What is leptin and what is its function?

Leptin is a protein produced by fat cells.
Leptin travels through the bloodstream to the hypothalamus.
When leptin levels are normal, people eat just enough to maintain their weight.
If leptin levels are high (because of more fat), appetite is decreased, metabolic rate increases, and the organism becomes more active.
If leptin levels are low (because of reduced weight), appetite increases and metabolic rate drops.
Leptin helps regulate body weight by strengthening neural circuits in the hypothalamus that reduce appetite and weakening circuits that stimulate it.
During a critical period in infancy, leptin helps to set the set point.