicals that they are unlikely to have any health effects. This debate is likely to rage for many years, because the research to prove or disprove health effects from low-dose exposures to common chemicals will be difficult, complicated, and time-consuming.
Because chemicals that are known or suspected endocrine disruptors are used for a wide variety of purposes, it is difficult for people to know how to avoid exposure. Some endocrine disruptors are used as pesticides on food and others are used in certain types of plastics such as polyvinyl chloride (PVC or vinyl). These chemicals are not just found industrial and agricultural products, but also in the runoff of pesticides from treated fields and in the discharge of waste from industrial operations. Certain endocrine disruptors are not used anymore, but their residues linger in the food chain and are consumed by humans in the form of fatty foods. Because it is difficult for people to make decisions as to how to avoid exposure to endocrine disruptors, many environmental health advocates urge the government to regulate these chemicals more strictly.
In 1996 Congress passed the Food Quality Protection Act, changing how the U.S. Environmental Protection Agency (EPA) regulates pesticides and requiring the EPA to develop an endocrine disruptor screening program. The EPA estimates that there are some 87,000 chemicals used in commerce, and admits there is not enough scientific data available to evaluate all potential risks.
see also Health, Human; Pesticides; Risk.
Bibliography
Colborn, Theo; Dumanoski, Diane; and Myers, John Peterson. (1996). Our Stolen Future. New York: Dutton.
Schettler, Ted; Solomon, Gina; Valenti, Maria; and Huddle, Annette. (1999). Generations at Risk: Reproductive Health and the Environment. Cambridge, MA: MIT Press.
Internet Resources
Colborn, Theo; Dumanoski, Dianne; and Myers, Jonathan Peterson. Our Stolen Future Web site. Available from http://www.ourstolenfuture.org.
Environmental Concepts Made Easy Web site. Center for Bioenvironmental Research, Tulane and Xavier Universities. Available from http://www.som.tulane.edu/ecme.
Solomon, Gina M., and Schettler, Ted. (2000). "Environment and Health: 6. Endocrine Disruption and Potential Human Health Implications." Canadian Medical Association Journal 163(11):1471–1476. Also available from http://www.cma.ca/cmaj.
Swan, S.H.; Elkin, E.P.; and Fenster, L. (1997). "Have Sperm Densities Declined? A Reanalysis of Global Trend Data." Environmental Health Perspectives 105(11): 1228–1232.
Gina M. Solomon and Annette Huddle