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Insulin (from the Latin, insu­la mean­ing island) is a pep­tide hor­mone pro­duced by beta cells in the pan­creas. It reg­u­lates the metab­o­lism of car­bo­hy­drates and fats by pro­mot­ing the absorp­tion of glu­cose from the blood to skele­tal mus­cles and fat tis­sue and by caus­ing fat to be stored rather than used for ener­gy. Insulin also inhibits the pro­duc­tion of glu­cose by the liv­er. Except in the pres­ence of the meta­bol­ic dis­or­der dia­betes mel­li­tus and meta­bol­ic syn­drome, insulin is pro­vid­ed with­in the body in a con­stant pro­por­tion to remove excess glu­cose from the blood, which oth­er­wise would be tox­ic. When blood glu­cose lev­els fall below a cer­tain lev­el, the body begins to use stored glu­cose as an ener­gy source through glycogenol­y­sis, which breaks down the glyco­gen stored in the liv­er and mus­cles into glu­cose, which can then be uti­lized as an ener­gy source. As a cen­tral meta­bol­ic con­trol mech­a­nism, its sta­tus is also used as a con­trol sig­nal to oth­er body sys­tems (such as amino acid uptake by body cells). In addi­tion, it has sev­er­al oth­er ana­bol­ic effects through­out the body. When con­trol of insulin lev­els fails, dia­betes mel­li­tus can result. As a con­se­quence, insulin is used med­ical­ly to treat some forms of dia­betes mel­li­tus. Patients with type 1 dia­betes depend on exter­nal insulin (most com­mon­ly inject­ed sub­cu­ta­neous­ly) for their sur­vival because the hor­mone is no longer pro­duced inter­nal­ly. Patients with type 2 dia­betes are often insulin resis­tant and, because of such resis­tance, may suf­fer from a “rel­a­tive” insulin defi­cien­cy. Some patients with type 2 dia­betes may even­tu­al­ly require insulin if dietary mod­i­fi­ca­tions or oth­er med­ica­tions fail to con­trol blood glu­cose lev­els ade­quate­ly. Over 40% of those with Type 2 dia­betes require insulin as part of their dia­betes man­age­ment plan. Insulin is a very old pro­tein that may have orig­i­nat­ed more than a bil­lion years ago. The mol­e­c­u­lar ori­gins of insulin go at least as far back as the sim­plest uni­cel­lu­lar eukary­otes. Apart from ani­mals, insulin-like pro­teins are also known to exist in Fun­gi and Pro­tista king­doms. The human insulin pro­tein is com­posed of 51 amino acids, and has a mol­e­c­u­lar weight of 5808 Da. It is a dimer of an A‑chain and a B‑chain, which are linked togeth­er by disul­fide bonds. Insulin’s struc­ture varies slight­ly between species of ani­mals. Insulin from ani­mal sources dif­fers some­what in “strength” (in car­bo­hy­drate metab­o­lism con­trol effects) from that in humans because of those vari­a­tions. Porcine insulin is espe­cial­ly close to the human version.