CHICAGO (ASRN.ORG) - Normally sedentary breast cancer survivors who completed an exercise program reduced the levels of insulin in their blood, revealing a likely link between physical activity and better outcomes, researchers from Dana-Farber Cancer Institute in Boston reported at the annual meeting of the American Society of Clinical Oncology in Chicago.

Although it was previously demonstrated that women who lost weight and became more active could lower their risk of breast cancer recurrence, scientists couldn't explain how it worked.

Jennifer Ligibel, MD, of Dana-Farber, and her colleagues' research findings support the hypothesis that exercise lowers levels of the hormone insulin in the bloodstream.

Studies have found an association between relatively high levels of insulin — often seen in obese and sedentary people — and an increased risk of breast cancer recurrence and breast cancer-related death.

"We know that women who are overweight at the time of breast cancer diagnosis have a higher risk of recurrence than lean women, but the reasons for this have not been clear," said Ligibel, of the Breast Oncology Center at Dana-Farber.

 The association with insulin has emerged from recent observations.

In the new study, researchers measured insulin and blood glucose levels in 101 women as well as their weight, body composition, and circumference of the waist and hips.

Half were assigned to a 16-week regimen of mixed cardiovascular and strength training, while the other half had normal care.

At the end of the experiment, the women who exercised had lowered their insulin measurements by an amount that approached statistical significance.

There was also a statistical trend to improved insulin sensitivity (how the body responds to insulin) in the exercise group.

The more-active women had a reduction in hip circumference as well.

"Our findings suggest that the effects of exercise on breast cancer prognosis may be mediated, at least in part, through changes in insulin levels and/or changes in fat mass or deposition," said Ligibel. "Exercise has benefits all through treatment and afterwards. It's an investment in a woman's health and hopefully will prove to lower her risk of dying from breast cancer."

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