Surely weve all be told at one point that cooking with such and such an oil is bad for you and this oil is has a better smoke thresh hold. But whos right? Well, lets start with what all the talk is about. All oils used for cooking are a blend and combination of different types of fats. Studies and research have found that heating up oils (using them to cook things in) can see the oil turn rancid or release high levels of aldehydes, a chemical that is said to be connected with conditions such asheart disease and dementia.