Leptin: the report from your fat
Fat is not just storage — it is an endocrine organ. The more fat you carry, the more leptin it releases into the blood. So leptin is essentially a fat gauge read by the brain: high leptin says “stores are full, you can relax appetite and spend energy freely.” This makes leptin one of the classic adipokines, hormones secreted by adipose tissue.
Crucially, leptin matters most when it is low. When you lose fat, leptin falls, and the brain reads this as a famine warning: it cranks up hunger and dials down energy expenditure. Evolution tuned this system to protect against starvation far more carefully than against overeating — which is part of why losing weight is harder than gaining it.
Ghrelin: the pre-meal alarm
If leptin is the slow background gauge, ghrelin is the fast foreground alarm. It is a peptide hormone made mostly by the stomach. Ghrelin rises before meals, peaking when you are used to eating, and falls after you eat. It is the only well-established hormone that actively increases appetite, which is why it is nicknamed the hunger hormone.
Ghrelin's daily rhythm is partly learned: eat lunch at noon for a week and ghrelin will start spiking near noon. This is one reason regular meal timing makes hunger more predictable, and why skipping a habitual meal feels worse than never having had one.
A day of appetite signals (schematic)
Ghrelin (hunger) /\ /\ /\
/ \ / \ / \
______________/ \____/ \________/ \___
^breakfast ^lunch ^dinner
(peaks just before each meal)
Leptin (fullness) ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
tracks total body fat — moves over days/weeks,
not minute to minute. Slightly higher overnight.Two timescales, one decision
Appetite regulation blends these timescales. Ghrelin and other gut signals govern when a single meal starts and stops — the moment-to-moment sense of hunger and satiety. Leptin sets the long-run baseline, biasing the whole system toward eating more or less depending on how much fat you have stored.
Both hormones ultimately report to the same place: a cluster of neurons in the hypothalamus that we will open up in the next guide. Leptin and ghrelin do not act directly on your willpower — they reshape the wiring of hunger from below.