In both mice and humans, there is a natural process called apoptosis through which cells with DNA too damaged to repair—such as cancer cells—can be forced to destroy themselves. The study showed some antioxidant vitamins may suppress that process, reducing cancer cell death.

"Antioxidant vitamins are not just good or bad. They may protect some normal cells [from developing into cancers], but also may reduce tumor cell death from apoptosis [once a cancer has formed]," said the study’s lead author, Rudolph I. Salganik, MD, PhD, of the University of North Carolina’s School of Public Health.

"People take antioxidants to protect themselves from free radicals," added Dr. Salganik, referring to a form of oxygen that can damage cells. "But free radicals are not only bad guys. Without free radicals, apoptosis can’t occur. That’s why free radicals are important."
 
How the Study was Performed

Dr. Salganik and colleagues gave a group of 14 mice a standard mouse diet containing normal amounts of antioxidant vitamins A and E, and 13 other mice the same diet minus the vitamins A and E. The mice are of a type that usually develop brain tumors spontaneously.

Only about one-sixth as many tumor cells were killed by apoptosis in the mice with vitamins A and E in their diets. And tumors in the vitamin-deficient mice were about half as large as those in mice in the other group. Vitamin C, also an antioxidant, was not studied because mice bodies make their own, unlike humans who need to get vitamin C from their diets.

Clinical Studies in Humans Needed

Dr. Salganik said clinical studies are needed to learn how the findings might apply to humans. "We can’t transfer our animal data to humans," he said. "But there is a serious concern about whether high levels of these vitamins might promote cancer. It’s time for serious population studies, clinical trials, and a look at individual differences in antioxidant needs and levels."

The new study raises concern partly because it comes just months after another mouse study suggesting that tumor cells need high amounts of vitamin C, and two human studies suggesting smokers with more of the antioxidant beta carotene in their blood have higher risks of lung cancer.

And experts familiar with the way chemotherapy and radiation work say antioxidant vitamins may also interfere with those therapies, making them less effective. Radiation and many chemotherapy drugs are meant to work by producing unbalanced atoms, called ions, that damage cancer cells so badly that apoptosis takes over, finishing the job of killing the cancer cells. But antioxidant vitamins may neutralize those ions chemically, so apoptosis isn’t called into play.

Cancer Experts Urge Caution

As a result of the recent research, many cancer experts are strongly cautioning against self-prescribing of vitamins.

Harmon Eyre, MD, Executive Vice President for research and medical affairs for the American Cancer Society (ACS), suggests cancer patients take the following precautions in light of all the evidence:

• If your doctor has not prescribed vitamins, do not take any on your own.
• It is safest to avoid taking high doses of antioxidant vitamins during chemotherapy or radiation treatment. Ask your doctors when it might be safe to start such vitamins after treatment is finished.
• Eat a balanced diet. A balanced diet usually supplies the necessary vitamins and nutrients.