Mothers who eat fish and other foods rich in omega-3 fatty acids during pregnancy and while nursing may reduce the risk of breast cancer in their daughters by as much as 40 percent, a new study of mice found.

The researchers also found that feeding female offspring a diet rich in omega-3 fatty acids after weaning reduced their risk of breast cancer by 40 percent.

And consuming omega-3 fatty acids through foods or supplements at any point in life can reduce the rate for breast cancer in female offspring significantly, said lead researcher W. Elaine Hardman, an assistant professor at Louisiana State University's Pennington Biomedical Research Center.

However, eating omega-6 fats, which are commonly found in Western diets, could increase female offsprings' risk of breast cancer, according to the study, presented Wednesday at the American Association for Cancer Research annual meeting in Anaheim, Calif.

Both omega-6 and omega-3 fatty acids are essential for good health. But in Western diets, the amount of omega-6 fatty acids is much greater than omega-3 fatty acids. Omega-6 acids are found in meat, eggs, poultry, cereals, breads, baked goods, most vegetable oils and margarine, the researchers said.

Sources of omega-3 fatty acids are fish, especially tuna, salmon and mackerel. They're also found in canola and flaxseed oils, soybeans and nuts.

"In mice genetically programmed to develop breast cancer, we found that if we fed omega-3 fatty acids to the mice, we could prevent them from developing cancer," Hardman said.

Hardman's team compared the rates for breast cancer in the offspring depending upon how much omega-6 fatty acids or omega-3 fatty acids they and their mothers consumed.

All the mouse pups exposed only to omega-6 fatty acids -- in the uterus, while nursing and after weaning -- developed mammary gland tumors by six months after birth, which was expected, according to the presentation on Wednesday.

However, less than 60 percent of the female offspring with diets rich in omega-3 fatty acids either in the uterus or after weaning developed mammary tumors by eight months.

Hartman speculated that because omega-3 fatty acids reduce the amount of estrogen, which is important in mammary gland development, this helps reduce cancer risk. "Somehow changes are going on in the breast tissue of the mice before they're born that makes a difference in their risk for developing breast cancer later on," she said.

Hartman said people need to include more omega-3 fatty acids in their diet. "Particularly pregnant women need to eat more fish or take an omega-3 supplement to help reduce the risk of cancers in the next generation," she said.

One expert said this study may be relevant to humans.

"Does this prove the same is true in humans? By itself, no. But in the context of all we know about dietary fats, hormones and health outcomes in people, it is very suggestive," said Dr. David L. Katz, associate clinical professor of public health at Yale University School of Medicine.

"I generally encourage my patients to increase their intake of omega-3 fatty acids, and emphasize this in particular during pregnancy and breast-feeding," added Katz, director of the university's Prevention Research Center.

Katz noted that cancer typically develops slowly, often over decades. So, tracing the root causes, or tracking down all the clues to prevention, can be a challenge. "In this case, mice are providing an important clue, and teaching a lesson I believe we should heed," he said.