Reused, Reheated Cooking Oil May Trigger Breast Cancer Progression, Metastasis

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We’re all aware that fried foods aren’t part of a healthy diet — but it’s more than just “bad for you.” A study out of University of Illinois at Urbana-Champaign found that reused fry oil can cause cancer cells to metastasize more aggressively.

Restaurants habitually reuse oil when deep frying foods.

Oil that has been reused and reheated to high temperatures repeatedly is called “thermally abused” oil. For this study, researchers compared the effect of thermally abused frying oil (TAFO) on tumor cells and compared it to using fresh soybean oil.

They found that TAFO acts as a toxicological trigger that can cause changes to tumors at the cellular level and lead to metastasis, increased cell growth, and changes in how fats are broken down in the body.

Photo: Wikimedia Commons/Biswarup Ganguly

The study was led by Professor William G. Helferich, with Anthony Cam and Ashley B. Oyirifi serving as lead authors. It was published in the journal Cancer Prevention Research.

Mice were fed a low-fat diet for one week, then given either unheated, fresh soybean oil or thermally abused frying oil (TAFO) for 16 weeks. Soybean oil is often the oil of choice in restaurants for deep frying foods.

Then the mice were injected with aggressive breast cancer cells, called 4T1, which have a high rate of metastasis.

Photo: Adobe Stock/vitanovski

After 20 days, researchers found that the primary tumors in the mice who ingested the thermally-abused oil had four times as much growth as the fresh oil group. In addition, the TAFO group had double the amount of tumors in their lungs, and those tumors were more aggressive than the fresh oil group.

Thermally-abused oil contains a carcinogenic chemical called acrolein and fatty acids, which are created when triglycerides are broken down. These are referred to as polar materials, and the more an oil is used and degraded, the more polar materials are produced.

Photo: Wikimedia Commons/Tony Webster

Some countries have regulations on the amount of polar materials allowed in frying oil, with the standard maximum being 24% to 27%.

There are no regulations on polar materials in oil in the United States.

For this study, researchers used TAFO that had 15% polar materials — well under the standard limit in other countries. Fresh oil contains 4% or less polar materials.

Photo: Wikimedia Commons/Silar

Deep-frying food is incredibly common in the United States and around the world, and metastasis is a huge fear among cancer survivors. While metastasis doesn’t stem from one cause alone, this study shows that there’s reason to be cautious when it comes to eating fried foods.

“Many cancer biologists are trying to understand what’s happening at metastatic sites to prime them for tumor growth,” Oyirifi said. “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.”

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C. Dixon likes to read, sing, eat, drink, write, and other verbs. She enjoys cavorting around the country to visit loved ones and experience new places, but especially likes to be at home with her husband, son, and dog.
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