In pre-clinical studies, cancer researchers at Memorial Sloan-Kettering Cancer Center in New York have found that vitamin C appears to substantially reduce the effectiveness of anticancer drugs.

The study found that when human cancer cells were treated with a form of vitamin C in lab dishes, chemotherapy drugs killed 30% to 70% fewer tumour cells than usual and they say the same mechanism may affect patient outcomes as vitamin C supplements may undercut the effectiveness of cancer drugs.

The team led by Dr. Mark Heaney also implanted human cancer cells into mice, and found that when mice were given vitamin C supplements two hours before chemotherapy, the tumours grew more quickly.

The research team, which included researchers from Columbia University, then delved into the mechanism by which vitamin C may be protecting these cells and discovered that it was not because the nutrient was neutralizing oxygen-free radicals.

Five commonly used chemotherapy drugs doxorubicin, cisplatin, methotrexate, vincristine and Gleevec, also known as imatinib, were tested and Dr. Heaney says while the vitamin C did not neutralize the effects of the chemotherapy drugs, it blunted their effects.

The drugs work in different ways to combat tumours and Dr. Heaney says while vitamin C is something everyone needs to have in their diet, he does not recommend taking supplemental vitamin C during chemotherapy treatment and it is the use of large doses of over-the-counter vitamin C that is a concern.

Dr. Heaney believes the protective effect vitamin C has on mitochondria, which generate energy for a cell within cancer cells, may be the culprit rather than the antioxidant properties of the vitamin.

Chemotherapy drugs damage mitochondria in cancer cells and Dr. Heaney says when mitochondria are damaged, they can send signals to the cell to die and that is one of the ways that the chemotherapy drugs exert their beneficial effects and vitamin C helps to preserve the health of the mitochondria, preventing chemotherapy agents from working properly.

The researchers have been researching the connection between vitamin C and cancer therapy for some time, and these new findings expand on their earlier observation that vitamin C seems to accumulate within cancer cells more than in normal cells.

Use of vitamin C during cancer treatment has been controversial and some studies have suggested that because vitamin C is an antioxidant it might be beneficial to cancer patients, but studies in which vitamin C pills were given to treat cancer failed to show a benefit.

Heaney suspects that vitamin C is good for the cells of normal tissue because it provides more protection for the mitochondria, and therefore probably extends cell life.

But he says this is not what you want when you are trying to eliminate cancer cells.

The research is published in the journal Cancer Research.