Pasture turnout can help type-1 PSSM horses. | Thinkstock

A. Horses with the GYS1 mutation (type-1 polysaccharide storage myopathy, or type-1 PSSM) accumulate abnormally large amounts of glycogen (the storage form of glucose), as well as the abnormal sugar amylase-resistant polysaccharide, in their muscle tissue. This mutation is inherited, exists in many equine breeds, and affects as many as 35% of individuals within certain breeds.

Researchers posit we might have unwittingly selected horses with this gene defect. That’s because when humans historically used horses for activities such as pulling ploughs and carriages, individuals with an ability to store more muscle glycogen would likely have performed better. The difference today is that we manage our horses differently. We tend to work our horses less intensely and feed them beyond their needs. This results in the development of clinical signs in those horses with the gene mutation. Nutrition is, therefore, a key component in successfully managing horses with type-1 PSSM. In fact, about 50% of horses with type-1 PSSM are reported to show improvement with diet changes alone. This might increase to 90% with additional changes in exercise protocol.

The interesting thing about horses with type-1 PSSM is that despite having 1.8 times more glycogen in their muscle tissue than normal horses, they suffer from energy deficits when exercised. The GYS1 mutation causes the enzyme glycogen synthases to be overly active, particularly in the presence of insulin. This can result in an almost constant formation of glycogen. As the storage form of carbohydrate, glycogen is used by muscle tissue as a source of adenosine triphosphate   (ATP, the cell’s energy currency). Glycogen can be used during low-intensity aerobic work as well as during more intense work when muscles need to work anaerobically