One French research team raised safety questions, but their much-criticized 2012 study linking genetically modified corn to rat tumors was retracted in 2013 by the scientific publisher, who cited weak evidence supporting the conclusions.
Even the food police say they are safe: The Center for Science in the Public Interest, a well-known critic of food companies and artificial and unhealthy ingredients in foods, has not opposed genetically modified foods, on the basis that there's no evidence they are harmful.
Though what we are eating now appears safe, the main concerns for the future would be new genetically engineered foods — from the United States or abroad — that somehow become allergenic or toxic through the engineering process. The FDA says the foods they have evaluated to this point have not been any more likely to cause an allergic or toxic reaction than foods from traditionally bred plants.
Unlike animals, the FDA is not required to approve genetically engineered crops for consumption. However, most companies will go through a voluntary safety review process with FDA before they put them on the market.
There are clear benefits for the agricultural industry — the crops that are engineered to ward off pests or to tolerate herbicides, for example. And companies like Monsanto that produce modified seeds say their technologies will be needed to feed a rising world population as they engineer crops to adapt to certain climates and terrains.
While most modified foods have so far been grown to resist chemicals, pests or disease, advocates envision engineering crops to make them more nutritious as well. Food animals have been engineered to be bred to be free of diseases, be cleaner in their environments or grow more efficiently, though none has yet been approved in the United States.
There is an escalating political fight between the labeling advocates and the food industry, which has dug in against labeling. In the absence of a federal labeling standard, GMO opponents have gone to the states to try to get a patchwork of labeling laws approved — a move that could eventually force a national standard.
Ballot measures in California and Washington state failed, but the legislative effort prevailed in Vermont. Maine and Connecticut also have passed laws requiring labels, but they don't take effect unless other states follow suit. The food industry is widely expected to challenge the Vermont law in court.
The state efforts aren't slowing down. According to the National Conference of State Legislatures, there are 85 pending GMO labeling bills in 29 states.
In Congress, the food industry is pushing a House bill that would head off efforts to enact mandatory labeling of genetically modified ingredients by proposing new voluntary labels nationwide — an attempted end run around the state-by-state laws.
Currently, the FDA says labeling of genetically modified foods isn't needed because the nutritional content is the same as non-GMO varieties.
Safe or not, consumers are increasingly interested in what is in their food, including GMOs.
"There's so much confusing speech on food packaging and food advertising that consumers are often buying things they think are GMO free when they are not," says Scott Faber of the Environmental Working Group, which is pushing for the labels.
Faber and other labeling proponents say it's about transparency, not technology. They say there is precedent, like orange juice labels that say whether the juice is from concentrate.
David Ropeik, the author of the book "How Risky Is It, Really? Why Our Fears Don't Always Match the Facts," says he thinks the food industry should endorse labeling so it can move past the debate.
"By supporting labeling, companies would say, 'There's no risk, we have nothing to hide,'" he says.
He says people rightly or wrongly fear GMOs because they are more concerned about man-made risks than natural ones and the food industry's control over what we eat.
"It's a surrogate for a values war they have against big companies," he says.
AP Science Writer Seth Borenstein and AP Medical Writer Lauran Neergaard contributed to this report. Follow Mary Clare Jalonick on Twitter at http://twitter.com/mcjalonick
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