Scottish physiologist who was awarded a Nobel Prize for Physiology or Medicine in 1923 with Frederick Banting for their part in the discovery and isolation of the hormone insulin, the hormone in the pancreas that reduces blood glucose (sugar) levels. Since its discovery, insulin has been used extensively as the main treatment for diabetes.
Macleod was born in Cluny, Scotland, and studied medicine at the University of Aberdeen in 1898. He went on to study biochemistry at the University of Leipzig and published various papers on biochemical aspects of respiration and carbohydrate metabolism. While professor of physiology in Toronto (from 1918 onwards) he invited Banting to work with him and his colleague Charles Best in their search for the hormone in the pancreas that reduces blood glucose levels. The hormone responsible for this, insulin, was first isolated in Macleod's laboratory by Banting and Best in 1921 while Macleod was away on holiday.
However, when Banting and Best wanted to present their work to the American Physiological Society in December 1921 they were unable to, because neither of them were members of the Society. Macleod, who was a member, attached his name and the resulting communication was published under the three names in 1922. In 1923, the Nobel Prize for Physiology or Medicine was awarded to Macleod and Banting ‘for their discovery of insulin’. Banting expressed his disapproval by sharing half of his prize with Best, and Macleod thereupon gave half of his prize to Collip, who characterized insulin after its discovery.