Leptin and ghrelin: the hormones behind hunger and fullness

Ghrelin and leptin are the two hormones that frame hunger and fullness, but they work on different timescales. Ghrelin is a fast signal from an empty stomach that rises before meals and makes you want to eat now. Leptin is a slow signal from your fat stores that tells the brain roughly how much energy you have banked over weeks and months. The twist that explains a lot of failed diets is this: in common obesity the problem is almost never too little leptin, it is a brain that has stopped listening to it, which is why injecting extra leptin does nothing for most people and why "leptin reset" supplements are selling you a fiction.

Two hormones, two timescales

Ghrelin is made mainly by cells in the stomach lining and is the only well-established circulating hormone that actively drives the urge to eat. Its levels climb in the hours before a meal, peak when you are hungry, and fall sharply once food arrives. That pre-meal rise is partly learned: eat lunch at the same time each day and ghrelin will start to surge in anticipation, which is why hunger often feels like a clock rather than a fuel gauge.¹

Leptin is made by fat cells (adipocytes) in rough proportion to how much fat tissue you carry. It is not a meal-to-meal signal. Its job is to report your long-term energy reserves to the brain. When fat stores are healthy, leptin is high and quietly says "we have enough, no emergency". When you lose fat, leptin falls and the brain reads that as a threat to be corrected. Leptin was discovered in 1994 by Jeffrey Friedman and colleagues at Rockefeller University, who positionally cloned the mouse ob gene and its human equivalent, work that won a share of the 2010 Lasker Award.²

How they act on the hypothalamus

Both hormones converge on the same small command centre: the arcuate nucleus of the hypothalamus, which sits where the blood-brain barrier is leaky enough to sample circulating hormones. Two opposing neuron populations live there. AgRP/NPY neurons drive hunger when activated; POMC neurons promote fullness and energy expenditure. Ghrelin stimulates the AgRP/NPY hunger neurons, turning appetite up. Leptin does the reverse: it activates POMC neurons and suppresses AgRP/NPY, turning appetite down and nudging energy use up.¹

This is why thinking of either hormone as a simple "on switch" for weight is misleading. They are inputs into a balance, and that balance is wired to defend body fat far more aggressively against loss than against gain. The system evolved when starvation was the threat, not an oversupply of cheap calories. If you want the wider picture of how the brain reads internal state, our hormones and physiology library covers the related signalling systems.

Leptin resistance, not leptin deficiency

Here is the single most important and most misunderstood fact about leptin. When it was discovered, the logic looked irresistible: leptin tells the brain you have enough fat, so giving people more leptin should switch off appetite and melt away obesity. It did not work. People with common obesity already have high leptin, in proportion to their larger fat mass, yet their brains behave as if it is low. This is leptin resistance.³

A randomised, dose-escalation trial of recombinant leptin in obese adults was disappointing: meaningful weight loss required leptin concentrations many times above normal physiology, and the effect was modest even then.³ The proposed mechanisms of resistance are several and probably overlap: reduced transport of leptin across the blood-brain barrier into the hypothalamus, low-grade inflammation in hypothalamic tissue, endoplasmic reticulum stress inside neurons, and increased expression of signalling brakes such as SOCS-3 that dampen the leptin receptor's downstream JAK2-STAT3 pathway.⁶ Note that these mechanistic details come mostly from animal and cell studies, so treat the exact pathway claims as plausible biology rather than settled human fact.

The contrast that proves the point is congenital leptin deficiency, a rare condition where a gene fault means the body makes almost no functional leptin. These children are intensely hungry and severely obese from infancy. In landmark work led by Sadaf Farooqi at Addenbrooke's Hospital in Cambridge, daily injections of recombinant leptin produced dramatic, sustained falls in appetite and fat mass, and also restored normal puberty and immune function.⁴ Leptin replacement is genuinely curative there, because the problem really is a missing signal. For the other 99 plus percent of people with obesity, the signal is loud and the receiver is jammed, which is why adding more signal achieves so little.

Why sleep loss makes you hungrier

Two influential studies established that short sleep tilts these hormones toward hunger. In a tightly controlled crossover experiment, Karine Spiegel and colleagues (Annals of Internal Medicine, 2004) restricted twelve healthy young men to four hours in bed for two nights, with food and activity held constant. Compared with a well-rested condition, leptin fell by about 18 percent, ghrelin rose by about 28 percent, and the men reported roughly 24 percent more hunger, with the strongest cravings for calorie-dense sweet and starchy foods.⁵

The same year, Shahrad Taheri and colleagues (PLoS Medicine, 2004) looked at over a thousand people in the Wisconsin Sleep Cohort and found the same pattern in real life: habitually short sleepers had lower leptin, higher ghrelin and higher body mass index, with the effects most pronounced around five hours of sleep.⁸ These are real, replicated findings, but keep two caveats in mind: the experimental study was small and short, and the cohort study is observational, so it shows association, not proof that the hormone shifts alone cause the extra weight. Still, the direction is consistent and the practical message is sound. Protecting sleep is appetite management. We go deeper into this in our writing on sleep, stress and the nervous system.

