Dieting complex carbohydrates and fats, for energy. The first source to which the body turns is glycogen (by glycogenolysis). Glycogen is a complex carbohydrate, 65% of which is stored in skeletal muscles and the remainder in the liver (totaling about 2,000 kcal in the whole body). It is created from the excess of ingested macronutrients, mainly carbohydrates. When glycogen is nearly depleted, the body begins lipolysis, the mobilization and catabolism of fat stores for energy. In this process, fats, obtained from adipose tissue, or fat cells, are broken down into glycerol and fatty acids, which can be used to generate energy.
Glycogen-branching enzyme deficiency caused by an autosomal recessive mutation to the GBE1 gene, which leads glycogen branching enzyme activity that is reduced to absent. Subsequently, glycogen molecules are produced with few branches, which greatly decreasing the number of nonreducing ends, drastically slowing the rate at which the molecule can be synthesized or broken down. This causes low levels of muscle glycogen that is very resistant to amylase. Lacking proper glycogen storage, the horse's brain, heart muscle, and skeletal muscles cannot function, leading to rapid death. Most foals with glycogen-branching enzyme deficiency (GBED) are aborted (usually in the second trimester), or are stillborn, and