The genetically engineered corn that is linked to a widening web of food recalls has not been approved for human consumption because it contains a protein that is not normally part of the human food supply and that shares some characteristics of known food allergens. But there is no evidence that the corn actually does cause allergies, and some experts say that consumers need not worry about eating products that might contain the corn.
"I do not believe it's likely to cause any allergies," said T. P. King, professor emeritus at Rockefeller University in New York, who was on a scientific advisory panel formed by the Environmental Protection Agency to evaluate the potential allergenicity of the corn, which is known as StarLink.
For its part, the environmental agency said on Thursday that it "believes the risks, if any, are extremely low."
Still, the case illustrates what some critics of biotechnology say is a potential risk of genetically engineered foods -- that it is difficult to determine whether such foods will cause allergies. "We don't know and the E.P.A. doesn't know and the allergists don't know," said Jane Rissler, senior staff scientist at the Union of Concerned Scientists in Washington.
Indeed, the advisory panel examining StarLink concluded earlier this year that "there is no evidence to indicate" that the protein "is or is not a potential food allergen."
StarLink, made by Aventis SA, is one of several genetically modified corn strains known as BT corn, so named because they contain a gene from the Bacillus thuringiensis bacteria that causes the corn to produce a toxin that kills insects. BT toxins derived directly from the microbes have been used for decades as pesticidal sprays and are a favorite of organic farmers because they are natural.
While there is at least one report of farm workers' developing antibodies to the toxin, these sprays have generally been given a clean bill of health for farm workers and consumers. And that is one reason the environmental agency has been willing to approve several types of crops -- corn, potatoes and cotton -- with BT genes. But the StarLink BT toxin, known as Cry9C, is from a different strain of bacteria than the others and has not been used in sprays.
In some cases, allergenicity can be tested in advance. Several years ago, a seed company transplanted a gene from the Brazil nut into soybeans in order to make a more nutritious bean. But tests done on the blood of people with known allergies to Brazil nuts found that those people would have also suffered reactions to the genetically modified soy. That soybean was never marketed.
But because Cry9C comes from a type of bacteria that has not been part of the human diet, there are no people with known allergies to the bacteria who could provide blood for allergy tests. And there are no validated animal tests that can predict human allergies.
So scientists try to decide safety based on the physical and chemical characteristics of the protein. Many proteins that cause food allergies are not digested readily by the acids and enzymes in the stomach, are heat stable, have attached carbohydrates and are present in food in high levels, according to the agency's review documents. But none of these characteristics are absolute predictors of causing allergies, and there are proteins with one or more of these characteristics that are not allergens.
The Cry9C has two of these characteristics. It does not break down quickly when exposed to an acid bath simulating stomach conditions and also can withstand heat treatment at 90 degrees Celsius for 10 minutes.
On the other hand, the protein is in low concentrations in the corn, and the amino acid sequence of Cry9C does not resemble the sequence of any other known allergens. Aventis maintains that there is no evidence that carbohydrates are attached to the protein, although the federal report says there is some evidence of this.
Some experts say that even if Cry9C is a potential allergen, people might not suffer reactions now because the corn is new to the food supply. Usually, people must be exposed to an allergen over time to become sensitized to it. "In my opinion, there is virtually no risk associated with the ingestion of StarLink corn in this situation," Steve L. Taylor, professor of food science and technology at the University of Nebraska, said in a statement issued by Kraft Foods when it recalled its taco shells found to contain StarLink.
Dr. King of Rockefeller University also noted said that proteins were most often allergenic in their natural form. But corn is consumed only after cooking or food processing, which could destroy or change the shape of the protein.
Indeed, none of the tests done so far have actually detected the Cry9C protein -- the potential allergen -- in any of the food products that have been recalled. The tests have detected the Cry9C gene, which is made of DNA. It is possible that in food processing the DNA survived but the protein did not, some experts said. However, the protein in this case is stable when heated, increasing its chances of surviving intact.
Environmental groups and the Food and Drug Administration say they have received some reports of people claiming to have gotten sick or suffered allergic reactions after eating the taco shells. But none of these reports have been confirmed.
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Last Updated on 10/18/00