BOSTON (CBS) — A new study says obesity can have a negative effect on young breast cancer patients.

Karen Hackett has shed 30 pounds since being diagnosed with breast cancer two years ago.

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“I’ve made exercise become a priority,” she says. “It’s just part of my day. I try to go sometime around the lunchtime hour and if I don’t, I can go in the evening.”

A new study finds obesity leads to poorer outcomes for women like Hackett who were pre- or peri-menopausal when they were diagnosed with hormone-sensitive breast cancer.

“That’s the novel finding here, we used to think that this association was really seen primarily in older women, here it’s being seen in young women,” said Dr. Clifford Hudis of Memorial Sloan Kettering Cancer Center.

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Doctors still aren’t sure how obesity and cancer are linked.

“The most popular theory is that obese women have more fat cells in their body and these fat cells do produce a little bit of estrogen which can in fact stimulate these hormone-sensitive cells,” said Dr. Freya Schnabel of NYU’s Langone Medical Center.

Researchers also suspect obese woman have higher levels of inflammation in their blood which allow cancer to thrive. It’s unclear if losing weight after diagnosis will improve a patient’s outcome, but Hackett isn’t taking any chances.

“Anything you can do to prolong your life is worth doing,” she says.

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The study doesn’t link obesity to poorer outcomes for post-menopausal patients but doctors say it’s always important to maintain a healthy weight.