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Insulin is a hormone that is secreted by the pancreas in response to an increase of blood sugar. Insulin increases the ability of sugars to pass through cell walls into cells where it can be metabolized or converted to fat for storage.

In some individuals, the pancreas are unable to produce enough insulin to allow the sugar to pass into cells to be metabolized. The sugar builds up in the blood leading to diabetes mellitus, which when untreated is inevitably fatal.

However, insulin was isolated from dog's pancreas in the early part of the 20th century by a Canadian team led by Frederick Banting. It was found that the same extract from the pancreas of slaughtered pigs would approximate human insulin. Human patients injected with insulin would immediately recover from their symptoms. Banting's first patient, a 14-year old named Leonard Thomspon, went on to live to the age of 27 and died of pneumonia unrelated to his diabetes.  The first American patient, Elizabeth Hughes, daughter of politician Charles Evans Hughes, was also 14 when she was first treated and lived to be 74, receiving over 42,000 injections.

Insulin is digested by stomach acid and as a result must be injected, usually twice a day before meals.

Insulin at Wikipedia