Ghrelin

Ghrelin is produced mainly in the stomach lining and is the counterpart to leptin: it rises when your stomach is empty, tells your brain “time to eat” and drops again after a meal. It was discovered in Japan in 1999; its name comes from the Indo-European root “ghre”, meaning “to grow”, because it also stimulates the release of growth hormone.

Ghrelin follows a learned rhythm. If you eat at 12 noon every day, your ghrelin level rises shortly beforehand – your stomach rumbles out of habit, not out of need. That's good news: regular meals train your hunger to show up at reliable times, and a few days after a change, the hormone adapts.

After weight loss, however, ghrelin stays elevated: in a well-known study (Cummings 2002), levels after a diet were around a quarter higher than at the start. Your body is, so to speak, campaigning for a return to your old weight. Knowing this helps you understand why the time after losing weight needs at least as much attention as the weight loss itself – and why the hunger you feel then isn't a failure of willpower.

What keeps ghrelin in check in everyday life: protein-rich meals lower it more, and for longer, than meals high in carbohydrates or fat. Fibre and volume (vegetables, soups) stretch the stomach and dampen the signal. Sleep is crucial: in one study, after two nights of only four hours' sleep, ghrelin was 28% higher and leptin 18% lower – lack of sleep makes you measurably hungrier, especially for sweet and fatty foods. A glass of water and a bit of distraction can also help you ride out a ghrelin peak until it subsides on its own.

As for children: their ghrelin works beautifully – regular family meals give hunger a rhythm, while constant snacking throws it off.