This fat may aid spread of breast cancer.

Last year, researchers noted that diets high in linoleic acid -- a nutrient the body needs but must obtain from foods -- appear to lower the risk that nascent prostate cancer will spread (SN: 10/9/93, p.229).

If that observation prompted you to load up your diet with linoleic-rich fare such as nuts, corn oil, and most margarines, here's a sobering follow-up.

New animal data indicate that diets high in this same polyunsaturated fat increase the risk that a breast cancer will spawn potentially deadly satellite tumors.

David P. Rose and his coworkers at the American Health Foundation in Valhalla, N.Y., injected mice with cells cultured from either of two types of human breast cancer lines. They then compared how different proportions of linoleic acid -- accounting for approximately 8 percent or 50 percent of dietary fat -- influenced tumor development. (Each mouse derived roughly 40 percent of its calories from fat, a proportion similar to that of the typical U.S. diet.)

Primary tumors -- those that start in mammary tissue -- grew faster in animals eating the diet containing more linoleic acid, the scientists report in the Dec. 15 CANCER RESEARCH. How much faster the tumors grow depended on the cell line.

But more important, notes coauthor Jeanne M. Connolly, were data on the ability of developing tumors to spread. When cancer victims die, it's usually from metastases -- secondary tumors that form when renegade cells begin colonizing other parts of the body.

"Linoleic acid enhances metastasis in both of the cell lines," Connolly notes -- a finding she says suggests this invasiveness is not a fluke.

The data also point to a mechanism underlying this effect: Linoleic acid can boost the cancer cells' production of an enzyme that allows them to snip through barrier membranes and invade neighboring tissues or the bloodstream.