Glycogen metabolism, as well. In hypoglycemia caused by excessive insulin, liver glycogen levels are high, but the high insulin levels prevent the glycogenolysis necessary to maintain normal blood sugar levels. Glucagon is a common treatment for this type of hypoglycemia. Various inborn errors of metabolism are caused by deficiencies of enzymes necessary for glycogen synthesis or breakdown. These are collectively referred to as glycogen storage diseases. Long-distance athletes, such as marathon runners, cross-country skiers, and cyclists, often experience glycogen depletion, where almost all of the athlete's glycogen stores are depleted after long periods of exertion without sufficient carbohydrate consumption. This phenomenon
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