The combination of eating too much and exercising too little appears to significantly increase the risk of breast cancer, particularly in postmenopausal women, according to a new study .

Investigators in the U.S. and China found that women who had an unhealthy energy balance - meaning they weren’t working off the calories they consumed - were more than twice as likely as lean and active women to develop breast cancer. After menopause, both unhealthy habits appeared to up the risk of the cancer by nearly five-fold.

In contrast, simply overeating appeared to do little to increase breast cancer risk, the authors report in the journal Cancer Epidemiology, Biomarkers & Prevention.

“It’s both (eating poorly and not exercising) together that really magnifies the risk, and makes it worse,” Dr. Alecia S. Malin of Meharry Medical College in Nashville, Tennessee, told .

During the study, Malin and her team interviewed 1,459 women diagnosed with breast cancer, and 1,556 women of similar age without breast cancer, and asked them to recall their activity levels in recent years. All women were living in Shanghai.

Women who were relatively overweight appeared to have no higher risk of breast cancer if they also exercised, while inactive women appeared to escape the disease if they stayed lean.

Women who consistently showed the highest risk of cancer were those who were relatively inactive, heavier, and followed a higher-calorie diet.

These findings suggest that it’s the combination of the two unhealthy habits - overeating and inactivity - that appear to put women at risk, Malin said in an interview. “Women who didn’t work out, who had zero activity, and ate more than 2100 calories were at risk,” she said.

She added that these findings serve as another “warning” for women about how they put their health at risk if they don’t eat right and exercise.

“These are the healthy things to do, and here’s what happens if you don’t do them,” Malin noted.

The researcher said that one of her co-authors, Charles E. Matthews, has also found that women who eat more than they work off are also at higher risk of endometrial cancer.

To investigate the relationship between energy balance and cancer, she and her colleagues plan to study how the body changes with an unhealthy energy balance, and if those changes can trigger cancer.

SOURCE: Cancer Epidemiology, Biomarkers & Prevention, June 2005.