November 18 (HealthDay News) -- Scientists believe they have found out why diet and exercise affect a women's chance of breast cancer after she's past menopause, a new study says.

Researchers at the University of Texas at Austin found that cutting calories and exercise affect pathways to mTOR, a molecule that integrates energy balance with cell growth and can contribute to various human diseases when it is not functioning properly.

The research team, expected to present its findings Nov. 18 at the American Association for Cancer Research's annual conference on cancer prevention research, in National Harbor, Md., said these pathways are different, though. Calorie restriction affects more upstream pathways, which may explain why cutting calories delays tumor growth better than exercise when tested on animals.

"One of the few breast cancer modifiable risk factors is obesity," study lead author Leticia M. Nogueira, a research graduate assistant at the University of Texas, said in a news release issued by the conference organizers. "Our study may provide a good scientific basis for medical recommendations. If you're obese, and at high risk for breast cancer, diet and exercise could help prevent tumor growth."

Past research has suggested that consuming fewer calories or increasing exercise levels creates a "negative energy balance" where less energy is taken in than expended, and this lowers the risk of postmenopausal breast cancer associated with obesity. While scientists have thought hormones may play a part in this, it has never been proven.

For the new study, researchers studied 45 obese mice that had their ovaries surgically removed to model the post-menopausal state. After eight weeks, mice fed a calorie-restricted diet had significantly lower blood levels of leptin, a hormone that plays a role in fat metabolism, than those mice only put on an exercise program or those allowed to eat at will with no forced exercise. The calorie-restricted mice also had increased levels of adiponectin, a hormone produced in fat tissue that regulates some metabolic processes, the researchers said.

Some of the cell signaling pathways these hormones manage converge at mTOR, and the researchers found that the key proteins found downstream of mTOR were less active in both the calorie-restricted and exercised mice compared to the controls.

"These data suggest that although exercise can act on similar pathways as caloric restriction, caloric restriction possesses a more global effect on cell signaling and, therefore, may produce a more potent anti-cancer effect," Nogueira said.

SOURCE: American Association for Cancer Research, news release, Nov. 18, 2008