Polysaccharide Storage Myopathy (PSSM, EPSM) is a neuromuscular disorder that affects a wide range of breeds, from quarter horses to warmbloods to drafts. Low carbohydrate and low glycemic index feedstuffs, combined with frequent small meals is the best feeding program for horses with this condition. Why?
Since PSSM involves the deposition of a non-bioavailable form of glycogen into the muscle tissue, the best diet is one that minimizes glycogen formation. If your horse needs a calorie source in addition to the cellulose provided by pasture hay to maintain body condition score, you can consider vegetable oil or sugar beet pulp.
Vegetable oil is converted to “energy dense” volatile fatty acids in the hindgut, and bypasses glucose metabolism. An adult horse of average weight can be given as much as 1 cup (240 ml) three times per day if needed to maintain weight; however this amount of oil must be gradually increased from one third cup (80 ml) per feeding.
Sugar beet pulp is another low glycemic index feedstuff that, when digested by the hindgut microbes, will provide a safe calorie source. Sugar beet pulp is the product that remains after the sugar has been extracted and is a high cellulose feeding stuff for horses. The cellulose is converted to usable energy in the hindgut of the horse and therefore bypasses sugar and glycogen formation.