Clinical trials have indicated the harm of high dosage in certain cases

High-dose antioxidant supplements can accelerate the progression of lung cancer in high-risk groups such as smokers and those with chronic obstructive pulmonary disease, suggests a study carried out in mice.

Molecules known as 'reactive oxygen species' (ROS) are generated in cells through normal metabolic processes as well as in response to stress, such as exposure to toxins and radiation. These highly reactive chemicals can damage DNA and other cellular structures.

As DNA damage can lead to cancer, it has been thought that giving antioxidants, which neutralise ROS, could offer a way of reducing the risk of cancer. Clinical trials have, however, given mixed results, with some indicating that antioxidant supplementation could be harmful in certain cases.

In research published in Science Translational Medicine today (January 30), Swedish scientists examined the effect of two commonly given antioxidants, vitamin E and N-acetylcysteine, on mice with lung tumours. (The mice had been genetically modified to carry one of two tumour-causing genes.)

“What we found is that antioxidants caused a three-fold increase in the number of tumours and also tumour aggressiveness,” observed Martin Bergo of the University of Gothenburg in Sweden, one of the senior authors of the paper.

The antioxidants caused the mice to die twice as fast, he said during a telephonic press briefing. The effect was dose-dependent, with the tumours worsening when the mice were given a higher dose of the antioxidants.

Take-home message

“The take-home message of this study is that antioxidants may be harmful and should probably be used with caution by patients with lung cancer and people with an increased risk of developing the disease,” such as smokers and those with chronic obstructive pulmonary disease, remarked Per Lindahl, another of the study authors, at the press briefing.

However, “it's important to remember that this is a basic science study,” he cautioned. Directed clinical studies were needed to answer the important questions it raised.

Whether N-acetylcysteine, which was often given to those with chronic obstructive pulmonary disease to ease their breathing difficulties, might increase their cancer risk had, for instance, needs to be carefully evaluated. The effect of other antioxidants too needed study.

The scientists took the view that consumption of antioxidant-rich foods was unlikely to have a harmful effect as the levels of those chemicals in them were far lower than in dietary supplements that were prescribed.

The experiments they carried out showed that the antioxidants acted by lowering levels of a vital protein, p53, inside tumour cells. This tumour-suppressing protein could detect damaged DNA and then stop such cells from dividing or even induce their death. With less p53 around, tumour cells were able to escape this sort of scrutiny and could continue to proliferate.

This study only showed what could happen when antioxidants were given after a lung tumour had developed in mice, remarked T.R. Santhosh Kumar, a cancer researcher at the Rajiv Gandhi Centre for Biotechnology in Thiruvananthapuram. Whether this held good for other sorts of tumour too had to be examined. Besides, it was possible that antioxidants might be beneficial in preventing cancers from setting in.