Michelle Holmes, a cancer expert at Harvard University, said people might wrongly think their chances of getting cancer are more dependent on their genes than their lifestyle.
"The genes have been there for thousands of years, but if cancer rates are changing in a lifetime, that doesn't have much to do with genes," she told The Associated Press in a phone interview from Cambridge, Massachusetts.
Breast cancer is the most common cancer in women. In Europe, there were about 421,000 new cases and nearly 90,000 deaths in 2008, the latest available figures. The United States last year saw more than 190,000 new cases and 40,000 deaths.
A woman's lifetime chance of getting breast cancer is about one in eight. Obese women are up to 60 percent more likely to develop any cancer than normal-weight women, according to a 2006 study by British researchers.
Many breast cancers are fueled by estrogen, a hormone produced in fat tissue. So experts suspect that the fatter a woman is, the more estrogen she's likely to produce, which could in turn spark breast cancer. Even in slim women, exercise can help reduce the cancer risk by converting more of the body's fat into muscle.
Yet any discussion of weight and breast cancer is considered sensitive, for some people may misconstrue that as the medical establishment blaming victims for getting breast cancer. Victims themselves could also feel guilty, wondering just how much the issue of weight factored into their own cancer case.
Tara Beaumont, a clinical nurse specialist at Breast Cancer Care, a British charity, said her agency has always been very careful about issuing similar lifestyle advice. She noted that three of the major risk factors for breast cancer — gender, age and family history — are clearly beyond anyone's control.
"It is incredibly difficult to isolate specific factors, therefore women should in no way feel that they are responsible for developing breast cancer," she told the AP on Thursday.
Yet Karen Benn, a spokeswoman for Europa Donna, a patient-focused breast cancer group, said it was impossible to ignore the increasingly stronger links between lifestyle and breast cancer.
"If we know there are healthier choices, we can't not recommend them just because people might misinterpret the advice and feel guilty," she said. "If we are going to prevent breast cancer, then this message needs to get out, particularly to younger women."
Other patient advocates agreed.
"We hope that no one comes away from these studies with the idea that they're an attempt to 'blame' anyone for breast cancer," said Diana Rowden, a vice president at Susan G. Komen for the Cure, a breast cancer group in Dallas. Rowden said the research was essential to warn people of their potential risks for developing breast cancer.
Other lifestyle factors like smoking and spending time in the sun have long been implicated in lung cancer and melanoma. Experts say there is now increasing evidence that what people eat and how much they weigh can contribute significantly to whether or not they develop cancers including those of the colon, stomach, and esophagus.
La Vecchia cited figures from the International Agency for Research on Cancer, which estimated that 25 to 30 percent of breast cancer cases could be avoided if women were thinner and exercised more.
That means staying slim and never becoming overweight in the first place. Robert Baan, an IARC cancer expert in Lyon, France, said it wasn't clear if women who lose weight have a lower cancer risk or if the damage was already done from when they were heavy.
The recommendation to stay slim applies only to breast cancer in post-menopausal women, as there isn't enough evidence to know whether this applies to younger women.
Drinking less alcohol could also help. Experts estimate that having more than a couple of drinks a day can boost a woman's risk of getting breast cancer by four to 10 percent.