THURSDAY, March 6 (HealthDay News) -- New research shows that women who experienced a recurrence of their breast cancer had almost twice as much estrogen in their blood as women who remained cancer-free after treatment.

This indicates that circulating estrogen levels contribute to a recurrence as much as the initial malignancy does.

That information is not entirely new, said Dr. Jennifer Wu, an obstetrician/gynecologist at Lenox Hill Hospital in New York City. "That's the reason we use drugs that help to lower estrogen levels. Estrogen causes increased cell division; we think it can perhaps start breast cancer," she said. "But this is a good study in that it has a lot of patients and proves that they have a demonstrable increase in estrogen levels over patients who don't have a recurrence."

Where there's a problem, there's also often a solution.

"Anti-estrogen drugs can only have so much impact," said study author Cheryl Rock, a professor of family and preventive medicine at the University of California, San Diego, School of Medicine. "There are two things apart from these drugs that can help to lower estrogen, or we believe it can, because it can in the general population. One is moderate to vigorous exercise, and the other is healthy weight management, achieving an ideal weight."

The hormone estrogen is produced not only by the ovaries, but also by fat tissue.

Previous research has shown that estrogen contributes to the risk of primary breast cancer in postmenopausal women, but there has been less evidence of the role of estrogen in cancer recurrence.

"The relationship between circulating estrogen and risk for primary breast cancer is very well-established, but there were surprisingly few studies in which estrogen levels have been measured in breast cancer survivors," Rock explained.

This study, published in the March issue of Cancer Epidemiology, Biomarkers & Prevention, followed 153 pairs of women who had had breast cancer (one in each pair experienced a recurrence, while one did not) for more than seven years.

Two-thirds of the participants were using tamoxifen, a drug which interferes with estrogen's activity in the body.

In the end, women with more circulating estrogen were more likely to have a recurrence.

There may be other factors at play also, Rock said. For instance, sex hormone-binding globulin basically makes estrogen available to get into tissue. "If estrogen is bound to that protein, it's not going to float right over to the cell," Rock said. "When people are overweight, they have higher blood levels of insulin, which suppresses synthesis of that protein, so exercise not only is related to actually helping weight management but, because it lowers insulin, it might make the hormonal situation look better."

And don't rule out existing anti-estrogen drugs, experts added.

"This study justifies the use of drugs that help decrease estrogen levels like tamoxifen and aromatase inhibitors," Wu said. "[In the future], we may want to titrate different levels of anti-estrogen medications. Right now, we have a standard dosage for everyone, whereas women who are heavier or other women who may have higher estrogen levels for one reason or another may need larger doses."

More information

Visit the National Cancer Institute for more on breast cancer.