Before the discovery of insulin during the 1920s, children who developed diabetes rarely lived into their twenties. The lack of insulin meant that they were unable to use their food for energy and for repairing and growing their body tissues and organs. A typical diabetic looked very thin and starved and could die within months of being diagnosed.
There were some efforts to try and get diabetics to gain weight by feeding them diets that contained only protein and fat ~ they cut out carbohydrates completely. However, this only put off the inevitable for a few months and it was difficult to maintain.
Once insulin was discovered, it was literally a wonder drug and turned around the condition of the first diabetics it was tested on within weeks.