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A genetically modified corn should be removed from sale in New Zealand following a study showing it may be causing cancer and other health problems, the Green Party says.
The Journal of Food and Toxicology has published a two-year study showing rats which ate a GM maize treated with the herbicide RoundUp were more likely to develop a range of cancers.
Female rats who ate the maize were two to three times more likely to die from large mammary tumours than subjects used in a control experiment.
Males developed large tumours, liver problems and kidney deficiencies. Each experiment group contained 10 rats.
Green Party genetic engineering spokesman Steffan Browning said the GM maize used in the study had been approved for sale in New Zealand a decade ago.
It could be causing increased cancers and other health issues among New Zealanders, he said.
"There are now huge concerns over the safety of this corn. Eating this corn has now been proven to cause the growth of tumours, so why was it approved a decade ago without the necessary evidence that it was safe to eat?"
Mr Browning said 70 GM corns available in New Zealand were approved for sale on the back of a 90-day health trials.
The two year study had proved those trials were not long enough, he said.
He called for Food Standards Australia New Zealand to immediately recall any products which had the GM corn from the study as an ingredient.
It should also implement and enforce a rigorous labelling system for GM products, he said.
"New Zealanders don't want to be part of a science experiment. They rely on the Government to make sure these products are safe before they are released for human consumption."
However, scientists have picked out flaws in the study's design and conclusions.
Professor David Spiegelhalter of the University of Cambridge said the results were "well below" the standard he expected from a rigorous study.
Placing only 10 rats in each experiment group meant it was difficult to find trustworthy statistical trends, he said.
"The numbers are so low they do not amount to substantial evidence. I would be unwilling to accept these results unless they were replicated properly."
John Innes Centre senior scientist Wendy Harwood said the scientists behind the study had not included full data in their report - including results for the control group.
"We have to ask whether a diet with this level of maize is normal for rats. Another control with an alternative diet should have been included...
"Without access to the full data, we can only say that these results cannot be interpreted as showing that GM technology itself is dangerous."
Liggins Institute researcher Mark Vickers said the study was not robust.
Key data on the rats' fluid intake and growth were missing and the strain of rat used was prone to developing tumours with advancing age, he said.
"It is surprising that the paper was accepted for publication without such data."
Otago University genetics director Peter Dearden said the paper's results were "intriguing" but very preliminary.
The number of rats used was low and the controls for the experiments, which were vital to understand their significance, were not well reported, he said.
"In my opinion this is interesting work, but with major flaws, with an outcome that needs to be followed up with robust, well described experiments."
But Mr Browning said the study was rigorous enough to show there was a health issue with GM maize.
It should be proved safe before being allowed to be sold to New Zealanders, he said.
"There is enough there to say there is a problem here."