Iowa is playing center stage in a global debate over whether people should be warned when the genetic makeup of their food has been altered.

A national advocacy group believes consumers would demand that genetically modified foods be labeled if they knew just how much is being changed in labs. The Campaign to Label Genetically Engineered Foods is pushing presidential candidates to support making labeling the law - with some success.

Leading Democrats Hillary Clinton and John Edwards agree to the organization's proposal, as do candidates Bill Richardson and Dennis Kucinich. Top Republican candidates have not taken positions.

"We want to make food safety a defining issue of this election," said Anne Dietrich, the Fairfield, Ia.-based executive director of the campaign. "Once this becomes the law of the land, then Monsanto, Syngenta, Kraft and Kellogg's will reformulate their products. Iowa is the best place to start."

But the group's efforts have met resistance from Iowa industry leaders and global experts in genetic engineering. Many of them are gathered in Des Moines this week for the World Food Prize, an event that honors innovations in increasing the world's food supply.

While Dietrich and her supporters argue that genetically engineered foods threaten human health and the environment, biotechnology leaders say the foods are safe and vital to feeding the world, especially amid growing demand for crop-based biofuels.

James Greenwood, a speaker at the event and president of the Biotechnology Industry Organization, said opponents of genetic engineering use scare tactics.

"They hope by using these scare tactics they can persuade policymakers to alter labeling, and they can use the label to drive people away."

Although years of debate have yielded no public consensus on the issue, one thing is certain: Genetic engineering or modification increasingly affects Iowans at the supper table and in the field. Ninety-four percent of soybeans and 78 percent of corn planted in Iowa are genetically engineered varieties, according to the U.S. Department of Agriculture.

Those who modify the foods use DNA to take a trait from one species and introduce it into the genes of another. The process can make a species grow better, yield more or resist pests and disease.

Prior attempts in Congress by Ohio Rep. Dennis Kucinich to pass similar legislation requiring labels have failed. But supporters of labeling believe this election year is different. Recent concerns about food safety in the United States, they say, create an opportunity to inject genetic engineering into the presidential debates and create consumer support.

Yet Greenwood cautioned candidates against trashing genetics in front of farmers, who profit from genetically engineered crops.

"I wouldn't want to be a presidential candidate going into Iowa ... and extolling the virtues of labeling their corn in a way that might make consumers not want to buy it," he said.

Perhaps the worst biotech black eye in the United States happened in 2000, when Aventis' StarLink corn was found in taco shells. The incident prompted a nationwide recall and caused farmers and others in the grain industry to lose money.

The genetically engineered StarLink was allowed in animal feed, but it was not approved for human food because of concerns it could trigger allergic reactions. Federal regulators never proved that it did.

In fact, biotech leaders say no studies have ever shown their foods to be dangerous to consumer health. The industry, they say, has stopped using antibiotics in gene work because of concerns about people developing immunity.

Opponents say studies have shown that genetically modified foods expose people to new allergies and generate new toxins.

Fairfield attorney Steven Druker said the government allows genetically modified food on the market without adequate testing to determine its true risk. Druker sued the FDA to release files that he says show how agency bureaucrats silenced government scientists who doubt genetic engineering.

"These foods are not to be presumed safe," he said, adding they shouldn't be on the market - with or without a label.

The two camps also wield dueling research in other related areas:

- Opponents blame genetic engineering for ruining habitat and killing off certain animals and insects, including the monarch butterfly, and robbing the soil of nutrients. Biotech leaders dispute those claims and say genetically engineered crops actually help the environment because they lessen the need for pesticides.

- Proponents say genetic engineering creates reliable crops that can grow in parts of the Third World and other areas where it's difficult to farm, and provide more suitable and nutritious food for impoverished families. Opponents say that better distribution of food could help poor families in other countries, and that the benefits of growing altered crops don't outweigh the long-term risks.

A study war continues, with both sides alleging that existing research is flawed, biased or incomplete.

The U.S. Food and Drug Administration has deemed genetically engineered foods safe.

According to its rules, "there must be something tangibly different about the food product - not the process by which it's made - for the FDA to require labeling."

Tom West, vice president in biotech affairs with Pioneer Hi-Bred International, contends that genetically engineered foods are "the most tested foods in the history of mankind."

"There's not a single documented case of an illness or allergic reaction to a biotech food," he said.

Governments require labels in other parts of the world, including Europe, where consumers are more opposed to genetically modified food.

Although genetically engineered foods in the United States lack labels, some companies have chosen to market foods as "GMO-free." Consumers also can look for the USDA "organic" label because genetic modification is banned in organic food.

Leigha Bitz, a West Des Moines jewelry designer who blogs about buying organic food for her two young children, believes genetically modifying food robs it of its natural nourishment.

"God didn't make it that way," she said. "Everything we put in our bodies gets broken down by our bodies in special ways. If you change its molecular structure, it's not going to work as well."

About 60 percent of Americans don't believe they have eaten genetically engineered food, even though almost every American has, according to a study done last year by the Pew Initiative on Food and Biotechnology.

The same survey showed that 46 percent of respondents were opposed to genetically engineered foods, which is down from a high of 58 percent in 2001 (following the StarLink incident). And 54 percent said they were unlikely to eat foods that had been genetically modified.