The fats we consume come from many sources, both visible and invisible. Visible fats are ingredients such as olive oil or shortening, which are used in preparing foods, or extras such as butter, salad dressing, or cream cheese, added at the table. The fat you trim from your steak is also the visible variety. Invisible fats are an integral part of foods, including the fat found in meat, nuts, fish, or eggs, the butterfat in whole milk or cheese, and fat added in processing.
Fats contained in foods improve the texture (the marbling in a steak, for instance) and absorb and retain flavors, making meals more palatable. Fats have characteristics that cause them to remain in the stomach longer and prolong the “good-and-full” feeling. Most fats of either animal or vegetable origin are easily digested by healthy persons.
All of the fats naturally found in foods are made of mixtures of saturated and unsaturated fatty acids. In general, fats containing mainly saturated fatty acids do not melt at room temperature and are found in animal foods such as beef or lamb, lard, butter, and other dairy products. Fats containing mainly polyunsaturated fatty acids are usually liquid at room temperature and include corn, cottonseed, and safflower oils. Exceptions include poultry and fish oils, which come from animal sources but are high in unsaturated fatty acids despite being solid, and coconut oil, a vegetable oil high in saturated fatty acids despite being a liquid.
Some fat products such as shortening and margarine, are hydrogenated. This process adds hydrogen atoms to polyunsaturated fatty acids, creating a solid fat which is more stable and which can be stored at room temperature.