Large doses of antioxidants can actually encourage the growth and spread of skin cancer.

In the 1990s, researchers began singing the praises of antioxidants as effective weapons against everything from heart disease to cancer to age-related chronic conditions.

Now, some scientists are putting antioxidant supplements on the caution list for people who are battling cancer.

A study published today by the University of Gothenburg in Sweden concludes an excess of antioxidants may speed up the spread of skin cancer.

The researchers recommend that people with melanoma not ingest extra antioxidants outside of their normal diet.

“It is not far-fetched to suggest that antioxidant supplementation could increase metastasis in a patient with this disease and we would therefore recommend avoiding these supplements,” Martin Bergö, Ph.D., a professor in the university’s department of molecular and clinical medicine and co-author of the study, told Healthline.

However, Bergö — along with an American Cancer Society official — say antioxidants are probably fine in a regular diet, especially for people who don’t have cancer.

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In their study, Bergö and his team used genetically altered mice with melanoma. The mice developed tumors closely resembling those in humans.

The researchers then fed some of the mice water containing an antioxidant called N-acetylcysteine.

The antioxidant had no effect on the number and size of primary tumors, but it did enhance the migration and invasion of these tumors into other parts of the body.

They said the mice that drank the antioxidant-treated water had twice the number of metastatic tumors in lymph nodes compared to the other mice.

The researchers also exposed human melanoma cells in culture to N-acetylcysteine and another antioxidant and that produced similar results.

The scientists theorize the antioxidants activate a protein that regulates cytoskeletal changes in migrating cells called RHOA, encouraging the spread of the skin cancer.

Bergö noted this is important because the spread of melanoma is what makes it lethal.

“Growth of the primary tumor on the skin is not dangerous and this tumor is often removed by surgery,” said Bergö. “So, identifying factors that can affect metastasis is crucial.”

Cancer patients are more likely to supplement their diet with substances such as antioxidants than healthy people, making this an especially relevant discovery.

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Antioxidants can be found in most berries as well as some red beans, apples, cherries, russet potatoes, plums, and other foods.

For years, they were touted for their ability to absorb molecules called free radicals and reduce a person’s risk of cancer and other diseases.

Now scientists are cautioning against too much of a good thing.

A previous study done by Bergö and other researchers in 2014 concluded that antioxidants may increase tumor growth in people with lung cancer.

Another study linked antioxidants to the progression of prostate cancer, Brego said.

A review by researchers at the National Institutes of Health found that the results of studies on antioxidants and prostate cancer are “inconsistent,” and that there have been mixed results on antioxidants and cancer growth.

Victoria Stevens, Ph.D., the American Cancer Society’s strategic director of laboratory services, told Healthline she felt Bergö’s study was well done and had interesting results.

But, she notes, it was conducted on mice. “It’s pretty hard to extrapolate what it might mean in humans,” she said. “It’s too early to say specific antioxidants will affect specific cancers.”

She added that antioxidants still appear to have healthful effects as part of a regular diet, especially for people who don’t have cancer.

Anyone with cancer should consult their oncologist before taking any supplements, Stevens said.

It remains true that antioxidants can protect healthy cells from free radicals that can cause cells to become malignant, but they can also protect tumor cells once they have developed.

“For people with an increased risk of cancer this means that taking nutritional supplements containing antioxidants may unintentionally hasten the progression of a small tumor or premalignant lesion,” said Bergö.

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