Insulin is a protein that is synthesized by the beta cells of the islets of Langerhans in the mammalian pancreas.  Rate of biosynthesis and posttranslational modification are subject to many factors, among which are circulating levels of hormones such as growth hormone and steroids but also due to feeding status.  Insulin can be released in the blood as a result of certain triggers, e.g. concentration of glucose and certain amino acids in the blood. 
Insulin has a strong effect on energy metabolism in the body.  In particular insulin plays an important regulatory role in glucose metabolism, e.g. it activates some glucose transporters, stimulates lipogenesis and diminishes lipolysis, and increases amino acid transport into cells.  Energy metabolism depends also on several other factors, such as the level of glucagon or stress hormones in the blood.  The concentration of glucagon depends on its turn again on feeding status. 
Many subjects suffer from one or more health problems, which can either be solved, alleviated or prevented by interaction with the way a person deals with insulin or by administering insulin itself or compounds that mimic its action.  The former method has been predominantly applied in insulin-resistant persons, such as diabetes type II patients, persons suffering from the metabolic syndrome, severely ill persons or persons that experienced severe trauma like surgery or burns, while persons that suffer from severe pancreas damage like patients having diabetes Type I, severe pancreatitis or obstructions of the endocrine function or persons that are malnourished often benefit from the administration of insulin or its analogues. 
Because of the high interest of an appropriate action of insulin, many efforts have been made to influence or improve it.  Administration of insulin is frequently applied but often occurs parenterally, which is cumbersome to the patient.  In addition the effective time period is relatively short, which often makes multiple dosages per day mandatory. 
Increase of the sensitivity of mammalian cells to insulin is also frequently aimed for, but is difficult to achieve.  Active ingredients like biguanidines are claimed to achieve this.