Prediabetes refers to the syndrome of insulin resistance and elevated insulin levels that is usually associated with obesity and the accumulation of a certain kind of fat called abdominal, truncal, or visceral fat. This is fat tissue that is deep in the abdominal cavity and is very different from subcutaneous fat, which is deposited under the skin.
Abdominal obesity and fat tissue produces certain hormones and other substances that together cause insulin resistance. This was a major surprise in medicine when it was discovered only 10 or 15 years ago. Prior to that, fat tissue was considered to be "metabolically inert", which means that it was just a storage tissue and didn't affect metabolism. This was very far from the truth, and visceral fat is now considered to be very active and complex metabolically. It produces a host of hormones (for example, leptin, ghrelin, and adiponectin) and other factors (cytokines) that have major influences on metabolism.
The discovery that insulin resistance was the central "lesion" in type 2 diabetes led to a whole area of research that resulted in linking type 2 diabetes to high blood pressure, truncal or abdominal obesity, abnormal blood lipids (elevated triglycerides and low HDL cholesterol), and high waist to hip ratio (the "apple" body type). It was originally described by Gerald Reaven, MD a professor of medicine at Stanford University, who named it Syndrome X to underscore how little understood it was and how it was more than just elevated blood sugars that caused disease. He wrote a book for the general public on this called Syndrome X, published by Simon and Shuster in 2000. Syndrome X is now also called metabolic syndrome and prediabetes, and the majority of people with type 2 diabetes have some or all of its features.
