Discovering How Environment Contributes to Breast Cancer

By Jeffrey Norris, UCSF Today | August 21, 2006

Breast cancer incidence in the United States ranks near the top internationally. And just across the Golden Gate from UCSF -- in Marin County -- studies show that the rate at which new breast cancers arise is among the highest in the United States. Some Marin women touched by the disease have been driven to activism. They are working with scientists to plan and gain support for studies aimed at finding out why breast cancer rates are so high.

The trend toward increasing breast cancer incidence in the United States is indeed disturbing -- even though increased screening, early detection and better treatment have at the same time produced a trend toward fewer breast cancer deaths.

Some breast cancer risk is due to genetics. For instance, about 5 percent is due to rare mutations in just two genes called BRCA1 and BRCA2. But the variability of breast cancer incidence internationally and its overall increase over time suggest that environmental factors are primary causes. Among immigrant populations to the United States, for example, breast cancer incidence has increased markedly in just one or two generations. The gene pool does not change so quickly.

Environmental Exposures Are Not Limited to Pollutants
The public and breast cancer advocates may think of environmental toxins when they think of carcinogens. But to scientists, environmental influences include not only toxicants, but also diet and lifestyle.

Delayed childbearing is a lifestyle choice and a growing trend. Putting off having children -- or never bearing children -- is considered a "reproductive" risk factor. Reproductive risks increase a woman's exposure to her own estrogen. Estrogen clearly is an important, beneficial hormone, but it also helps foster breast cancer. Breastfeeding infants helps lower the risk. Other reproductive risks that increase estrogen effects are early age of first menstruation and late menopause -- not regarded as modifiable.

In addition to genes, breast cancer risks a woman can't control include increasing age and a family history of breast cancer.

Other known, modifiable breast cancer risks include being overweight after menopause and taking hormone replacement therapy. Alcohol consumption -- more than one drink per day -- also is a known risk factor for breast cancer. A woman can lower her risk for breast cancer by exercising. These modifiable risks all are considered environmental factors.


Read more at Jeffrey Norris, UCSF Today