Does Genetically Modified Corn Cause Cancer in Rats?

The last post introduced forms of genetically modified crops – why they were made and some details of just what the genetic modification is. This is a good moment to discuss this subject because part of the conversation has focused on a particular paper entitled: Long Term toxicity of a Roundup herbicide and a Roundup-tolerant genetically modified maize by a laboratory in France. Roundup is an herbicide discussed in a previous post. Opponents of genetically modified crops acclaimed the paper, thinking at last there was definitive proof of the harm caused by modified corn.  Many scientists found the paper inconclusive and were harsh in their criticism of the Journal. In the end, the Editor-in-chief of The Journal of Food and Chemical Toxicology, retracted the paper. He probably never imagined that he would be in the eye of this particular storm. Some people think the retraction reflected skullduggery by agribusiness lobbyists. No matter how it came about, retraction is a big deal in the scientific (or the journalistic) world.

So I downloaded and read the paper. You might say that I am not a plant biologist but the paper had little to do with plants. It had to do with rats that were fed genetically modified maize. You might say that I am a geneticist and prejudiced in favor of constructive use of genetics. Maybe so, but I have no vested interest in this debate. A former researcher in my laboratory is now head of research for a very large GMO seed company, but he is a large-minded guy and would not be angry if  I supported the Caen scientists. I have reviewed many such papers over the years so reading this one was not a problem. The problem is to explain it without putting readers to sleep.

Let’s start with the rats: The group in Caen ordered a large number of Sprague-Dawley rats from a breeder in France. They are an in-bred strain so that all of the rats were almost identical twins, except that half were male and half female. That eliminates variability, which is good. Unfortunately, individuals from this particular inbred line are known to be susceptible to cancer as they age, and the plan was to keep them alive for two years. This is a problem because cancer is a disease of aging in rats as well as humans. If you want to measure the effect of something (GM corn and Roundup in this case) it is better to measure a change from a low background. A small change is more likely to be random.

Let’s look at the data:  A group of 10 rats got non-GMO corn. Three out of ten of eventually died of cancer. Another group got relatively low doses of GMO modified corn diluted in normal corn. Five out of ten of them died. Interesting, you might say. But the next group got twice as much GMO corn and only one of them died. A third group got three times as much GMO corn and only one of them died. This result does not support the thesis that GMO crops cause cancer in rats. Adding Roundup to the drinking water, improved the results slightly, but there was still no convincing dose dependence, which we would expect. From a statistical point of view, ten rats receiving each treatment is too small a number to give definitive results. And the cancers almost all occurred at the end of the rats’ normal lifespan, when cancer usually occurs.

At various points in the experiment, which lasted two years, the researchers took blood samples and put them through a battery of tests much like a person’s blood would receive at a thorough annual physical. Astonishingly, the authors did not measure Roundup concentration. If they are making the case that the GMO crops cause cancer because of the Roundup, would they not want to see if it or the detergent in which it is often dissolved got into their blood?  For that matter, would you not want to know how much is in their feed? If there is none, how could it cause cancer?

The data are presented in a confusing way and then touted as definitive in a video, but they are not definitive. They are not even good experiments, although showing cancer -laden rats in a video scares people to death. Why not show cancer ridden control rats?

The biggest problem is to explain just how a few new proteins that do not look like toxins could cause the proposed effects. The study proved nothing. See their claims in a video.

Did GMO corn cause this rat's tumor? Probably not!

Are there problems with GM crops? Sure there are, but for a dispassionate view, read Nature Magazine’s analysis of a year ago, which assesses the problem of Round-up resistant weeds.