Dr Christopher Preston of the University of Adelaide, Australia, wrote this week that Jeffrey Smith's assertion that LibertyLink crops could pose a risk to human health were an "example how a few kernels of truth can be dressed up into a royal banquet".
Jeffrey Smith wrote on the Institute for Responsible Technology website, a source of anti-GM information, that a previously unidentified danger exists from Bayer's LibertyLink crops.
LibertyLink crops are genetically modified to be tolerant to glufosinate because they have been transformed with an enzyme that acetylates glufosinate.
"After you eat the GM corn, some inactive herbicide may become reactivated inside your gut and cause a toxic reaction," he wrote.
"In addition, a gene that was inserted into the corn might transfer into the DNA of your gut bacteria, producing long-term effects. These are just a couple of the many potential side-effects of GM crops that critics say put the public at risk."
According to Smith, whose forthcoming book on the health risks of GM food is out this autumn, the problem lies in NAG, a metabolite of the herbicide glufosinate that can be converted back to glufosinate and cause all sorts of problems.
Because glufosinate is a highly toxic compound causing all sorts of physiological and psychological problems, this poses a 'unique risk'.
Preston however does not fully agree with Smith's conclusions.
"Transgenic glufosinate resistant plants do convert glufosinate into NAG; however, very little if any NAG ends up in the grain and none in processed foods," he wrote in response on the AgWorldBio website, which provides information on agricultural biotechnology.
"The chances of consuming any significant amount of NAG are very low."
Preston however did accept that small amounts of NAG could be converted to glufosinate in the gut. Some rat experiments have shown this to be the case. However, he said that the conversion is fairly poor.
"It is true that transgenic glufosinate resistant plants metabolise glufosinate to NAG. It is also true that a small amount of NAG can be converted into glufosinate on passage through mammalian intestinal tracts.
"However, the rest of the steps required for Smith's 'unique risk' do not occur. NAG appears only at low concentrations, if at all, in grain from glufosinate-treated crops and not at all in processed foods.
"Therefore, the chances of consuming sufficient NAG to convert to sufficient glufosinate in the gut to produce any measurable effect must be exceptionally remote."
The issue of GM food has again been in the headlines following the discovery in the US of traces of unapproved GM rice in commercial batches. The EC adopted a decision last week requiring imports of long grain rice from the USA to be certified as free from the unauthorised GM (genetically modified) LL Rice 601from Bayer.