Frederick Banting upon the idea of obtaining insulin from the fetal pancreas. He removed the pancreases from fetal calves at a William Davies slaughterhouse and found the extracts to be just as potent as those extracted from the dog pancreases. By December 1921, he had also succeeded in extracting insulin from the adult pancreas. Pork and beef would remain the primary commercial sources of insulin until they were replaced by genetically-engineered bacteria in the late 20th century. In spring of 1922, Banting established a private practice in Toronto and began to treat diabetic patients. His first American patient was Elizabeth Hughes Gossett,
William Banting the Public" which contained the particular plan for the diet he followed. It was written as an open letter in the form of a personal testimonial. Banting accounted all of his unsuccessful fasts, diets, spa and exercise regimens in his past. His previously unsuccessful attempts had been on the advice of various medical experts. He then described the dietary change which finally had worked for him, following the advice of another medical expert. "My kind and valued medical adviser is not a doctor for obesity, but stands on the pinnacle of fame in the treatment of another malady, which, as