Vitamin C can help to prevent cancer, but not the way that scientists thought, according to a study published on Monday in the United States.

Scientists have long thought that Vitamin C and other antioxidants help to fight cancer growth by grabbing volatile oxygen free radical molecules and preventing them doing damage to DNA.

But researchers at Johns Hopkins University in Baltimore found that antioxidants play a different role in the fight: they destabilize a tumour's ability to grow under oxygen-starved conditions.

The researchers happened on this new role by observing mice which had been implanted with one of two types of cancer which produce high levels of free radicals, which can be suppressed by feeding the mice supplements of antioxidants, such as Vitamin C.

When the Hopkins team examined cancer cells from mice that had been implanted with cancer but not fed antioxidants, they noticed there was no significant DNA damage.

"If DNA damage was not in play as a cause of the cancer, then whatever the antioxidants were doing to help was also not related to DNA damage," said Ping Gao, one author of the study.

That conclusion led Gao and Dang to suspect that some other mechanism was involved, such as a protein known to be dependent on free radicals called HIF-1 (hypoxia-induced factor).

The researchers found that HIF-1 was abundant in untreated cancer cells taken from the mice, but disappeared in Vitamin C-treated cells.

"HIF-1 helps an oxygen-starved cell convert sugar to energy without using oxygen and also initiates the construction of new blood vessels to bring in a fresh oxygen supply," explained Chi Dang, who also worked on the research.

Some rapidly growing tumours cons me so much energy, they use up all the available oxygen nearby, and need HIF-1 to survive.

But HIF-1 can only work with a ready supply of free radicals. So antioxidants play a protective role by removing these free radicals and stopping HIF-1. In doing so, they also stop the tumour in its tracks.

The authors confirmed the importance of HIF-1 by creating cancer cells with a genetic variant of the protein that did not require free radicals to be stable. In these cells, antioxidants no longer had any cancer-fighting power.

But before health-conscious consumers rush out to stock up on Vitamin C supplements, the researchers stressed that their study was still in the early stages.