When I was a medical student at McGill in the late 1970s, we learned a straightforward explanation for the cause of Type 2 diabetes, the most common form of diabetes in adults, accounting for about 90 per cent of all diabetes cases. We were told the insulin resistance responsible for Type 2 diabetes was caused by high levels of insulin. Hyperinsulinemia–increased insulin levels in the blood–was said to “downregulate” insulin receptors, making cells with those receptors less responsive to the insulin message. From a physiology point of view, this makes perfect sense. It’s analogous to the development of tolerance that can happen with regular heroin use when a person no longer responds to the drug in the way they did initially.
Sometime in the 1980s this explanation for the cause of insulin resistance was abandoned. Instead, the medical community adopted a new theory that insulin resistance comes first, and is behind high insulin levels in Type 2 diabetes. To overcome insulin resistance, the pancreas secretes larger-than-normal amounts of insulin, resulting in so-called “reactive hyperinsulinemia.” The cause of this insulin resistance is never clearly explained, although obesity, chronic inflammation, and genes are all said to contribute.