Glucose homeostasis is regulated by hormones. Elevations in blood glucose levels during feeding stimulates insulin release from pancreatic beta cells through a glucose sensing pathway. Insulin is synthesized as a precursor molecule, proinsulin, which is processed prior to its secretion. A- and B-peptides are joined by a disulfide bond to form insulin, and the central part of the precursor molecule is cleaved and released as C-peptide. Insulin stimulates glucose uptake from blood into the skeletal muscle and adipose tissue. Insulin deficiency leads to type 1 diabetes mellitus.