Why dieting defends your old weight

If you have ever lost weight only to feel relentlessly hungry and slowly regain it, the biology is on your side, not against your willpower. As fat falls, leptin falls with it, and the brain interprets the lower leptin as a famine to be reversed. At the same time, ghrelin rises. The result is a coordinated defence of your previous weight that researchers call metabolic adaptation.⁷

The clearest human demonstration comes from Priya Sumithran and colleagues (New England Journal of Medicine, 2011). They put fifty people with overweight or obesity through a ten-week very-low-energy diet, then measured appetite hormones at baseline, at ten weeks, and a full year later. One year after the initial weight loss, ghrelin remained elevated, leptin remained suppressed (around 35 percent below baseline), and subjective hunger was still higher than before they started. The hormonal pressure to regain did not fade, it persisted for at least twelve months.⁷

This reframes weight regain as a predictable physiological response rather than a moral failing, and it is the honest, non-blaming core of this whole topic. It also explains why crash dieting so often backfires: the faster and larger the loss, the louder these defensive signals tend to become. Slower loss, adequate protein, resistance training to preserve muscle, and planning for long-term maintenance are not just nice-to-haves, they are the counterweight to a system built to pull you back.

Why "leptin reset" supplements are nonsense

Search "leptin" and you will quickly meet products promising to "reset", "detox" or "rebalance" your leptin so the fat falls off. The marketing is built on a basic category error, and it is worth being blunt about why.

• Leptin is a protein. Swallowed in a pill it would be digested like any other protein, never reaching the bloodstream intact. There is no oral leptin.⁹
• Most "leptin supplements" contain no leptin at all. They are typically blends of fibre, green tea extract, alpha-lipoic acid and similar ingredients relabelled as leptin support, with no evidence they alter leptin signalling.⁹
• The whole premise is backwards. In common obesity leptin is already high. Adding more, even if you could, addresses the wrong problem; the failure is in the brain's response, not the supply.³
• "Detox" has no mechanism here. Leptin is a normal regulatory hormone, not a toxin to be flushed. There is nothing to detox.

The levers that actually work

There is no supplement shortcut, but the genuine levers are unglamorous and well supported. The point is not to "hack" these hormones but to stop fighting them.

1. Prioritise sleep

Given the Spiegel and Taheri data, treating 7 to 9 hours as non-negotiable is one of the most direct ways to keep ghrelin and leptin out of hunger-promoting territory, and it costs nothing.⁵

2. Lead with protein and fibre

Protein blunts the post-meal ghrelin rebound more than fat or carbohydrate and improves satiety; fibre slows gastric emptying and feeds gut hormones that work alongside this system. Building meals around adequate protein is one of the more reliable ways to feel fuller on the same calories.¹

3. Do not crash diet

Because metabolic adaptation scales with the speed and size of loss, a moderate deficit with resistance training to protect muscle, plus an explicit maintenance plan, beats aggressive restriction that triggers maximal defence and rebound.⁷

4. Understand the GLP-1 context

The modern obesity drugs (GLP-1 receptor agonists such as semaglutide and tirzepatide) succeed where leptin failed precisely because they do not rely on a resistant pathway. They mimic gut satiety hormones and act on appetite circuits, reducing ghrelin-driven cravings and food preoccupation rather than trying to force a jammed leptin signal. They are powerful prescription medicines with real side effects and supervision requirements, not lifestyle add-ons, and weight tends to return if they are stopped, which is consistent with everything above about how hard the body defends its set point.¹⁰ If you are weighing options, our start here guide helps you sequence basics before medication, and the stack builder maps goals to evidence-based steps.

What to ask your GP

What to do next

This article is educational and does not constitute medical advice, diagnosis, or a treatment recommendation. Medication uses described as “off-label” are not licensed for that purpose in the UK and should only be considered under qualified clinical supervision. Always speak to your GP, pharmacist, or a registered specialist before starting, stopping, or changing any treatment. If you have severe or alarm symptoms - unintentional weight loss, blood in your stool, difficulty swallowing, persistent vomiting, a fever, or severe pain - seek urgent medical care.