Mice exposed to bisphenol A appear more sensitive to oestrogen.© Getty

A chemical found in plastics may put women exposed to it at greater risk of developing breast cancer, it seems. A study in mice has found that minute doses of the oestrogen-like substance increase breast tissue development, and higher density breast tissue is a risk factor for cancer.

Many hard plastics contain the compound bisphenol A, which can leach into food after heating. The chemical also appears in some dental fillings and the linings of tin cans. Industry began using bisphenol A in the 1950s, but in recent years scientists have documented how it mimics the hormone oestrogen.

Some scientists worry that because oestrogen plays such a crucial role in the development of a fetus's reproductive system and other organs, exposure to bisphenol A in the womb could cause problems. A recent study of mice exposed in this way found that the artificial compound caused abnormally high levels of growth in the male animals' prostate glands¹.

Now, another team of researchers has investigated the effects of this chemical on female mice: the results are reported in the journal Endocrinology².

Sensitive study

The scientists hypothesized that bisphenol A could make breast tissue more sensitive to the effects of oestrogen. So they exposed mouse fetuses to a daily dose of 250 nanograms per kilogram of their body weight, less than 1% the amount deemed safe for humans in the United States.

Four days after birth, the scientists stopped administering the chemical to the mice. They then waited until the animals reached puberty at 30 days of age, and removed their ovaries. This allowed them to deliver the mice's oestrogen themselves, so that they could measure its influence in a controlled way.

The mice exposed to bisphenol A while in the womb developed significantly more tissue in parts of their mammary glands than their control counterparts. The exposed mice had a fourfold increase in gland structures known as terminal end buds, according to Ana Soto, a cell biologist at the Tufts University School of Medicine, Boston, who led the study. "This is important because increased [breast tissue] density in humans is an established risk factor for breast cancer," she says.

"It's an important study because it's the first time someone's showed that prenatal exposure to bisphenol A causes alterations in mammary gland development," says Sohaib Khan, a molecular endocrinologist at the University of Cincinnati, Ohio, who has studied the chemical's impact on breast-cancer cells.

Although the findings focus on laboratory animals, they may have implications for people, Soto says.

"We have to be careful in the way we extrapolate," she says. But this research, along with previous studies, leads her to believe that bisphenol A probably increases the risk of breast cancer in humans, she adds.

Soto says that testing the effects of the chemical in humans would be difficult, as the study would have to monitor exposure over 50 years from birth to the age at which breast cancer more commonly occurs.

Lawmakers in California are currently considering legislation that would ban bisphenol A from children's toys. Soto believes that action is needed now. "Do we want to wait those 50 years?" she asks. 

University of Cincinnati, Ohio

• References

  1. Timms B. G., et al. Proc. Natl Acad. Sci. USA, 109. 7014 - 7019 (2005).
  2. Munoz-de-Toro M., et al. Endocrinology, published online doi:10.1210/en.2005-0340 (2005).