Barcelona, Spain ? Up to a third of breast cancer cases in Western countries could be avoided if women ate less and exercised more, researchers at a conference said Thursday, renewing a sensitive debate about how lifestyle factors affect the disease.

Better treatments, early diagnosis and mammogram screenings have dramatically slowed breast cancer, but experts said the focus should now shift to changing behaviors like diet and physical activity.

“What can be achieved with screening has been achieved. We can’t do much more,” Carlo La Vecchia, head of epidemiology at the University of Milan, said in an interview. “It’s time to move on to other things.”

La Vecchia spoke Thursday at a European breast cancer conference in Barcelona. He cited figures from the International Agency for Research on Cancer, which estimates that 25 to 30 percent of breast cancer cases could be avoided if women were thinner and exercised more. The agency is part of the World Health Organization.

His comments are in line with recent health advice that lifestyle changes in areas such as smoking, diet, exercise and sun exposure can play a significant role in risk for several cancers.

Dr. Michelle Holmes of Harvard University, who has studied cancer and lifestyle factors, said people might wrongly think their chances of getting cancer depend more 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 fuel breast cancer. Even in slim women, experts believe exercise can help reduce the cancer risk by converting more fat into muscle.

Yet any discussion of weight and breast cancer is considered sensitive because some may misconstrue that as the medical establishment blaming women for their disease.