Vegetable oils such as canola oil, safflower oil, corn oil, soy oil etc. are very reactive to oxygen, and go rancid when heated even at low temperatures. Once oxidized, these fats cause free radical damage in our bodies, which has been linked to cancer, heart disease and diabetes. According to the animal research of Dr. Kenneth Carroll, "...the more polyunsaturated fats were in the diet, the more they were cancer promoting; and the more saturated fats were, the more they were cancer reducing..." Yet we are told by such organizations as the American Heart Association that these polyunsaturated oils are the healthy oils to eat and cook with. In actual fact, we were only meant to consume vegetable oils by eating the nuts and seeds that they come in; the nuts and seeds also contain the antioxidants that prevent the oxidization of the fats.
Vegetable oils are refined with solvents and heat, making them toxic and rancid. Deodorizing compounds are added to cover up the smell, and voila - they will stay on the store shelf for months looking deceptively beautiful. Even in their unrefined form, these polyunsaturated oils cannot be recommended even in salad dressings, as they are sensitive to light. Polyunsaturated vegetable oils are also most frequently turned into trans-fats, so pretty much any time you see any vegetable oils listed on an ingredient list in a packaged food or a recipe, you would be doing yourself a huge favour in the long run by not eating it. The best fats for cooking are organic pasture-fed raw butter or ghee, organic virgin coconut oil, organic, pasture-fed beef tallow, and unrefined organic olive oil (low to medium temperatures only).
Enig, Mary; Know Your Fats: The Complete Primer For Understanding the Nutrition of Fats, Oils, and Cholesterol Bethesda Press, Silver Spring, MD, 2003.
Fallon, Sally and Enig, Mary; Nourishing Traditions, Revised 2nd Edition NewTrends Publishing Inc., Washington, D.C., 2001