Insulin is a peptide (protein) anabolic hormone that is secreted by the β cells of the pancreatic islets of Langerhans. Insulin regulates the supply of energy to the cells and affects the balance of macronutrients. Its main task is to maintain a normal level of glucose (sugar) in the blood by facilitating the uptake of glucose by cells. Insulin regulates the metabolism of carbohydrates, proteins and lipids. It also supports the processes of cell division and growth and is responsible for intracellular glucose transport to insulin-dependent tissues, such as muscles and adipose tissue.

In the article “Insulin and insulin resistance”, the author indicates that insulin signals the supply of external energy to the cells and thus inhibits the process of fat decomposition in adipose tissue. In muscle cells, glucose enables the synthesis and storage of glycogen, and carbohydrates, not fatty acids (or amino acids), are used as the immediately available energy source for muscle contraction. Insulin therefore supports the synthesis of glycogen and lipids in muscle cells, while inhibiting lipolysis and gluconeogenesis from muscle amino acids. In the presence of an adequate supply of amino acids, insulin has an anabolic effect on the muscles.

Source: Wilcox G.. Clin Biochem Rev. 2005 May;26(2):19-39. PMID: 16278749; PMCID: PMC1204764.