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Diabetic boy after treatment with insulin. Two images, taken two months apart, showing a diabetic boy before and after treatment with insulin. Diabetes mellitus type 1 is a metabolic disorder caused by a lack of the hormone insulin. Without insulin the body cannot use sugar to make energy. Instead it starts to break down fat and muscle to make energy, leading to emaciation. Daily insulin injections and a strict diet can control the disease. Insulin, extracted from cows, was first used in the 1920s. (KEYSTONE/SCIENCE PHOTO LIBRARY/NATIONAL LIBRARY OF MEDICINE/SCIENCE PHOTO LIBRARY)