More than two decades ago, the U.S. Food and Drug Administration (FDA) granted GMOs “generally regarded as safe” status, meaning the industry had no obligation to conduct long-term safety studies. And while GMOs don’t have what scientists call “acute” effects, what about “chronic” effects — those that come on gradually and can’t easily be tied to one cause? The French study — the most comprehensive GMO safety assessment ever conducted — highlights that concern. It involved 200 rats and spanned two years, the life expectancy of the species of rat used. Previously, the longest study had lasted 240 days, says Michael Hansen, senior scientist at Consumers Union and an expert on GMO research. Industry-funded studies typically last just 90 days.

The researchers investigated how eating Monsanto’s Roundup-Ready corn (and any Roundup herbicide traces that may come with it) affected rats’ health. They separated rats into 10 groups: Three had part of their standard diet replaced at varying levels with Roundup-Ready corn that had been treated with Roundup in the field; three received the same feed protocol, but with untreated Roundup-Ready corn; three ate no GM corn but had tiny amounts of Roundup herbicide in their drinking water; and one control group ate two-thirds standard rat chow and one-third non-GM corn. Each group contained 10 females and 10 males.

The researchers say their results show “severe adverse health effects, including mammary tumors and kidney and liver damage, leading to premature death” from Roundup-Ready corn and Roundup herbicide, whether they were consumed separately or together. Almost all of the ill effects manifested after 90 days. By the end of the study, 50 to 80 percent of the females had developed large tumors, compared with 30 percent developing tumors in the control group. In males, liver congestion and necrosis were 2.5 to 5.5 times higher than in the control group, and there were 1.3 to 2.3 times more instances of kidney disease. Overall, among the rats receiving GM corn and/or Roundup, up to 50 percent of males and 70 percent of females died prematurely, compared with only 30 percent and 20 percent in the control group.

So does the study close the case? Are GMOs dangerous? Predictably, industry-aligned scientists are questioning the study, but even longtime critics of GMOs, including Hansen, have concerns. Hansen says that while the new study was longer and better designed than any of the industry GMO safety studies, the sample size — 10 males and 10 females per group — was too small to draw conclusions from.

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Hansen says, however, that while the individual comparisons may not be statistically significant because of sample size, the results still paint a troubling picture. The study made 54 comparisons between treated rats and control rats, and in all but four, the treated rats showed worse outcomes. “That’s suggestive that there’s something going on and that there should be further research,” Hansen says, adding that a possible reason the researchers didn’t use a greater number of rats to get more robust results is because multiyear rat studies are extremely expensive.

That fact highlights how little funding goes to GMO safety studies. Between 1992 and 2002 — a period of rapid GM crop development — the USDA spent about $1.8 billion on biotech research, of which only 1 percent went to safety testing. Meanwhile, the ag industry uses its patent power to maintain tight control over who researches what, and it dominates the research agenda at U.S. agriculture universities. The French study didn’t fully illuminate the situation, but it’s a start.