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.