October 31, 1920

Discovery of insulin

Frederick Banting records his pancreatic-duct ligation idea

Banting began the entry by misspelling "diabetes" — and first dated it 30 October before crossing that out and writing 31.

A note written in the night

Unable to sleep after preparing a lecture on the pancreas and reading Moses Barron's paper on pancreatic-duct obstruction, Frederick Banting wrote a proposed experiment into a small notebook in London, Ontario, on 31 October 1920.

He proposed ligating dogs' pancreatic ducts, waiting for the enzyme-producing acini to degenerate while the islets remained, and then extracting the islets' hypothesised internal secretion to relieve sugar in the urine.

He first dated the entry 30 October and corrected it to 31 October, and he opened it by writing "Diabetus".

No insulin that night

Nothing was isolated or administered. The immediate product was a research hypothesis, which Banting took to the physiologist J.J.R. Macleod eight days later.

Macleod supplied laboratory space, guidance, dogs and an assistant at the University of Toronto. The animal experiments did not begin until 1921, with Charles Best, who was not present when the notebook entry was written.

The duct-ligation mechanism at the centre of Banting's idea was ultimately shown not to be necessary for extracting insulin.

What "fifteen months later" compresses

The first injection into Leonard Thompson, aged 14, on 11 January 1922 used a crude Banting–Best extract. It was largely ineffective and caused an abscess; James Collip's purer preparation produced the first unambiguously successful treatment on 23 January.

Calling Banting and Best the first to isolate insulin also overstates the work: their early preparations were crude pancreatic extracts, earlier investigators had produced active extracts, and clinically usable purification required Collip and the wider Toronto team.

Insulin was not the first peptide hormone discovered either — secretin had been established as a hormone in 1902.

Sources

Researched 24 Aug 2026 5 sources not yet audited

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What the sources leave uncertain

The physical notebook supplies strong evidence for October 31, but Banting first wrote October 30 and corrected it to October 31, reflecting that the idea was recorded during the night around the calendar boundary.