Scientists vary in their opinions on why this link exists, and what it means.
Some believe that obesity may make tumors harder to detect, so a woman's breast cancer will be further developed before it is discovered.
"It could be because there's more breast tissue, a lump would be less evident," Ligibel said.
Researchers also believe that the systemic effects of obesity might do something to spur cancer on. For example, obesity or overweight can lead to fluctuations in hormone levels in the body.
"When women are heavier, their estrogen levels are higher," Ligibel said. "That could be a pathway through which weight affects breast cancer. Other studies have shown that when insulin levels are high, there's more chance a cancer will come back."
Another link to obesity was found in a study from the University of North Carolina at Chapel Hill that showed that obese women are more likely to skip screenings for breast and cervical cancer. Without those screenings, women are less likely to catch breast cancer at a more treatable stage.
Debbie Saslow, director of breast and gynecologic cancer at the American Cancer Society, said it's not completely clear what role obesity plays in breast cancer risk.
"For obesity, which is independent of breast size, I would think two factors would come into play," Saslow said. "One, a positive, is that the breasts may be fattier, which would make a mammogram easier to read. The second, a negative, is indirect: Obese women are less likely to go to a doctor."
Menopause appears to be a critical time, Ligibel said. Obesity creates a greater risk for breast cancer post-menopause, while pre-menopausal women actually have a reduced risk.
"Gaining weight around the time of menopause is a risk factor in developing breast cancer," Ligibel said.
The increased risk of develo
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