Ghrelin is a clock, and leptin is an alarm that only rings on the way down#
Ghrelin is made mostly by the stomach and climbs before you eat. Leptin is made by fat tissue and reports how much energy you have in storage. That much is textbook, and it is where most explanations stop — ghrelin cast as the hunger switch, leptin as the fullness switch, one turning appetite on and the other turning it off.
The experiments refuse that symmetry. Delete ghrelin from a mouse entirely and it eats a normal amount of food. Give leptin to a person born without any and their appetite collapses; give the same leptin to a person who already has plenty and very little happens. So ghrelin behaves less like a switch than like a clock, rising in anticipation of meals it has learned to expect. And leptin behaves less like a fullness signal than like a low-fuel alarm — loud on the way down, close to silent on the way up. If you arrived from how sleep controls your hunger hormones, this is the chemistry that page audits; here the subject is the hormones themselves, with sleep set aside.
Ghrelin peaks before the meal, not when the stomach runs empty#
The finding that named ghrelin a hunger hormone came from watching it across an ordinary day. Ten healthy adults on fixed meal schedules gave 38 blood samples over 24 hours; plasma ghrelin rose nearly twofold immediately before each meal and fell to a trough within an hour of eating, in a pattern reciprocal to insulin1. Three meals, three clean peaks, three collapses.
Two details in that study get dropped from the retelling, and both matter. First, the meals were on a fixed schedule, so the rise arrived ahead of a meal the body had been trained to expect — it is anticipation at least as much as it is a fuel gauge reading empty. Second, ghrelin had a separate overnight peak at around 1 a.m. and its lowest point of the day at 9 a.m., which is not what a simple emptiness signal would do; a stomach that has been empty since dinner should be shouting at 7 a.m., not whispering. The hormone runs partly on a daily rhythm of its own, set by the body clock rather than by the contents of your gut.
The study also cannot prove the rise causes the meal. Ten people who knew their lunch was coming at a fixed hour is a design that can show a pattern beautifully and settle causation not at all. For that you have to add ghrelin and take it away.
Ghrelin can create hunger — but hunger does not need it#
Adding it works. Nine healthy volunteers received an intravenous ghrelin infusion or saline in a randomized double-blind crossover, then ate from a free-choice buffet. Every single participant ate more on ghrelin, by an average of 28 ± 3.9% (p < 0.001), with higher appetite ratings and no change in gastric emptying — so this was appetite, not a mechanically faster stomach2. Ghrelin was the first circulating hormone ever shown to make humans eat more.
Taking it away does almost nothing. Mice engineered without the ghrelin gene are not the anorexic dwarfs the pharmacology predicted: size, growth rate, food intake, body composition and behavior were indistinguishable from normal littermates, and their body fat percentages were identical3. The obvious defense is that an animal born without a hormone builds a workaround. So a later group destroyed the ghrelin-producing cells in adult mice, after normal development. Food intake averaged 3.6 ± 0.1 g/day in both the ablated and control groups; body weight did not diverge, and the ablated mice put on weight on a high-fat diet exactly like controls4.
Ghrelin can manufacture hunger you did not have. The body does not appear to need it in order to be hungry. That gap between sufficient and necessary is the whole space in which "block your hunger hormone" marketing operates.
Be careful in both directions. The removal experiments are rodent work, and mice are not small people. The addition experiment is nine humans given an intravenous drip — a blunter intervention than anything your own physiology does to itself in a day. What survives both caveats is the shape of the result: appetite has redundant machinery, and ghrelin is one input to it rather than the master control.
Leptin is loud on the way down and quiet on the way up#
Leptin's asymmetry is even sharper, and it is best seen at the extremes.
At the bottom: a handful of children are born unable to make leptin at all, and they are relentlessly, dangerously hungry. Three of them were given daily leptin injections for up to four years. More than 98% of the weight they lost was fat mass, and within two months their energy intake at a standardized test meal had fallen by 45–84%5. Restoring a missing signal did not nudge their appetite; it rewrote it.
At the top: giving leptin to people who already have normal or high levels is a far weaker lever, producing modest average losses with enormous person-to-person spread — the reason that story gets its own accounting in leptin resistance explained.
The most instructive case sits in between. Ten inpatients were studied at their usual weight, then again holding a 10% reduced weight, then a third time at that reduced weight while receiving twice-daily leptin sufficient to restore their pre-diet levels. The energy-expenditure drop, the increased skeletal muscle work efficiency, the dampened sympathetic tone and the fallen thyroid hormones all returned to pre-weight-loss values, leading the authors to describe the weight-reduced state as "a condition of relative leptin insufficiency"6. Falling leptin is what your body reads when you diet — the physiology behind why maintenance is harder than the loss, and a more precise account of the phenomenon usually mangled as starvation mode.
This is where a genuine argument sits in the field, not a manufactured one. In a 2017 commentary, Jeffrey Flier and Eleftheria Maratos-Flier — the former a longtime leptin researcher and a former dean of Harvard Medical School — accepted that falling leptin is an established starvation signal, then pointed out that leptin's other famous job has never actually been demonstrated: the idea that rising leptin from overfeeding "provides a physiologic signal that orchestrates resistance to obesity" is, in their words, "still widely believed, and possibly true in some circumstances," but "has not been experimentally demonstrated"7. Half of leptin's reputation rests on evidence; the half that describes it as your fullness hormone is the half under dispute.
