Leptin

Leptin is a hormone that your fat cells produce and release into the blood – the more fat, the more leptin. In the brain, specifically in the hypothalamus, it acts as a fuel gauge: lots of leptin means “stores are full, you can eat less and burn more”. It was discovered in 1994 by the US geneticist Jeffrey Friedman and fundamentally changed our understanding of hunger and weight.

The tricky part when you're losing weight: as soon as you eat less, your leptin level drops significantly within a few days – more than the fat loss alone would explain. The brain reads this as an impending shortage and responds with more hunger, less satiety, less desire to move and slightly reduced energy expenditure. This protective mechanism got our ancestors through times of famine; today it makes maintaining a lower weight a task that takes patience.

In people with a lot of excess weight, the opposite is often the problem: leptin levels are high, but the brain barely responds to them any more – this is called leptin resistance, similar to insulin resistance. The body no longer “sees” its full stores and keeps hunger high. Chronic inflammation, a diet very high in fat and sugar, and lack of sleep appear to promote this resistance.

What helps in everyday life? A moderate rather than drastic deficit, so that leptin doesn't crash; enough sleep, because even a few short nights lower leptin and boost the hunger hormone ghrelin; meals with protein and fibre, which also work through other satiety signals; and planned phases without a deficit, during which your hormones can recover. Leptin as an injection or pill doesn't help healthy people – it only works in very rare conditions such as congenital leptin deficiency.