Most replayed moment: The hormone that controls your weight | Dr Andrew Jenkinson

**Jonathan Wolf** (0:01)
Hello, and welcome to ZOE Recap, where each week we find the best bits from one of our podcast episodes to help you improve your health.
Floating through all of our bodies is a complex network of hormones. You can think of it as an internal postal service, delivering vital messages and instructions to different organs and systems. But what happens if these messages get intercepted, mixed up, or worse, not delivered at all? In this episode, Andrew Jenkinson tells us about a hormone that plays a crucial role in weight regulation and the undesirable consequences when it isn't functioning properly. He also shares three simple tips you can do tomorrow to make sure your hormones support your health.

**Andrew Jenkinson** (0:46)
So there's one hormone, the master controller, leptin. Skimmed over in medical school, not too many doctors really understand it too much, but it's like absolutely crucial in human weight regulation. And mammalian weight regulation. If we're in an environment where there's no food, we're sort of okay for 60 days. That's a long time to try and find some food. And then we start eating into our muscle and whatever and start to become very, very sick. So 60 days is what a human is hardwired to carry.
We're not hardwired to carry 120 days. We don't need it. It's going to drag you down. You're not going to be as fast. You're going to be, from an evolutionary point of view, at risk of predation.
So those people who were big, they didn't pass their genes on. So how is that regulated? Because it is. Because it's really, really important from a survival perspective, this homeostasis, this regulation of that 60 days. Basically, it all evolves around leptin. Leptin is a hormone that is produced by our fat cells. The more fat cells that we have, the higher the leptin level. So leptin comes from the fat, goes into the blood. That really important area of our brain, the hypothalamus, which is the master controller of our weight, will sense the level of leptin in our blood. And it will think, okay, the leptin level is quite high.
I think we've got maybe 70 days or 80 days storage. The tank is really, really full. We can ratchet down the appetite. We can be not that interested in food. We can increase the metabolism, that dimmer switch.
Seamlessly, without any thoughts, your weight will go back down towards the weight set point, basically. And the same thing happens the other way. If we lose weight, if we go on a calorie restriction diet, or if we get sick and we don't eat much and we lose weight, or if we're in a famine environment, that level of leptin will decrease because our fat has decreased, and there'll suddenly be an alarm. So that the hypothalamus will dip into the blood. It will see the leptin level has gone down. Think, oh God, okay, we're in a famine situation. Ratchet down the metabolism so we don't burn off too much. Go into battery-saving mode, but actually be looking for food. And this, from an evolutionary perspective, is the thing that kept us within that target range of around about 60 days of energy storage.
Now, if that's working, why does anyone become fat or store far too much energy? Sometimes a year's worth of energy. What is the point of that? That is a disease. Something's gone wrong, okay? So something's gone wrong with leptin. It's called leptin resistance. So if you measure someone who's really suffering with obesity, and you measure their leptin level, it is sky high. It's like triple what ours would be. So why is that not working? Why is the hypothalamus not able to see it? And it's because it's being blocked by this other hormone that we all know, insulin. Insulin and leptin share a receptor pathway in the hypothalamus. So the more insulin we have, the more dilutes the leptin signal.
So you can be walking around with very, very high leptin levels. But actually, because your Western diet has foods that stimulate high insulin, so sugar, highly refined carbohydrates and a snacking culture, which we have, your insulin levels are, on average, raised. That dilutes the leptin signal and the brain can't see the leptin anymore. Despite you carrying far too much weight, it can't see it. You know, it's blocked. It's leptin resistant. And then you get the opposite signals or responses from the hypothalamus that you should get. Because it can't see leptin, it thinks that you're fading away rather than you're carrying far too much weight. So you will get totally inappropriate signals of hunger and low metabolism. And this is the crucial thing, Jonathan. So, you know, when you understand leptin resistance, these factors that society and doctors blame on obesity as poor character traits, so being a little bit too hungry, a little bit too greedy and a little bit too lazy, that is not the cause of the condition. That is the symptom of the condition. So when you got leptin resistance, you become hungry and you become quite tired because your metabolic rate has been turned down.