Hunger vs satiety

Where do feelings of fullness come from? Katie Haylor spoke to Giles Yeo...

Giles - Feeling hungry is different to feeling full. You might think "well they're related". But they're two different processes. So the feeling fullness, interestingly enough, tends to come from the gut hormones. So I think our gut releases - and now I'm going to get this wrong. And people are going to @ me - at last count around 22 to 23 different hormones that we have identified from our guts. Okay. Now of these call it 22, it's going to be plus or minus, 20 of them - so the vast majority of them - make you feel full. Okay. And those are the full signals. Now, the fat signals, because they're long term, they're there to tell you, "okay, well, this is what the bank account looks like". Rather than the change in your pocket. So those signals, the signals from fat tend to signal starvation. Because clearly you're not going to be starving if you're feeling hungry, but you have plenty of fat, because you're not starving. So I think the feeling fullness comes from your gut typically, whereas the starvation signals - all the neuroendocrinology, all of the hormonal milieu that turns on when you're actually starving, those are going to come from your fat or lack or lack thereof.

Katie - In terms of the communication that goes along on the vagus nerve between the brain and the gut, I know it's two way, but am I right in thinking there's quite a big sort of bias, a big percentage of nerve fibers in the vagus nerve go from the gut to the brain?

Giles - You're right about the bias. It tends to go northward. So the stretch signals and any number of things will go from the gut directly, hardwired directly to the hindbrain. So the vagus tends to be directly, so the back of the head, tends to come up via that route. That is one set of signals that come from the gut. The ones that make you feel full, however, those signals tend to be hormonal. So in other words, they are hormones that are secreted into the blood, circulate, and then - so they are not hardwired, they are fly-by wired - like homing devices hormones.

Katie - Are we talking about ghrelin?

Giles - Ghrelin is one of them. Now, interesting you should mention ghrelin. I told you about the 22 different gut hormones, most of them making you feel full. Ghrelin is one of the two that make you feel hungry. There are two, the other one is called incident like peptide five and it's not as powerful as ghrelin. But ghrelin is one of the key "hunger hormones".