LOS ANGELES, CA (Catholic online) - "We think that the likelihood is that since this change has been so marked over just a couple of decades, that it's something external, a modifiable lifestyle-related risk factor or perhaps an environmental toxic exposure, but we don't know what," Dr. Rebecca Johnson at Seattle Children's Hospital and the University of Washington says.

Johnson and her colleagues found the rate of metastatic breast cancer, in particular had risen about two percent each year between 1976 and 2009 among younger women.

Overeating and lack of exercise may be driving up early-life metastatic breast cancer rates, Johnson says. The use of hormonal birth control could also be playing a role. Johnson also advocates for more research into the potential effects of hormones in meat or plastic in bottles.

Johnson's team analyzed data from cancer registries run by the National Cancer Institute. As expected, they found that the number of early breast cancer diagnoses increased among middle-aged and older women during the study period, likely due to widespread screening.

The only other change in cancer incidence was among the youngest women, between ages 25 and 39, where women diagnosed with metastatic breast cancer, which has spread to the bones, brain or lungs rose from one in 65,000 in 1976 to one in 34,000 in 2009.

More of the increase appeared to be in cancers that are sensitive to estrogen, which is "comparatively fortunate." Those cancers are somewhat more responsive to treatment and have longer average survival rates in general.

Still, metastatic cancer is the most dangerous kind, with less than one-third of women surviving at least five years after diagnosis, Johnson's team wrote.

There are dissenting voices. "It is intriguing data, but I think that it's going to have to be validated in some other datasets," Surgeon Dr. Julie Margenthaler, who has studied breast cancer in young women at the Washington University School of Medicine in St. Louis, says.

Margenthaler said the new study was limited by a lack of data on women's family history, including which ones were carriers of BRCA gene mutations.

However, since the overall rate of cancer in young women is still low, Johnson said the findings shouldn't cause alarm - but should prompt further research.

"We're certainly not advocating any changes in screening mammography practices. This is an increase, but it's small on a population level," she told Reuters Health. "There's no reason that because you're 35 and see this report, you need to go out and get a mammogram right away."

© 2013, Distributed by NEWS CONSORTIUM.