Lipids are naturally occurring, organic compounds, that are insoluble in water. Of primary importance in equine diets are fatty acids, triglycerides, and fat-soluble vitamins (such as vitamin A, D, E, and K).
There are many sources of lipids able to be fed to horses, including (but not limited to): vegetable oils, cereal grain brans, and oilseeds. Oilseeds and cereal grain brans are usually included in extruded concentrate supplements, or meal diets. Some examples of this are rice bran and canola meal being included in rations. Vegetable oils can be included in a formed ration, or added to any feed that is being given to the horse. Common oils that are supplemented are canola oil, soybean oil, corn oil, linseed oil, and more recently, hemp oil. Forages and cereal grains contain very low amounts of lipids, thus traditionally fats have represented a small portion of the equine diet.
Most lipids are digested in the small intestine, specifically the duodenum, by enzymatic process. The enzymes responsible for digestion of lipids are called lipase's and they are released from the gastric chief cells (zymogenic cells) contained in the stomach. The majority of fats are absorbed in the ileum, and very small amounts of (if any) reach the large intestine.
Fats are the most energy dense of the energy providing macronutrients. They contain 38 MJ/Kg of digestible energy, which is 2.25x higher than that of carbohydrates or proteins. The primary purpose for adding lipids to the equine diet is for dramatically increasing energy content of the feed, without having to increase the amount of feed consumed by the horse. This is of great importance for animals that are under a heavy work load and expending a large amount of calories in their daily activities.