Researchers from the University of Illinois at Urbana-Champaign tested "thermally abused frying oil," which is cooking oil that has undergone reheating to high temperatures multiple times, in laboratory mice and found that it increased metastatic breast cancer growth.

The team reported these findings in the journal Cancer Prevention Research.

The scientists fed all of the lab mice a low-fat diet for a week. Then, they gave some of the mice unheated fresh soybean oil for 16 weeks while the rest ingested thermally abused oil instead.

They chose to use soybean oil because the restaurant industry commonly uses it for deep frying.

To simulate breast cancer, they injected 4T1 breast cancer cells into a tibia of each mouse. These breast cancer cells are very aggressive and have a high rate of metastasis to multiple distant sites. As a result, they often appear in the lymph nodes, liver, and lungs.

The effects of reused oil

At 20 days after the injection of the tumor cells, there was a notable difference in the rate of metastatic growth between the two groups of mice. In the mice who had eaten thermally abused oil, the metastatic growth of the tibia tumors was four times greater than that of the tumors in the mice who consumed the fresh oil.

There were also more lung metastases in the former group. Lead researcher William G. Helferich, a professor of food science and human nutrition, noted that there were twice as many lung tumors, which were also more aggressive and invasive than those in the fresh-oil group.

"I just assumed these nodules in the lungs were little clones — but they weren't," says Helferich. "They'd undergone transformation to become more aggressive. The metastases in the fresh-oil group were there, but they weren't as invasive or aggressive, and the proliferation wasn't as extensive."

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