Playing with our food: get the latest on genetically modified organisms and learn how they may harm the body, not to mention the earth.(PERSONAL CARE: CONSCIOUS SHOPPER)

Insect-killing genes from bacteria in your mashed potatoes? Fish genes in your ketchup? It's all possible in the weird-science world of genetic engineering. Proponents say it's a surefire way to increase crop protection and yield, but some studies have suggested scary health effects, including increased allergic reactions and possible organ damage. While no one knows the long-term effects of genetic engineering on humans and the environment, the potential dangers are staggering.

"You have to consider a broad range of possible harmful effects, including creating unknown allergens and increasing natural toxins in food," says Margaret Mellon, PhD, director of the food and environment program at the Union of Concerned Scientists. "With genetic engineering, you're operating in a different, unknown world with different risks. And because organisms are highly complex and dynamic, you can't predict what the end result will be."

In genetic engineering, the genes and DNA--the building blocks of organisms--are manipulated, creating a genetically modified organism (GMO) that would never occur in nature. Genes from bacteria, viruses, animals and even humans have been inserted into plants like soybeans, corn, canola, cotton and rice. The process allows the exchange of information between completely unrelated species--say, from fish to fruits, or insects to grains--and was originally intended to increase crop protection and yield. One example: An "antifreeze" gene that allows flounder to survive in very cold water can be inserted into tomatoes to boost their tolerance to frost. Or insect-killing genes from bacteria may be inserted into corn or potatoes to increase their defenses against pests.

But the practice of shuffling genes between species raises plenty of scary possibilities. In 1998, Arpad Pusztai, a researcher at Rowett Research Institute in Aberdeen, Scotland, performed the first study analyzing the effect of genetically engineered (GE) food on mammals. The results: an unmarketed version of GE potatoes caused intestinal inflammation in rats. Further, it appeared that the actual process of genetically altering an organism--not just the toxins created--led to the organ damage.

"There is mounting evidence that GE foods damage animal health and human health," says Ronnie Cummins, national director of the Organic Consumers Association. "It's a risk we just don't need to be taking." Other potential hazards of shuffling genes between species:

1. Creating allergenic foods. Genetically altering a food creates new proteins that may be allergenic. A recent study found that moving a gene from a bean, which is not an allergen, to a pea, created an allergenic plant. "Even under normal circumstances, eating unaltered food, it's hard enough to track down an allergen," Mellon says. "Genetic engineering makes it nearly impossible."

2. Increasing toxins in plants. Most plants produce natural toxins at low levels, as part of their defense mechanisms against pests. Genetic engineering can affect the metabolism of the plant, causing it to kick up production of these naturally occurring toxins to possibly harmful levels. For example, canola contains glucosinolates that, in high quantities, can impair metabolism and lead to goiter, and high levels of naturally occurring lectins in soybeans can cause intestinal inflammation.