Although there have been more than two thousand studies documenting that GMOs do not pose an unusual threat to human health, questions about the safety of genetically modified foods remain in the minds of many consumers.

Gilles-Eric Séralini, in his retracted GMO corn study (later republished in a pay for play journal without peer review), claimed rats fed genetically engineered corn developed grotesque cancerous tumors—the kind no farmer would miss among his animals if the cause-effect was genuinely in place.

Anti-GMO crusader Jeffrey Smith, on his personal website, the Institute for Responsible Technology, lists more than a dozen cases in which he claims test animals fed GMOs exhibited abnormal conditions, including cancer and early death. He also references his own self-published book and anecdotal activist web site posts in claiming that pigs fed GM feed turned sterile or had false pregnancies and sheep that grazed on BT cotton plants often died.

“Nearly every independent animal feeding safety study on GM foods shows adverse or unexplained effects,” he writes. “But we were not supposed to know about these problems either—the biotech industry works overtime to try to hide them. Industry studies described above, for example, are neither peer-reviewed nor published.”

The American Academy of Environmental Medicine—an alternative medicine group that rejects GMOs and believes that vaccines are dangerous, and characterized as a “questionable organization” by Quack Watch and Science Based Medicine—claims, “Several animal studies indicate serious health risks associated with GM food,” including infertility, immune problems, accelerated aging, faulty insulin regulation, and changes in major organs and the gastrointestinal system.”

Leveraging these allegations, anti-GMO groups regularly post blogs alleging that animals fed GMOs have developed health problems that could show up in humans. “Monsanto’s GMO Feed Creates Horrific Physical Ailments in Animals,” screamed a typical headline, in AlterNet, a popular fringe alternative site. It touted “new research” but as is typical of many such articles, it neither cited a study nor linked to any independent research.

Is there any basis to these allegations? After all, globally, food-producing animals consume 70 percent to 90 percent of genetically engineered (GE) crop biomass, mostly corn and soybean. In the United States alone, animal agriculture produces over 9 billion food-producing animals annually, and more than 95 percent of these animals consume feed containing GE ingredients. The numbers are similar in large GMO producing countries with a large agricultural sector, such as Brazil and Argentina.

Estimates of the numbers of meals consumed by feed animals since the introduction of GM crops 18 years ago would be well into the trillions. By common sense alone, if GE feed were causing unusual problems among livestock, farmers would have noticed. Dead and sick animals would literally litter farms around the world. Yet there are no anecdotal reports of such mass health problems.

But we don’t need to depend on anecdotes to address these concerns. Writing in the Journal of Animal Science [NOTE: article behind paywall], in the largest study ever conducted, Alison Van Eenennaam and Amy E. Young, geneticists with the Department of Animal Science at the University of California-Davis, reviewed 29 years of livestock productivity and health data from both before and after the introduction of GE animal feed. The field data represented more than 100 billion animals.

What did they find?

There were no indications of any unusual trends in the health of animals since 1996 when GMO crops were first harvested. Considering the size of the dataset, it can reasonably be said that the debate over the impact of GE feed on animal health is closed: there is zero extraordinary impact.

The authors also address the implications of their study on human health.

No study has revealed any differences in the nutritional profile of animal products derived from GE-fed animals. Because DNA and protein are normal components of the diet that are digested, there are no detectable or reliably quantifiable traces of GE components in milk, meat, and eggs following consumption of GE feed.

The authors go on to warn about the fractured regulatory process in which countries eager to export “second generation” GE crops are likely to get approvals before import countries give their okay to receive the new varieties.

“GE crops with altered output traits for improved livestock feed [are] in the development and regulatory pipeline,” they write. “Additionally, advanced techniques to affect targeted genome modifications are emerging, and it is not clear whether these will be encompassed by the current GE process-based trigger for regulatory oversight.”

They argue for the harmonization of both regulatory frameworks for GE crops and governance of advanced breeding techniques to prevent widespread disruptions in international trade of livestock feedstuffs in the future.

Jon Entine, executive director of the Genetic Literacy Project, is a senior fellow at the World Food Center, Institute for Food and Agricultural Literacy, University of California-Davis. Follow @JonEntine on Twitter