The Controversy Over Whether Monsanto’s GM Corn Gives Rats Cancer Continues

A scary paper about what happened to rats that ate genetically modified food was initially retracted. Now it’s been re-done. The results are still inconclusive, but warrant another look for anyone concerned about GM food.

The Controversy Over Whether Monsanto’s GM Corn Gives Rats Cancer Continues
[Image: Rat via Shutterstock]

In 2012, a study was released that seemed at first glance to justify every anti-GMO advocate’s worst fears: eating genetically modified food gives you cancer. Now, after a long period of controversy, the study is back, albeit somewhat diminished.


The research, led by molecular biologist Gilles-Eric Séralini, claimed that rats fed Monsanto’s Roundup-resistant corn for a two year period developed more tumors and died significantly faster than controls. Rats given drinking water contaminated with Roundup, which is an herbicide manufactured by Monsanto, also developed tumors. The backlash from the scientific community didn’t take long.

The sample size, 10 male and 10 female rats in each group, was too small, outside scientists said. Plus, the type of rats used in the study, known as Sprague-Dawley rats, are prone to developing tumors. Ted Schettler, science director of the Science and Environmental Health Network, told Mother Jones in an interview at the time:

Overall, he said, he’s “intrigued” by the results, but isn’t convinced. “I don’t want to trash” the study, he said, “but I just don’t see enough there that’s very persuasive to me at this point.” He added: “It does suggest to me that we need longer term feeding studies with GM foodstuff, in a standardized way with the right number of animals in each group so we can pick up the changes and be confident that they exist.” He stressed that using enough rats to show robust, statistically significant results would be very expensive.

Less than a year later, in the fall of 2013, the study was retracted by the journal Food and Chemical Toxicology, which originally published the research. Elsevier, the publisher of the journal, offered up many of the same points that scientists had been complaining about:

Unequivocally, the Editor-in-Chief found no evidence of fraud or intentional misrepresentation of the data. However, there is a legitimate cause for concern regarding both the number of animals in each study group and the particular strain selected. The low number of animals had been identified as a cause for concern during the initial review process, but the peer review decision ultimately weighed that the work still had merit despite this limitation. A more in-depth look at the raw data revealed that no definitive conclusions can be reached with this small sample size regarding the role of either NK603 or glyphosate in regards to overall mortality or tumor incidence. Given the known high incidence of tumors in the Sprague-Dawley rat, normal variability cannot be excluded as the cause of the higher mortality and incidence observed in the treated groups.

This week, Séralini and his fellow researchers republished their study, “Long term toxicity of a Roundup herbicide and a Roundup-tolerant genetically modified maize,” along with commentary on the retraction, in the journal Environmental Sciences Europe. The new version, edited for clarity, makes it clear that it isn’t a carcinogenesis study (a study on cancer generation).

Our research represents the first chronic study on these substances, in which all observations including tumors are reported chronologically. Thus, it was not designed as a carcinogenicity study. We report the major findings with 34 organs observed and 56 parameters analyzed at 11 time points for most organs.

Those findings include kidney and liver toxicity, earlier death, and, of course, tumors.

“The authors have more carefully crafted this [version]. It’s designed to look at toxicity endpoints, not cancer endpoints,” says Laura Vandenberg, an assistant professor of environmental health sciences at the University of Massachusetts, Amherst.


Vandenberg first became familiar with the research because it cited a study she wrote with a number of other experts on the state of the science in endocrine disruption. “They cited it because they found that some of their lower dose treatment groups had significant effects but the higher doses didn’t. They were using it to say how Roundup could be acting as an endocrine disrupter,” she explains.

She is skeptical about criticisms of the use of Sprague-Dawley rats in the research (“There are plusses and minuses to every strain,” she says) but still believes that its implications are limited, even in the republished study. “They didn’t go back and do another experiment, so it’s still limited in its ability to tell us anything definitive about whether these products are carcinogens,” she says.

The researchers admit as much, noting in their commentary on the study’s retraction that they never actually used the word “cancer” in their paper. They believe that the retraction was motivated largely by conflict of interest (they note, for example, that a former Monsanto employee became assistant editor for biotechnology at Food and Chemical Toxicology after the study was published), since no fraud or “intentional misinterpretation” occurred. Séralini did not respond to our requests for comment.

“From my perspective as a young scientist, there’s a lot of politics that goes on when you study a chemical or a product that has such a large economic impact. That’s not an issue related to Roundup, that’s with everything,” says Vandenberg.

As the researchers note in their revised paper, their study was designed as a follow-up to a 90-day study done on the same strain of GMO maize conducted by Monsanto. They write:

Our study is the first and to date the only attempt to follow up Monsanto’s investigation and to determine whether the differences found in the NK603 GM maize-fed rats, especially with respect to liver and kidney function, were not biologically meaningful, as claimed, or whether they developed into serious diseases over an extended period of time.

The study’s results are still inconclusive, but at the very least, they still warrant further research.

About the author

Ariel Schwartz is a Senior Editor at Co.Exist. She has contributed to SF Weekly, Popular Science, Inhabitat, Greenbiz, NBC Bay Area, GOOD Magazine and more.