SUNDAY, Nov. 2, 2008 (HealthDay News) -- Obesity can wreck a person's health for many reasons. But for women, too much weight tacks on an additional danger: Studies have linked obesity and breast cancer in a variety of ways.
Doctors aren't sure why this link exists and are trying to figure out what ties weight gain to breast cancer. But they are more and more convinced the link is there, and they are urging women to watch their weight and increase their exercise to help stave off what is the most common cancer among females, nonmelanoma skin cancer aside.
"There are a lot of factors we need to figure out," said Dr. Jennifer A. Ligibel, of the Dana-Farber Cancer Institute in Boston. "There are a lot of things we don't know."
An estimated 182,500 women in the United States will be found to have invasive breast cancer in 2008, according to the American Cancer Society, and about 40,480 women will die from the disease this year. Currently, there are about 2.5 million breast cancer survivors in the United States.
Studies have found that, in general, obesity is linked to cancer. The higher a person's body-mass index (BMI, a ratio of weight to height), the more likely she or he will develop cancer, according to recent research by scientists at the University of Manchester in England. Other studies have found similar links to increased body fat.
Still other studies have found that women with breast cancer are more likely to live shorter lives and suffer a recurrence of their cancer if they are overweight.
For example, in a recent study conducted at the University of Texas M.D. Anderson Cancer Center in Houston, more than two-thirds of women with stage III locally advanced breast cancer were either overweight or obese. The study also found that a greater proportion of obese patients were likely to be diagnosed with a rare and more deadly form of breast cancer, known as inflammatory breast cancer.
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 developing breast cancer and dying of it after menopause is believed due to increased levels of estrogen in obese women, said Colleen Doyle, director of nutrition and physical activity with the American Cancer Society.
There is good news. Studies have shown that exercise -- 30 minutes to 60 minutes a day of moderate-to-high intensity physical activity -- decreases breast cancer risk, Doyle said.
"Physical activity reduces breast cancer risk both directly, by decreasing circulating estrogens, and also indirectly, by helping with weight control," she said. "Women are so concerned about breast cancer risk. Communicating that there are key things you can do to reduce risk -- watch your weight and be more active -- are valuable messages."
Ligibel agreed, noting that exercise might be valuable enough to counteract the strain on the body caused by obesity.
"You might not need to lose weight if you exercise," Ligibel said. "Exercise could affect the hormone levels and help keep cancer from occurring or recurring."
To learn more visit the American Cancer Society.