How do we know when we’re hungry or full? The truth is, we’re not very good at noticing those signals from our body. We often make choices whether or not to eat based on external cues, like the mere presence of a box of doughnuts in the break room.
But the signals are there, they do affect how we eat, and therefore how our body manages weight. Where do those signals come from? Understanding how ghrelin and leptin work is a great place to start.
The most basic thing to remember is that ghrelin is the body’s primary hunger hormone, while leptin, conversely, indicates fullness.
Ghrelin is produced by cells in our digestive system and travels to the hypothalamus in the brain. An empty stomach triggers ghrelin to be released. As the stomach fills, ghrelin levels are reduced until they disappear with a full stomach. Usually, ghrelin levels stay low for about three hours after a meal.
Leptin, unlike ghrelin, is produced in fat cells and acts to reduce hunger. It is similar to ghrelin in that its action occurs in the hypothalamus of the brain.
In individuals with obesity, however, prolonged releases of leptin lead to decreased sensitivity towards the hormone’s effects. In other words, even though leptin is being released, a body with excess fat tissue doesn’t recognize it. This leads to prolonged and increased hunger signals despite higher levels of leptin.
This explains why leptin injections and supplementation are not effective for most individuals trying to lose weight. Leptin pills wouldn’t be effective anyways, as the digestion process would break down the substance before its absorption; so ingesting leptin doesn’t actually add any leptin to the body. Leptin injections, theoretically, should be more effective because they aren’t digested; however, the leptin resistance built up by most individuals who would use them negates the effects.
Leptin being produced in fat cells does help to explain why weight loss is generally difficult to sustain. As we lose weight, we lose body fat – this means less leptin will be produced. With less leptin, we will likely be hungrier throughout the day.
An extreme example of this came from research done on participants from the show The Biggest Loser. As the participants lost dramatic amounts of weight, their leptin levels plummeted to almost nothing. With slowed metabolisms and ravenous appetites as a result of their drastic weight changes, weight regain occurred in almost all of the show’s participants after the show’s conclusion.
This isn’t to say that long-term weight loss isn’t achievable. Participants in the National Weight Control Registry prove that it is. It is important, however, to be aware of the factors that make long-term weight loss challenging, and understanding the roles of hormones like ghrelin and leptin are an important part of that equation.