In Toronto, Canada, Leonard Thompson becomes the first person to receive insulin as treatment for diabetes. Diabetes has been recognized as a distinct medical condition for more than 3,000 years, but its exact cause was a mystery until the 1920s. In the early 20th century, the only way to treat the fatal disease was through a diet low in carbohydrates and sugar and high in fat and protein. Instead of dying shortly after diagnosis, this diet allowed diabetics to live for about a year. A breakthrough came at the University of Toronto in the summer of 1921, when Canadians Frederick Banting and Charles Best successfully removed the pancreas-secreted protein insulin from test animals, producing diabetic symptoms, and then began a program of insulin injection that returned the animals to normalcy. This experiment confirmed their theory that diabetes was caused by a lack of insulin, which metabolizes sugar. With the aid of other scientists, Banting and Best extracted insulin from the pancreases of cattle from slaughterhouses and began treating Leonard Thompson. The teenager improved dramatically. By 1923, insulin had become widely available, saving countless lives around the world.