Scientists know a lot about how insulin works in people with type 2 diabetes, but they still don’t have much direct information on how leptin behaves in the bodies of lean or obese people—even after many years of studying it.

This study says that even after 30 years of studying leptin, scientists still don’t fully understand how it works in people’s bodies, especially when comparing lean and obese individuals—unlike insulin, which we know a lot about in diabetes.