BARCELONA, Spain (AP) -- 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 on breast cancer said Thursday.

While better treatments, early diagnosis and mammogram screenings have dramatically slowed the disease, 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, told The Associated Press. "It's time to move onto other things."

La Vecchia spoke Thursday on the influence of lifestyle factors at a European breast cancer conference in Barcelona.

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.

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.

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, 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.

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.

After studies several years ago linked hormone replacement therapy to cancer, millions of women abandoned the treatment, leading to a sharp drop in breast cancer rates. Experts said a similar reduction might be seen if women ate better -- consuming less fat and more vegetables -- and exercised more.

Michelle Holmes, a cancer expert at Harvard University, said changing things like diet and nutrition is arguably easier than tackling other breast cancer risk factors.

"Women who have early pregnancies are protected against breast cancer, but teenage pregnancy is a social disaster so it's not something we want to encourage," she said in a phone interview. "But there's no downside to reducing obesity and increasing physical activity."

She also said people may mistakenly 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 said.

In the 1980s and 1990s, breast cancer rates steadily increased, in parallel with the rise in obesity and the use of hormone replacement therapy, which involves estrogen.

La Vecchia said countries like Italy and France -- where obesity rates have been stable for the past two decades -- show that weight can be controlled at a population level.

"It's hard to lose weight, but it's not impossible," he said. "The potential benefit of preventing cancer is worth it."

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