For decades, common nutritional wisdom proclaimed that fat and oil were bad, bad, bad. And then nutritionists and doctors determined that not all fat is bad; in fact, some of it is not only good for you, but essential.
Trans fat is is the worst kind of fat – the one that lowers HDL (good cholesterol) levels, raises LDL (bad cholesterol) levels, and ultimately clogs your arteries because there is no way for your body to break it down. Trans fats are found in margarine and shortening and products made with these, particularly fried foods and those containing partially hydrogenated oil. The vast majority of trans fats are created during the production of these foods, though a small amount of trans fat is found naturally in meat and dairy.
Saturated fat raises levels of both HDL and LDL; the overall effect is unhealthy. Saturated fat is high in butter, lard, seafood, and certain kinds of oil (coconut, palm, and palm-kernel). Saturated fats from plants are much healthier than those from animals.
Polyunsaturated fat also raises both HDL and LDL and is a much better alternative than saturated and trans fats. Polyunsaturated fat contains the omega-3 and omega-6 fatty acids. Flaxseed, hemp, safflower, and sunflower oil are very good sources (more than two thirds of their fat is polyunsaturated). Other oils with a significant amount of polyunsaturated fat are corn, pumpkin seed, sesame, soybean, and walnut oil.
Monounsaturated fat is very good for you, as it raises good cholesterol levels and lowers bad cholesterol. However, heat can destroy these beneficial properties, so these oils are healthiest when not used for cooking. Olive and avocado oil are the highest in monounsaturated fat; other good sources are canola, flaxseed, grape seed, hemp, palm, peanut, sesame, and sunflower oils.
All oils contain some combination of the above types of fat, so you should always read the nutritional labels. Please read my article on olive oil for additional information about choosing oil.