Insulin

Where it comes from

Made in

Beta cells of the pancreatic islets

Sent out by

Rising blood glucose after a meal, helped along by the gut incretins GLP-1 and GIP and by amino acids

Acts on

• Liver
• Skeletal muscle
• Fat tissue
• Brain

What it does

• Opens muscle and fat cells to glucose ¹
• Tells the liver to store glycogen and to stop making glucose ¹
• Holds back the breakdown of fat and protein

In medicine

• Insulin therapy since 1922: human insulin and analogues such as lispro, aspart, glargine and degludec
• Type 1 diabetes is its absence; type 2 begins as resistance to it ¹
• C-peptide, cut from the same precursor, is measured to tell how much insulin a pancreas still makes

Beyond the body’s own use

• Pumps joined to a glucose sensor, the “closed loop” or artificial pancreas, now adjust the dose themselves and raise the time spent in range in randomised trials; inhaled insulin is approved and little used
• A once-weekly insulin, icodec, is approved in Europe (2024) and the United States (2026) ³; a “smart” insulin meant to switch itself on as glucose rises has worked in animals and has not been given to people
• Insulin as a nasal spray was tested for memory in Alzheimer’s disease, on the idea that the brain’s own insulin signalling fails; a 289-person randomised trial found no cognitive or functional benefit over a year ²
• It is banned in sport at all times ⁴ and is misused alongside growth hormone to build muscle; the danger is severe hypoglycaemia, and no trial supports that use