The study, which appeared in the journal Food and Chemical Toxicology, was embraced by opponents of genetically altered foods, including backers of Proposition 37, which if approved by California voters in November would require most foods with genetically modified ingredients to bear a label.
However, the report was sharply criticized by geneticists, who cited its small size and other methodological problems.
The research was led by Gilles-Eric Seralini, a professor at the University of Caen in France and founder of the nonprofit Committee for Research and Independent Information on Genetic Engineering. It is the latest in a series of papers by his group to report abnormalities linked to genetically modified organisms, or GMOs.
The authors studied 200 rats clustered into groups of 10. Some groups ate chow containing various quantities of Monsanto Co.'s Roundup-resistant corn; others ate the same chow and were fed Roundup in drinking water with it. Still other rats received chow made from conventional corn, with various levels of Roundup.
During the two-year study, the scientists found that up to 50% of the male rats and 70% of females that ate genetically modified corn, Roundup or both died prematurely. That compared to a 30% (male) and 20% (female) mortality rate among control rats that ate conventional corn without Roundup. The scientists also reported greater numbers -- and earlier development -- of mammary tumors in female rats exposed to genetically modified corn, Roundup or both than were found in control animals.
The authors also reported organ abnormalities, including 2.5 to 5 times higher rates of liver problems in animals exposed to genetically modified corn, Roundup or both.
The findings "present an overwhelming case for further research" as well as for labeling and reform of regulations about genetically modified food, Seralini said in a news conference Wednesday that was organized by Britain's Sustainable Food Trust, a nonprofit group that campaigns for changes in the food supply.
However, other scientists saw problems with the study. They said the type of rat used in the research is predisposed to develop tumors, which makes the results hard to parse -- especially since the small numbers of animals in each group makes it impossible to know whether observed differences occurred by chance.
Another red flag was that tumor rates didn't increase in line with the dose of GMOs fed to animals, as scientists would expect to see if the genetically engineered corn were to blame, said Kevin Folta, a plant molecular biologist at the University of Florida in Gainesville. Instead, "you are likely seeing variation of normal tumor incidence in a small population of rats," he said.
Bob Goldberg, a plant molecular biologist at UCLA, said the study did not offer any credible explanation for why the engineered corn would cause tumors. The changes made to plants to help them resist Roundup involve a protein that is naturally present in all plants people eat every day, he said.
The results are out of line with other long-term studies that have investigated the safety of GMOs fed to a range of animals including chickens, rats, mice, quail, monkeys and fish, said Agnes Ricroch, a geneticist at the University of Paris XI and Pennsylvania State University, who co-wrote a review of 24 such studies that was published this year.
Still, Gary Ruskin, campaign manager of the Yes on Proposition 37 effort, released a statement saying that the study's findings "underscore the importance of giving California families the right to know whether our food is genetically engineered and to decide for ourselves whether we want to gamble with our health by eating GMO foods."