Macleod, John James Rickard (1876-1935) was a Scottish physiologist who shared the 1923 Nobel Prize in physiology or medicine with Fredrick Grant Banting for their discovery of insulin in 1922. Insulin is a hormone produced by specialized cells in the pancreas that regulates the body's use of sugar and other food. Some diabetics lack insulin.

Macleod was born on Sept. 6, 1876, near Dunkeld, Scotland. His father was a clergyman. The family moved to Aberdeen, and Macleod earned a degree in medicine with honors from the University of Aberdeen in 1898. He taught physiology at universities in Cleveland, Toronto, and Aberdeen. His early research was on circulation, and between 1902 and 1922, he published numerous papers on the control of respiration.

Macleod became interested in carbohydrate metabolism and especially in diabetes. He published several papers on carbohydrate metabolism and glycosuria, a condition in which glucose (sugar) is present in the urine, as in diabetes. He believed that the pancreas was the organ involved in diabetes but was unable to prove exactly what part it played. Although research suggested that a hypothetical substance, “insuline,” controlled the metabolism of sugar, nobody had been able to prove its existence.

Banting and Macleod, with Charles Herbert Best and James Bertram Collip, announced their discovery of insulin in 1922. They patented a process for extracting insulin from animals that could be used for the treatment of human patients. They then sold the patent to the University of Toronto, which made it available to pharmaceutical companies. Insulin eventually became available as a manufactured product.

In the 1930's, Macleod worked on carbohydrate metabolism and the possible existence of a diabetogenic center in the brain. He also researched air sickness, electric shock, and the bacteria that cause tuberculosis. He wrote or co-wrote Physiology and Biochemistry in Modern Medicine (1918), Diabetes: Its Pathological Physiology (1913), Carbohydrate Metabolism and Insulin (1926), and The Fuel of Life (1928), and other books.