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WASHINGTON, June 27 -- Young women with high body fat have a decreased chance of developing breast cancer before menopause, according to a study published on Wednesday in the journal JAMA Oncology.

"It is well known that women who gain weight, particularly after menopause, carry an increased risk of postmenopausal breast cancer," said Dale Sandler, the paper's co-senior author and head of the Epidemiology Branch at the National Institute of Environmental Health Sciences (NIEHS), part of the National Institutes of Health.

"Our finding that breast cancer risk is not increased in obese premenopausal women, and in fact decreases, points to the possibility that different biologic mechanisms are responsible for causing breast cancer in younger women," said Sandler.

Sandler and other researchers pooled data from 19 different studies, comprising 758,592 women from around the world.

The participants ranged in age from 18 to 54 at the beginning of study. Volunteers for each individual study filled out several rounds of questionnaires, which included height, weight, and other health-related factors.

With this information, researchers evaluated the risk of developing breast cancer in relation to body mass index (BMI) in the following age ranges: 18-24, 25-34, 35-44, and 45-54. BMI is a way to measure the amount of body fat.

Overall, 13,082 participants, or 1.7 percent, developed breast cancer during the observed time periods.

The scientists determined that relative risk of premenopausal breast cancer was reduced 12 to 23 percent for each five-unit increase in BMI, depending on age.

The strongest effect was seen in relation to BMI at ages 18-24, with very obese women in this age group being 4.2 times less likely to develop premenopausal breast cancer compared to women with low BMI at the same age, according to the study.

While Sandler and her colleagues are unsure why young, premenopausal women with a high BMI appear to be protected against breast cancer, she cautioned that young women should not intentionally gain weight to lower their breast cancer risk.

"There are so many health risks associated with being overweight or obese," Sandler said. "We still believe it is important for women to maintain a healthy weight throughout life."