A scientific study published this week concludes that rats fed genetically modified (GM) corn grew massive tumors, but the research has also been criticized for its methodologies.

The study, published Sept. 19 in the peer-reviewed journal Food and Chemical Toxicology, was conducted by Gilles-Eric Seralini and others from the University of Caen in France and the University of Verona in Italy. Over the course of two years, the scientists fed mice a genetically modified corn called NK603, created by Monsanto to be resistant to the weed killer glyphosate (marketed by Monsanto under the brand name Roundup). One group of mice was fed corn that had been treated with Roundup, while others were fed untreated corn. Another group was given water with Roundup at levels of 0.1 parts per billion. The corn comprised 11 percent of their diet. According to the paper, the female mice developed large mammary tumors and disabled pituitary function; they died two to three times more than mice in a control group. Male experienced liver congestion and necrosis (tissue death) and tumors. Both genders experienced chronic kidney deficiencies.

The scientists said these conditions "may be due to an endocrine disruption linked to Roundup and a new metabolism due to the transgene," which is the genetic material transferred into the modified corn.

The team said this was the first study to look at the effects of GM corn over the two-year lifespan of mice, rather than the 90 day span of previous studies.

Dr. Michael Antoniou, a molecular biologist at King's College in London who was not affiliated with the study, told the Daily Mail that the research "shows an extraordinary number of tumors developing earlier and more aggressively — particularly in female animals. I am shocked by the extreme negative health impacts."

But some other scientists have been quick to criticize the study. "In my opinion, the methods, stats and reporting of results are all well below the standard I would expect in a rigorous study; to be honest I am surprised it was accepted for publication," professor David Spiegelhalter of the University of Cambridge, said in a collection of expert remarks gathered by the Science Media Centre in the United Kingdom. He said the study lacked proper statistical analysis and the control group of 10 male and 10 female mice was too small.

Dr. Wendy Harwood, senior scientist with the John Innes Centre, said it would be important to have a control group fed other types of food, since corn may not be a normal part of the mouse diet. She was also critical that the scientists did not release their full data set.

Both scientists called for replication of the study's results, and that may happen sooner rather than later. After hearing about the study, French Prime Minister Jean-Marc Ayrault said he will seek an immediate European Union ban on the import of the NK603 corn. "I've demanded a rapid procedure, in the order of a few weeks, which will allow us to establish the scientific validity of this study," Ayrault said today.