Scientists know that being overweight increases the risk of breast cancers fed by estrogen, but being too fat may also increase the risk of triple-negative breast cancers, a less common and far more deadly type, U.S. researchers said on Tuesday.

They said women who are overweight had a 35 percent higher risk of developing triple-negative breast cancers, an aggressive form of cancer that affects 10 to 20 percent of cases.

“The fact that we found an association with triple-negative breast cancer is unique because, biologically, this subtype is very different from other breast cancers,” said Amanda Phipps of the Fred Hutchinson Cancer Research Center in Philadelphia, whose study appears in the journal Cancer Epidemiology, Biomarkers & Prevention.

Breast cancer is the second-leading cause of cancer death among U.S. women, after lung cancer. It kills 500,000 people globally every year and is diagnosed in close to 1.3 million people globally.

Most breast tumors are called estrogen-receptor positive because they are fueled by the hormone estrogen. About 20 percent are HER2-positive, because a protein called HER2 is involved. A third type is driven by the hormone progesterone.

These types of cancer can easily be treated.

Then there are triple-negative tumors, so named because they lack estrogen, progesterone or HER2 receptors needed for most breast cancer drugs to work.

Studies have found that women who put on a lot of weight, they increase their risk of breast cancer, likely because estrogen accumulates in the fat and promotes tumors.

For her study, Phipps and colleagues analyzed data from the 155,723 women enrolled in a large study of the effects of hormone replacement therapy on menopausal women.

The team studied body mass index or BMI and physical activity among the 307 women in the study who had triple negative breast cancer and the 2,610 women who had estrogen receptor-positive breast cancers.

BMI is equal to weight in kilograms divided by height in meters squared. A person 5 feet 5 inches tall is classified as overweight at 150 pounds (68 kg) and obese at 180 pounds (82 kg).

Phipps and colleagues found women with the highest BMI in the study had a 35 percent higher risk of triple-negative breast cancers, and a 39 percent higher risk of estrogen-fed breast cancers.

The study raises new questions about what besides estrogen drives tumors, suggesting a possible role for other growth factors or inflammation, Amy Trentham-Dietz of the University of Wisconsin said in a statement.

Phipps said the effect was modest, but if confirmed by other teams, the study offers women new ways to reduce their risk of triple-negative breast cancers.

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By Julie Steenhuysen

CHICAGO