Scicurious is a PhD in Physiology, and is currently a postdoc in biomedical research. Follow on Twitter @Scicurious and read her blogs at Scientific American and at Neurotic Physiology.

A new toxicology study states that rats eating genetically modified food and the weedkiller Roundup develop huge tumors and die. But many scientists beg to differ, and a close look at the study shows why.

Genetically modified organisms (GMOs) have always been a controversial topic. On the one hand are the many benefits: the higher crop yields from pesticide- and insect-resistant crops, and the nutritional modifications that can make such a difference in malnourished populations. On the other side is the question that concerns many people: We are modifying the genes of our food, and what does that mean for our health? These are important question, but the new study claiming to answer them misses the mark. It has many horrifying pictures of rats with tumors, but without knowledge about the control rats, what do those tumors mean? Possibly, nothing at all.

The recent study, from the Journal of Food and Chemical Toxicology has fueled the worst fears of the GMO debate. The study, by Italian and French groups, evaluated groups of rats fed different concentrations of maize (corn) tolerant to Roundup or Roundup alone, over a two year period, the longest type of toxicology study. (For an example of one performed in the U.S., see here.) The group looked at the mortality rates in the aging rats, as well as the causes of death, and took multiple samples to assess kidney, liver, and hormonal function.

The presented results look like a toxicologists nightmare. The authors reported high rates of tumor development in the rats fed Roundup and the Roundup-tolerant maize. There are figures of rats with visible tumors, and graphs showing death rates that appear to begin early in the rats lifespan. The media of course picked up on it, and one site in particular has spawned some reports that sound like mass hysteria. It was the first study showing that genetically modified foods could produce tumors at all, let alone the incredibly drastic ones shown in the paper.

But can GMOs really produce such huge tumors? This paper isnt convincing. Following the release of the study, numerous scientists questioned the findings, citing anomalies throughout the paper that normally should have been corrected or resolved through the peer-review process. In particular, there are problems with the statistics performed on the data, the way the data were presented, and the numbers and types of animals used in the study.

First, the numbers. The authors examined groups of male and female rats in four different conditions: GMO food alone, GMO + Roundup, Roundup alone, and controls (normal food with no Roundup). For each experimental condition, there were three different doses of either the GMO maize (as a percent of the diet), Roundup, or both; the amount of doses of Roundup were all well below the approved doses. The 20 groups each contained 10 individuals, for a full total of 200 rats (100 male and 100 female). While 10 rats per condition might seem low, in a power analysis used to detect differences in response to, say a Roundup and non-Roundup condition, this would probably be OK. But how many final comparisons were the authors making? In the end, the authors compared each experimental condition to the same group of control rats, something that could severely bias the results. In most well-performed experiments, there would be a separate group of control rats for each condition, the GMO food alone, the GMO + Roundup, and the Roundup alone. The controls used for the study, as Anthony Trewavas, a cell biologist at the University of Edinburgh, pointed out in a press release response, are inadequate to make any deduction.

Then of course, there is the question of the animals themselves. Who were these rats? As it turns out, the rats used in the study were the Sprague Dawley rat strain, a widely used strain in biomedical and behavioral research. Unfortunately, this strain is prone to specific diseasesincluding the development of tumors. Up to 57% of female Sprague Dawley rats have been shown in other studies to develop tumors, especially mammary tumors, spontaneously. Males develop tumors at fairly high incidence as well. But in their striking mortality numbers for the study, showing the type and incidence of tumor development, the authors of the study do not show any of the control groups, and so we cannot actually compare the death rates of any of the GMO and Roundup exposures to controls. Tom Sanders, head of the Nutritional Sciences Research Division at Kings College London, pointed this fact out in the press-release response. Most toxicology studies are terminated at normal lifespan i.e. 2 years. Immortality is not an alternative. A careful read of the findings shows that the control group suffered a spontaneous death rate of 30% for males and 20% for females. But the authors do not state what caused the death. Did the dead animals develop tumors? Did control animals that survived develop tumors? We dont know. The authors did not show us.

Not only do they not show us, they do not present statistics to tell us the full story. In comparison to the 50% male death rate for the GMO maize dietis a 30% death rate in controls any better? There are no statistical analyses of how death rates compare between the different treatment groups and controls, only percentages. The way the data were analyzed is also unusual and highly complicated. This struck many of the scientists who read the study (including me) as odd. When comparing groups of doses as these authors did, there are simple enough statistical tests that will easily differentiate among the groups. Why were these tests not used? Why were the authors required to develop a highly convoluted analysis for something as simple as mortality rate?

In the end, while the results of the study look very drastic, there are too many issues to conclude that GMO maize and Roundup cause tumor formation. All we can really conclude is that rats who are prone to develop tumorsdevelop tumors, whether they are fed GMO maize, Roundup, both, or neither. In addition to the problems with the paper itself, the results contradicts a large amount of literature showing now difference in health consequences following consumption of GMOs. The potential health consequences of roundup exposure and GMOs should be carefully studied and evaluated, but studies like this one do not provide the answers, and only add to the hype.

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Under Controlled: Why the New GMO Panic Is More Sensational Than Sense | The Crux