A recent study in mice showed that reheated cooking oil might trigger cell changes that can promote late-stage breast cancer growth.

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."

Breast cancer statistics

Breast cancer occurs when cells in the breast grow out of control and form a tumor. If the cells are capable of invading the surrounding tissue or spreading to other areas of the body, doctors deem the disease to be malignant. Breast cancer is not exclusive to women—although it is rare, it can affect men as well. Breast cancer rates in the United States have increased by 0.4% a year over recent years.

About one in eight women in the United States will develop breast cancer during their lifetime, and the American Cancer Society (ACS) estimates that doctors will diagnose around 268,600 new cases of invasive breast cancer in 2019. In the same year, the ACS also expects almost 63,000 cases of carcinoma in situ, which is an early form of breast cancer that is noninvasive. The ACS recommends that women at average risk for breast cancer should start having annual routine mammograms at the age of 45 years. Even if a person has no signs or symptoms, mammograms can help detect breast cancer at an early stage, which is when treatment is most likely to be successful.

Why reheated oil may be bad

The repeated reheating of cooking oil changes its composition and releases acrolein, which is a toxic and potentially carcinogenic chemical. Restaurants and other food outlets often reuse soybean oil multiple times before replacing it with fresh oil in the frying vats. These factors are what led the researchers to investigate whether thermally abused oil could have any effect on breast cancer growth. Although the results are preliminary, they add to a knowledge base that will grow deeper as research in this area continues.

"Many cancer biologists are trying to understand what's happening at metastatic sites to prime them for tumor growth," notes co-lead author and graduate student Ashley W. Oyirifi.

"We're trying to add to this conversation and help people understand that it might not be just some inherent biological mechanism but a lifestyle factor. If diet provides an opportunity to reduce breast cancer survivors' risk, it offers them agency over their own health."

—Ashley W. Oyirifi