Unsaturated fats are liquid at room temperature. They are usually of plant origin, though fish oils may also be high in polyunsaturated fatty acids. Plant oils may be hardened by the addition of hydrogen atoms, converting double bonds to single bonds. This process is known as hydrogenation. Hydrogenated vegetable oils are often present in margarine and other processed foods.
Fats have a number of important functions in the body.
As well as being a concentrated source of energy, fats act as carriers for fat-soluble vitamins A, D, E and K. Fats are also essential for the structure of cell membranes and are precursors of many hormones. Two fatty acids are termed essential fatty acids. These are linoleic acid and a-linolenic acid. These must be present in the diet as the body is unable to make them itself. They are widely present in plant oils such as sunflower, rapeseed and soyabean oils.