U.S. researchers said on Thursday that high estrogen levels are directly linked with a higher risk of breast cancer recurrence. It was already known that the initial development of breast cancer was linked to more estrogen in the bloodstream, but it was previously unclear whether women treated with estrogen-blocking drugs like tamoxifen were still exposed.

The new study which will appear in the journal Cancer Epidemiology, Biomarkers and Prevention, has underlined that women who developed breast cancer have to take additional steps to lower their estrogen levels, including regular exercise and weight management.

"This is the largest study to date and the only one to have included women taking agents such as tamoxifen to reduce estrogen's effect on cancer growth," said Cheryl Rock, a professor of preventive medicine at the University of California, San Diego, quoted by Reuters.

The U.S. researchers found that women whose cancer recurred had more than the double the concentration of estrogen compared with women who remained cancer-free. Estrogen accumulates in the fat and promotes tumors, that is why regular exercise and weight management is needed to lower the risk of the cancer growing back.

"Women who have already been treated for breast cancer should do as much as they can do to reduce estrogen in their blood, such as exercising frequently and keeping weight down," Rock said in a statement.

Women in the United States have a 1 in 8 lifetime chance of developing invasive breast cancer and a 1 in 35 chance of dying from breast cancer. It is the second most common type of cancer after lung cancer and the fifth most common cause of cancer death.