How fat tissue reports energy stores to the brain
Leptin is secreted by adipocytes roughly in proportion to total fat mass, so larger fat stores generally mean higher circulating leptin. The hormone crosses to the hypothalamus, where it acts on neurons in the arcuate nucleus: it stimulates POMC neurons that reduce appetite and inhibits AgRP/NPY neurons that drive hunger. The net signal is a sense of energy sufficiency.
Leptin also moves with short-term energy balance. Levels fall during fasting and energy restriction faster than fat mass changes, which is part of why leptin is often described as a signal of energy availability, not just a fixed fat gauge.
What leptin does beyond appetite
Beyond hunger, leptin influences energy expenditure, reproductive signaling, and immune function, which is why very low leptin (as in extreme leanness or starvation) can disrupt menstrual cycles and other processes. This breadth reflects leptin's role as a marker that there is enough stored energy to support demanding functions.
These are well-established physiological roles, but leptin sits within a larger network of appetite and metabolic hormones rather than acting alone. This summary is general educational information and not medical advice.