Taking vitamin or mineral supplements could be feeding tumors and allowing them to grow, scientists believe.
Common antioxidants such as vitamins A, C, and selenium and zinc when taken additionally can all cause blood vessels in cancer to grow.
The discovery has come as a surprise as antioxidants were believed to be protective.
The researchers said that natural levels in food were fine but if people take supplements containing antioxidants as well then the extra amount can fuel tumour growth.
The study, published in the Journal of Clinical Investigation, was carried out by the Karolinska Institutet in Sweden.
It concluded that Vitamin C and other antioxidants stimulate the formation of new blood vessels in lung cancer tumors.
They say this could apply to all cancers and the spread of cancer.
Study leader Martin Bergö, professor at the Department of Biosciences and Nutrition, said: “We’ve found that antioxidants activate a mechanism that causes cancer tumours to form new blood vessels, which is surprising since it was previously thought that antioxidants have a protective effect.
“The new blood vessels nourish the tumors and can help them grow and spread.”
Antioxidants neutralize free oxygen radicals, which can damage the body, and are therefore commonly found in dietary supplements.
However, overly high doses can be harmful.
Professor Bergö added: “There’s no need to fear antioxidants in normal food but most people don’t need additional amounts of them.
“In fact, it can be harmful for cancer patients and people with an elevated cancer risk.”
The team discovered that antioxidants reduce the levels of free oxygen radicals but when extra amounts are introduced the drop in free radicals activates a protein called BACH1.
This then induces the formation of new blood vessels, known as angiogenesis.
Ting Wang, a doctoral student in Professor Bergö’s group said: “Many clinical trials have evaluated the efficacy of angiogenesis inhibitors, but the results have not been as successful as anticipated.
“Our study opens the door to more effective ways of preventing angiogenesis in tumors; for example, patients whose tumors exhibit high levels of BACH1 might benefit more from anti-angiogensis therapy than patients with low BACH1 levels.”
Using lung, breast and kidney tumors they found that when BACH1 was activated, either via ingested antioxidants or by overexpression of the BACH1 gene, more new blood vessels were produced but they were highly sensitive to angiogenesis inhibitors.
Wang added: “The next step is to examine in detail how levels of oxygen and free radicals can regulate the BACH1 protein, and we will continue to determine the clinical relevance of our results.
“We’ll also be doing similar studies in other cancer forms such as breast, kidney and skin cancer.”
Produced in association with SWNS Talker