Polyunsaturated fatty acids are degraded by oxygen, heat and light.
Fatty acids are the main components of lipids. The are characterised by the number of carbon atoms they possess, hence the expressions short, medium or long-chained fatty acids. They can be saturated (no double chemical bond between 2 carbons) or unsaturated (with 1 to 6 double bonds). Though the latter - more fragile and subject to rancidity - include many fatty acids that are essential to vital functions of the body.
Saturated fatty acids are exclusively energy sources. Short-chain saturated fatty acids (6- 10 carbon atoms) are a very good source of fast energy for sporting dogs, diabetic animals and newborn puppies. The function of polyunsaturated fatty acids is structural (in membranes or in blood lipoproteins); they include the omega 3 and omega 6 chemical series that have vital functions and cannot be synthesised by the body.
These are the same as for lipids: vegetable and animal oils and fats.