For the best summary of health risks, I will rely on Jeffrey Smith’s research. Jeffrey Smith is one of the most influential activists in the fight against genetically modified foods. He is the founder of the Institute for Responsible Technology and he has written two definitive books on the subject: Seeds of Deception, and Genetic Roulette (2007). It was reading Genetic Roulette, the reference book on the health risks of genetically engineered food, that got me inspired to become involved in this movement.
Smith’s strength is taking scientific studies and explaining them in layman’s terms. He does an excellent job of defining what genetically modified organisms are at the beginning of his interview with Functional Medicine Update:

“With genetically engineered foods you take single genes or combinations of genes, typically you make changes in the structure of them, and then you artificially force them into the DNA (the genome) of other organisms. So it is not natural, but it is rather a method of selecting certain traits, pulling it out of context, and transferring it into species that would never naturally contain those genes. The process itself also causes massive collateral damage in the DNA, causing mutations and changed gene expressions, etc.”

I will briefly summarize the major points in Genetic Roulette, on his website, and from his February, 2008 interview.

GE foods carry a risk of known and unknown allergens. The incidents of soy allergies in Europe increased by 50% after GE soy was introduced. Test results determined that there were known allergens in GE soy that are not in conventional soy. Also, after eating GE soy, someone who was not originally allergic to soy can become allergic to conventional soy. Allergies have also been reported in response to crops engineered to produce Bt toxin. The process of genetically engineering crops is inherently unpredictable, and the risk of known and unknown allergies is a serious concern.

Animals avoid GE feed, so why don’t we? There have been hundreds of anecdotal reports of livestock avoiding GE feed when they have the choice. To do testing on the first commercial genetically modified crop, Flavr-Savr© tomatoes, rats had to be force fed because they wouldn’t eat the tomatoes on their own. Several of the rats in the study developed stomach lesions, and 7 of the 20 rats fed the tomatoes died within two weeks. The USDA and Calgene (owned by Monsanto) still took the product to market, and the feeding study was only made public as the result of a lawsuit.

There is no benefit to consumer or food produces for using GE food. Approximately 80% of commercial crops are designed to be resistant to the firm’s brand of herbicide, the residue of which poses health risks to humans and is clearly harmful to the environment. Almost all other GE crops are engineered to produce Bt toxin, a pesticide, in all edible parts of the plant. Allergic reactions and other health risks have been associated with Bt toxin and Bt crops.

In sum, there is NO good reason to eat genetically engineered food. They are not produced for the benefit of consumers, and they are simply not worth the risk. 60% of Americans believe that they have never eaten GE food, but according to Greenpeace, 70% of food in grocery stores are genetically modified. To find out how to avoid GE food, read the Greenpeace GMO Guide, which lists products Greenpeace believes are GE-Free and those which are not. Also read Jeffrey Smith’s How to Buy Non-GM guide.