What is hunger?

What exactly is hunger? What does that rumbling feeling mean? And what's going on in the brain? Katie Haylor quizzed Cambridge University geneticist Giles Yeo...

Giles - So hunger is the drive to eat, right? Appetite drive to eat is what we're evolved to get so that we make sure we take on nutrients when we need nutrients. Okay. So let's go with that as hunger. But given that we only experience hunger within ourselves, then what is hunger? That feeling? And I think that's the real difficulty of it, you've almost asked a philosophical question! But in terms of signals, it's not just one signal. Your brain needs to know two pieces of information in order to modulate your food intake. The first piece of information your brain needs to know is, how much fat you are carrying on board? Why is this important? This is important because how much fat you're carrying on board is how long you will last in the wild without any food. Okay. Not a problem today. We have too much food. But a problem in the past where we never had enough food.

But then the second piece of information that your brain needs to know is what you are currently eating and what you have just recently eaten. So these are now your short term signals. And these short term signals are going to come from your gastrointestinal tract, your food to poop tube. And these signals are released because the moment we start eating and munching and swallowing, it goes through our stomach, our intestines, and out the other side, the entire tube gives off hormones, letting your brain know, not only how much you're eating, but what exactly are you eating? What the protein fats and carbs content is going to be. So your brain then senses these long and short term signals and influences your interaction with food. And so when you ask what is hunger, clearly hunger is when these signals - the short and long term signals - signal to your brain to say, uh, "I think you're going to need to eat some food now". That is hunger.

Katie - And what is the sensation that perhaps a lot of people will associate as being hungry? You know, when you've got that rumbling in your stomach and it's slightly uncomfortable, what actually is that?

Giles - The rumbling in your stomach and your intestines actually comes from the washing machine-like nature that your stomach does. Moving juices about. This process is called peristalsis. So in your stomach and moves things around like a washing machine around and around and around. In your small intestine, it moves stuff down in one direction towards the, towards the poop end. Now, what is interesting is that these actions actually happen all the time. However, the reason you hear it suddenly when you're hungry is because your stomach and your intestine is then empty. So the food that is normally there muffles the sound. And so if you're full now, because you've just had lunch, you won't hear it,  because everything is actually muffling it out. The reason you hear it is because you have an empty stomach. And so therefore it is often associated with being hungry because A) your stomach and your intestines are empty. And B) so therefore you then hear the sound. And we've now been conditioned to think, "Ooh, my stomach rumbling. I am hungry". And so it's a link between the two.

Katie - You mentioned there's the long term context and the short term context to being hungry, but how much of being hungry is motivated by, say, your body responding to depleted reserves, or responding to some sensory information, like, well, like smelling something delicious or walking past a cake shop or something like that?

Giles - So all of it's going to be integrated. So it's very difficult to say what percentage it is. It's a whole, all your sensations, all your senses, that are there will actually feed into this input. And secondly, it's going to differ from person to person. We know people for example, who love their food. And so are probably going to be far more sensitive, far more sensitive to all of the tactile, the smell, the vision, you know, and all of the accoutrements that, that surround food. Okay. That's me. Just FYI! But I've got friends who consider food as fuel. And my colleagues sits in my office next to me and he eats the same damn cheese sandwich every single day and has for the past 10 years that I've known him. But he clearly gets hungry when he's hungry, but he doesn't necessarily think about food all the time, like I do. He doesn't know what's going to happen for dinner. He thinks about dinner when it's time for dinner when he's hungry. And so I think each of us have different thresholds for all the sensations molecular, hormonal or just for your eyes and the smells. And it's this interaction which therefore guides and influences our interaction with food, a menu, the restaurant, the refrigerator.