Which is why obesity comes with high leptin and low ghrelin#
If the popular model were right, people with obesity should show too little of the fullness hormone and too much of the hunger hormone. The measured pattern is the reverse on both counts: circulating leptin is elevated in obesity, and ghrelin is decreased8. Leptin is high because it tracks fat mass and there is more fat tissue making it. Ghrelin is low in a way that looks like the system correctly turning down a meal-initiation signal in someone with abundant stores.
| Ghrelin | Leptin | |
|---|---|---|
| Made mostly by | The stomach | Fat tissue |
| Reports | Time until the next expected meal | How much energy is stored |
| Timescale | Minutes to hours | Days to weeks |
| Remove it | Mice eat normally3 | Humans are relentlessly hungry5 |
| Add it | Buffet intake up 28%2 | Little effect if levels are already normal |
| Level in obesity | Lower than in lean people8 | Higher than in lean people8 |
Read the last row as the summary of the whole page. A model that predicts the opposite of the measurement in the population it is most often invoked to explain is not a model you should be making decisions from.
What this changes about the hunger you actually feel#
Three things follow, and none of them require a lab.
Hunger arriving at your habitual lunchtime is partly a scheduled signal, not a measurement of depletion — which is why it recedes if you ride it out and why it reappears on time tomorrow. Change when you eat and, over days, the peaks move with you. That is a reason to treat mealtime hunger as information about your routine rather than as a verdict on your discipline.
Second, the food matters more than the hour. Ghrelin's suppression after a meal, and the satiety that follows it, depend heavily on what you ate, which is why protein does more per calorie for appetite than any timing trick. Third, and most usefully: the products aimed at this system almost all promise to raise leptin or block ghrelin, and both are aimed at the arm of the system that does not respond. Restoring leptin to someone who lacks it is transformative; adding more to someone who has plenty is not. Blocking ghrelin in an animal that has none already changes nothing about how much it eats.
The hormones are real, well characterized, and worth understanding. They are just not the two switches the diet internet has been describing — and the specific claim that a short night flips them is weaker than almost anyone assumes, which is the subject of how sleep loss raises ghrelin.
FAQ#
Why is ghrelin lower in people with obesity?#
Because ghrelin tracks the anticipated need for a meal, not the amount of body fat, and in a state of energy abundance the system turns that signal down. Reviews of the human data consistently find circulating ghrelin decreased and leptin elevated in obesity8. It is the opposite of the popular picture, and it is one reason "too much hunger hormone" is a poor explanation for weight gain.
Can you get your ghrelin and leptin levels tested?#
Assays exist, but a result would be close to uninterpretable. Leptin scales with fat mass, so a leptin value mostly restates your body-fat percentage in different units. Ghrelin nearly doubles before a meal and collapses within an hour after one1, so a single draw captures where you were in a meal cycle rather than a stable trait. Neither has a validated treatment threshold outside rare genetic deficiency.
Is hunger before a meal a sign your body needs food?#
Sometimes, but less often than it feels. On fixed meal schedules the ghrelin rise precedes the meal your body has been taught to expect, and it has a separate overnight peak at a time nobody is eating1. Genuine energy depletion produces sustained, escalating hunger; a scheduled peak crests and recedes on its own within roughly half an hour, which is a useful practical distinction.
Sources#
- Cummings DE, Purnell JQ, Frayo RS, Schmidova K, Wisse BE, Weigle DS. A preprandial rise in plasma ghrelin levels suggests a role in meal initiation in humans. Diabetes. 2001;50(8):1714-9.
- Wren AM, Seal LJ, Cohen MA, et al. Ghrelin enhances appetite and increases food intake in humans. J Clin Endocrinol Metab. 2001;86(12):5992.
- Sun Y, Ahmed S, Smith RG. Deletion of ghrelin impairs neither growth nor appetite. Mol Cell Biol. 2003;23(22):7973-81.
- McFarlane MR, Brown MS, Goldstein JL, Zhao TJ. Induced ablation of ghrelin cells in adult mice does not decrease food intake, body weight, or response to high-fat diet. Cell Metab. 2014;20(1):54-60.
- Farooqi IS, Matarese G, Lord GM, et al. Beneficial effects of leptin on obesity, T cell hyporesponsiveness, and neuroendocrine/metabolic dysfunction of human congenital leptin deficiency. J Clin Invest. 2002;110(8):1093-103.
- Rosenbaum M, Goldsmith R, Bloomfield D, et al. Low-dose leptin reverses skeletal muscle, autonomic, and neuroendocrine adaptations to maintenance of reduced weight. J Clin Invest. 2005;115(12):3579-86.
- Flier JS, Maratos-Flier E. Leptin's physiologic role: does the emperor of energy balance have no clothes? Cell Metab. 2017;26(1):24-26.
- Klok MD, Jakobsdottir S, Drent ML. The role of leptin and ghrelin in the regulation of food intake and body weight in humans: a review. Obes Rev. 2007;8(1):21-34.
