Here is an excerpt:
By Dr. Mercola
Anytime you cook a food, you run the risk of creating heat-induced damage. The oils you choose to cook with must be stable enough to resist chemical changes when heated to high temperatures, or you run the risk of damaging your health. One of the ways vegetable oils can inflict damage is by converting your good cholesterol into bad cholesterol—by oxidizing it.
When you cook with polyunsaturated vegetable oils (such as canola, corn, and soy oils), oxidized cholesterol is introduced into your system. As the oil is heated and mixed with oxygen, it goes rancid. Rancid oil is oxidized oil and should NOT be consumed—it leads directly to vascular disease.
Trans-fats are introduced when these oils are hydrogenated, which increases your risk of chronic diseases like breast cancer and heart disease. But the problems don’t end there.
The majority of these vegetable oils (at least in the U.S.) are made from genetically engineered crops, and they’re heavily processed on top of that. So not only are the polyunsaturated fats being oxidized, but these oils also contain other toxins, such as glyphosate and Bt toxin found in genetically engineered corn and soy. Glyphosate is the active ingredient in the broad-spectrum herbicide Roundup, which is used in very large amounts on all of these crops. So there are a number of reasons for avoiding vegetable oils, but the fact that they’re oxidized is clearly a high-priority one.