All animal tissues contain lipids or fats as essential components of cell membrane. It is also stored in certain tissues. Lipids are body's chief source of energy and are essential for diverse functions such as insulation, padding synthesis of steroid hormones and carriers of fat soluble vitamins. Many animals can live on little or no dietary fat because it can be formed from proteins as well as carbohydrates. But the synthetic ability of many animals is limited in respect to certain unsaturated fatty acids and cholesterol.
For instance, vertebrates can synthesise cholesterol readily: In humans cholesterol is considered harmful in diet because it is a major factor in the development of atherosclerosis or hardening of arteries. On the other hand, insects cannot synthesise cholesterol from their precursors. Therefore, it must be supplied in their diet. Studies on rats show that three fatty acids - linoleic, Linolenic and archidonic acids are not synthesised. Therefore, they are considered essential fatty acids. Many insects, birds and some mammals also reveal such a dietary requirement of fatty acids. It seems that animals in general have a better synthetic ability for lipids than for amino acids.