Ghrelin, Leptin and the Chemistry of Hunger

Why do you feel hungry at lunchtime, full after a big meal, and sometimes still want dessert? Hunger feels like a simple urge, but it’s controlled by a network of chemical messengers travelling between your stomach, intestines, fat tissue, pancreas and brain. Some say “eat now”; others say “that’s enough”; one reports how much energy you have stored for the long term. Understanding these molecules explains everyday hunger, why weight loss is hard to maintain, and how a new generation of medicines works.

The control centre: the hypothalamus

Appetite is coordinated in a small region at the base of the brain called the hypothalamus. One part of it, the arcuate nucleus, contains two groups of nerve cells with opposite jobs:

• “Hunger” neurons release signalling molecules (including neuropeptide Y and AgRP) that increase appetite and reduce energy use.
• “Fullness” neurons release molecules (including one derived from a protein called POMC) that reduce appetite.

The hormones below act largely by switching these two groups up or down. Other brain areas — including the reward system, which uses dopamine — add pleasure, habit and emotion to the mix.

Ghrelin: the “hunger hormone”

Ghrelin is a peptide of 28 amino acids, made mainly by cells in the stomach. It’s unusual in one chemical detail: to become active, one of its amino acids (a serine, third in the chain) must have an 8-carbon fatty acid (octanoic acid) attached by an ester bond. This fatty modification is added by an enzyme and is essential for ghrelin to bind its receptor. It’s a rare example of a hormone that is partly a protein and partly a lipid (see peptide bonds and fatty acids).

Ghrelin levels:

• rise before meals — especially at times you usually eat, as the body learns your schedule
• fall quickly after eating
• rise during dieting and after weight loss, and with sleep loss

Ghrelin activates the hunger neurons in the hypothalamus, making you want to eat. It’s the only well-known gut hormone that increases appetite; most others reduce it.

Short-term fullness signals from the gut

As food moves through the digestive tract, cells in the gut wall sense nutrients and release hormones that say “enough”:

• Cholecystokinin (CCK): released from the upper small intestine in response to fat and protein. It slows stomach emptying, stimulates bile and enzyme release, and signals fullness via the vagus nerve.
• GLP-1 (glucagon-like peptide-1): released from the lower small intestine. It boosts insulin release when glucose is high, slows stomach emptying and acts on the brain to reduce appetite.
• Peptide YY (PYY): released after meals in proportion to the calories eaten; reduces appetite.

Stretch sensors in the stomach wall also send signals to the brain as the stomach fills. That’s one reason high-fibre, watery foods (vegetables, soups) can be filling for relatively few calories.

Insulin: the meal-time messenger

Insulin, from the pancreas, rises after meals to help cells take up glucose (see insulin). It also crosses into the brain, where it acts as a fullness signal, reducing appetite. Its levels reflect both recent meals and, over time, the amount of body fat.

Leptin: the long-term fuel gauge

Leptin is a protein hormone of 146 amino acids made by fat cells. The more fat tissue a person has, the more leptin circulates in the blood. Leptin tells the hypothalamus how much energy is stored, rather than how much you’ve just eaten.

• High leptin → the brain reduces appetite and allows normal energy use.
• Low leptin → the brain senses “starvation”: appetite rises, energy use falls, and some reproductive and immune functions are turned down.

Leptin was discovered in 1994 by Jeffrey Friedman’s group, who found that a strain of extremely obese mice lacked a working leptin gene. Injecting leptin into these mice made them eat less and lose weight dramatically. A few people are born unable to make leptin; they have intense, constant hunger from infancy, and leptin injections transform their lives.

Why leptin isn’t a weight-loss cure

After the discovery, many hoped leptin injections would treat obesity. They didn’t work for most people. Most people with obesity already have high leptin — they make plenty — but their brains respond to it less. This is called leptin resistance, and it’s similar in principle to insulin resistance.

There’s also an asymmetry built into the system. Leptin is better at defending against weight loss than against weight gain. When you lose fat, leptin falls — often faster than fat mass — and the brain responds as if food were scarce: more hunger, lower energy expenditure. From an evolutionary point of view, this made sense: running short of energy was far more dangerous than having a little extra.

Why dieting is chemically hard

Put these signals together and the difficulty of keeping weight off makes chemical sense. After weight loss:

• Ghrelin rises (more hunger).
• Leptin falls (the brain senses reduced stores).
• PYY and other fullness hormones can fall.
• The body’s energy use drops a little more than expected from the lower body weight.

Studies have found some of these changes persist for a year or more after weight loss. This doesn’t make weight management impossible, but it explains why it isn’t simply a matter of willpower.

GLP-1 medicines

The newest weight-management drugs are based on GLP-1. Natural GLP-1 is broken down by an enzyme (DPP-4) within about two minutes. Chemists designed longer-lasting versions:

• Semaglutide has two amino acid changes, one of which protects it from DPP-4, plus a fatty-acid chain attached through a linker. The fatty chain makes it bind reversibly to albumin in the blood, which protects it from breakdown and slows its removal by the kidneys. Its half-life is about a week, so it can be injected once weekly.
• Tirzepatide acts on both GLP-1 receptors and receptors for a second gut hormone, GIP.

These medicines reduce appetite, slow stomach emptying and improve blood sugar control. They were first developed for type 2 diabetes. The chemistry of adding a fatty acid to extend a peptide’s life is a nice echo of ghrelin’s own fatty modification (see how drugs are designed).

Other influences on appetite

• Sleep: short sleep tends to raise ghrelin and lower leptin, increasing hunger (see the chemistry of sleep).
• Stress: cortisol can increase appetite, particularly for energy-dense foods (see cortisol and stress).
• Protein: tends to be more filling per calorie than fat or carbohydrate, partly through gut hormones.
• Reward: highly palatable foods activate the brain’s dopamine reward system, which can override fullness signals — the chemical reason there’s always “room for dessert”.

Common misconceptions

• “Hunger just means an empty stomach.” It’s controlled by hormones from many organs, plus habit and reward.
• “Leptin shots cure obesity.” Only in the rare people who lack leptin; most people with obesity have leptin resistance.
• “Weight regain is purely lack of willpower.” Hormonal changes after weight loss push appetite up.
• “Ghrelin is bad.” It’s a normal signal that also helps regulate growth hormone and blood sugar.

Key takeaways

• The hypothalamus balances hunger and fullness signals.
• Ghrelin (stomach, with an attached octanoyl fatty acid) increases hunger before meals.
• CCK, GLP-1, PYY and insulin signal fullness after eating.
• Leptin from fat cells reports long-term energy stores; leptin resistance is common in obesity.
• After weight loss, hormones shift to increase appetite; GLP-1 drugs use chemistry to extend a gut hormone’s action.