Antioxidants are often marketed in products from foods to skin creams as a way to ward off aging and cancer.

While antioxidants seem to have some benefits, ongoing research shows that although they don’t cause cancer they may fuel growth of the disease once it does appear.

The latest research from an international team of scientists shows that, at least in mice, antioxidants used in diabetes medication can spur the spread of existing cancers, specifically liver and colon cancers.

The new research, published Wednesday in the journal Science Translational Medicine, suggests that, when combined with diabetes drugs, antioxidants protect cancers from oxidative stress.

This, in turn, boosts the cells’ ability to migrate from their source and invade other parts of the body.

Researchers note that this highlights antioxidants’ potential to fuel the spread of cancer.

Earlier this week, new research revealed that global rates of diabetes have quadrupled since 1980. As of 2014, 422 million adults worldwide have either type 1 or type 2 diabetes.

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Too Many Free Radicals

Scientists believe that diabetes is caused by oxidative stress that occurs when the body has an imbalance between reactive oxygen species, known as “free radicals,” and the body’s ability to get rid of them.

This kind of oxidative stress is why antioxidants are often added to diabetes drugs. They can be helpful in ushering out free radicals, which have also been linked to cancers.

According to the American Diabetes Association, age, tobacco use, being male, white or black, overweight, or inactive increase a person’s risk of both diabetes and cancer. Worldwide, the numbers of people with both diabetes and cancer are rising.

As researchers are still trying to explain the link between diabetes and cancer, the new research suggests there may be cause to suspect diabetes drugs containing antioxidants may help cancer cells spread once they’ve formed.

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Testing It Out on Mice

The new research, led by Hui Wang of the department of endocrinology at Xinqiao Hospital in Chongqing, China, began with mice with liver and colon cancers.

Researchers gave the mice diabetes drugs with antioxidants to see the effects on cancerous tumors.

The research team, comprised of scientists in China, Germany, and the U.S., found that while these antioxidant-infused drugs didn’t raise the risk of developing cancer, they did speed up the spread of the tumors. It did this, they say, by protecting the cancer cells from oxidative stress.

This, in turn, activated what scientists call the NRF2 signaling pathway and triggered the production of proteins that promote metastasis. Researchers noted that deleting or blocking NRF2 reduced the cancer’s ability to spread.

Besides mice, which aren’t always as reflective of humans as researchers would like, the team analyzed liver tumor samples from human patients. They found the expression of NRF2 correlated with the spread of cancer cells.

The researchers concluded that in light of their results, further studies should be done on the safety of antioxidant-containing diabetes drugs, especially those being used in patients with cancer.

However, before any meaningful clinical decisions can be made, these studies need to be replicated in human patients.

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Other Connections of Antioxidants to Cancer

The new research is far from being alone in suggesting antioxidants have a role in the spread of cancer.

A 2014 study, also published in Science Translational Medicine, showed a connection when mice were given vitamin E, which has antioxidant properties, and the drug N-acetylcysteine (NAC). NAC is used as a treatment for chronic obstructive pulmonary disease (COPD).

In that study, a team of Swedish researchers found the more antioxidants given to mice, the quicker they died from lung cancer. The presence of antioxidants caused a three-fold increase in tumor growth, researchers found.

The same researcher, Martin Bergö, Ph.D., a professor in the University of Gothenburg’s department of molecular and clinical medicine, concluded in 2015 that an excess of antioxidants might speed up the spread of skin cancer.

While supplementing your diet with antioxidants may appear to do more harm than good for cancer patients, the research is still preliminary and results are based on animal models.

The American Cancer Society (ACS) notes that cancer patients, especially smokers, can have worse outcomes when supplementing their diets with antioxidants.&

“Additional large randomized controlled trials are needed to provide clear scientific evidence about the potential benefits or harms of taking antioxidant supplements during cancer treatment,” the ACS website states. “Until more is known about the effects of antioxidant supplements in cancer patients, these supplements should be used with caution.”