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Blood glucose levels in the individual without diabetes remain remarkably stable, approximately between 4 and 7 mmol/l, even if the individual fasts for several hours or consumes a large sugary meal. This level is maintained by a relationship between insulin, which lowers blood glucose, and the counter-regulatory hormones (primarily glucagon), which cause a rise in blood glucose (Table 3.1). The efficiency of this system is often not appreciated until the prescriber tries to support the patient with diabetes to mimic these same effects and achieve near normoglycaemia with the various manufactured insulins available!
Insulin is produced by the beta cells in the islets of Langerhans, embedded in the pancreas gland (which also produces pancreatic digestive juices, a function that is not affected in diabetes). After a carbohydrate load in the gut following eating, starch and sugars are broken down into glucose by digestive juices. Glucose is then absorbed from the gut into the blood circulation. Receptors in the beta cells monitor the prevailing blood glucose. In the individual without diabetes, the resulting rise in blood glucose after a meal stimulates the beta cells to produce a burst of insulin, which prevents blood glucose from rising much above 7 mmol/l. As the blood glucose falls to normal as feeding finishes, beta cell stimulation is reduced and insulin production also falls in response.
During periods of fasting, low blood glucose concentrations result in minimal stimulation of the beta cells, so production of insulin is low.