LATE-stage cancer patients could be thwarting their own treatment by taking multi-vitamin pills containing antioxidants, the Nobel Prize-winning scientist James Watson has warned.
The benefits of supplements containing antioxidants such as vitamins A, C and E are the subject of fierce debate.
While some studies suggest that they could offer moderate protection against cancer, Professor Watson, who with Francis Crick discovered the ''double helix'' structure of DNA in 1953, argues that the pills could be doing more harm than good.
In a new paper, he claims that the reason late-stage cancers often become untreatable is that they produce high levels of antioxidants which stop treatments such as chemotherapy and radiotherapy from working.
In healthy people, antioxidants can be helpful because they attack molecules known as ''free radicals'' which can damage DNA. But many cancer treatments use free radicals to kill tumour cells, meaning antioxidants could prevent them doing their job.
Professor Watson said studies should be carried out to test his theory, which he described as ''among my most important work since the double helix''.
Writing in the Royal Society's Open Biology journal, he said: ''For as long as I have been focused on the understanding and curing [of] cancer, well-intentioned individuals have been consuming antioxidative nutritional supplements as cancer preventatives if not actual therapies.
''In light of the recent data strongly hinting that much of late-stage cancer's untreatability may arise from its possession of too many antioxidants, the time has come to seriously ask whether antioxidant use much more likely causes, than prevents, cancer.
''Blueberries [which are high in antioxidants] had best be eaten because they taste good, not because their consumption will lead to less cancer.''
Professor Nic Jones, Cancer Research UK's chief scientist, said: ''We know from many large studies that, far from being potent cancer-fighters, antioxidant supplements seem to be ineffective for cancer prevention in healthy people, and some can even slightly increase the risk of cancer. This should give people good reason to think twice about relying on them.''
Steve Williamson, cancer spokesman for Britain's Royal Pharmaceutical Society, added: ''I always advise patients not to take antioxidants while they are having chemotherapy in case it counteracts it.''
Cancer Council Australia's chief executive Ian Olver said the comments ''had merit''.
''Multi-vitamins are something that may be helpful in prevention, but they may not be helpful for treatment,'' Professor Olver said.
With a study last year showing that more than 50 per cent of male cancer patients in Australia were using ''complementary and alternative medicines'' while receiving chemotherapy or radiotherapy, Professor Olver said patients and doctors should ensure that alternative medicine use is out in the open.