You don't have to look hard to find genetically modified food on supermarket shelves: More than 85 percent of the corn and soy grown in the United States comes from seeds whose DNA has been rejiggered (to increase yields), and those two crops play starring roles in countless processed foods, from soda to salad dressing to bread. Advocates say genetically modified (GM) foods allow farmers to produce more with fewer chemicals—which means a cleaner environment and cheaper groceries for us all. But the question remains: What impact do GM foods have on our health?

The answer is, no one really knows. GM foods have been on the market only since 1994, and research on their long-term effects on humans is scarce. To date most of the studies have been done on animals; worryingly, though, some of those studies link GM foods to altered metabolism, inflammation, kidney and liver malfunction, and reduced fertility. In one experiment, multiple generations of hamsters were fed a diet of GM soy; by the third generation, they were losing the ability to produce offspring, producing about half as many pups as the non-GM soy group.

In addition, allergy sufferers worry that, as genes are transferred between plants, allergenic proteins (from, say, peanuts or wheat) will pop up in unexpected places (like soy or sugar). Richard Goodman, PhD, a professor of food science and technology at the University of Nebraska-Lincoln and a former scientist for Monsanto, says that seed companies run sophisticated tests to prevent that kind of mistake from happening. But inserting new genes into a seed's delicately constructed genome is always a gamble because scientists can't predict all the consequences. There is, for example, the possibility of creating brand-new allergens.

Despite the potential health implications, more GM foods appear each year. In 2011 the USDA approved the planting of genetically enhanced sugar beets (sucrose) and alfalfa (hay for livestock). The FDA is expected to okay a fast-growing salmon in the near future. And possibly on the horizon: pigs designed to produce omega-3s.

Yet because GM foods are not required to be labeled as such, it's impossible for consumers to tell them apart from regular foods. Gary Hirshberg, chairman and cofounder of Stonyfield, the organic yogurt company, thinks that's not right. Last October he partnered with Just Label It, a national coalition of nearly 450 organizations that's currently petitioning the FDA to give consumers a choice. More than 600,000 people have already signed. (To add your name, go to

"The status quo is innocent until proven guilty," says Ashley Koff, a registered dietitian who studies GM foods, "as it was for trans fats, DDT, and countless other harmful chemicals. A labeling requirement would motivate seed companies to prove to consumers that their products are safe, to protect their sales." Nearly 50 other countries—including China, Brazil, and most European nations—have mandated that genetically modified organisms (GMOs) be marked, and in an MSNBC online poll, 96 percent of more than 45,000 respondents said that they should be labeled. At press time, 18 states had introduced legislation to promote GMO labeling.

Hirshberg, for one, remains impatient. "I'm 57," he says. "I don't want to wait another 30 years to see what the science says. At this point, I want to decide for myself."

Next: Five ways to lessen your exposure to GMOs

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