Scientists believe they may have uncovered a key reason why obese people have a raised risk of health complications such as type 2 diabetes.
They blame a specific protein - pigment epithelium-derived factor (PEDF) - which is secreted by fat cells.
The Australian and US research on mice suggests blocking some of PEDF's action may reverse some complications - raising hopes of new drug treatments.
The study appears in the journal Cell Metabolism.
Because PEDF is produced by fat cells people who are overweight have higher levels of the protein in the bloodstream.
The latest research shows that the protein sends a signal to other tissues in the body, triggering development of insulin resistance - a condition that often leads to type 2 diabetes - in the muscle and liver.
Raised PEDF levels were also linked to a release of fats into the bloodstream, raising the risk of complications such as heart disease.