Appetite and the gut microbiome

Now, we know we’re not alone in our bodies. We’ve evolved with all sorts of micro-organisms living on us and in us. And the gut, of course, has an important role to play in the process of digestion. Katerina Johnson researches connections between the microbiome, the brain and our behaviour, at Oxford University, and Katie Haylor asked Katerina how the microbes in the gut could be influencing our appetite...

Katerina - We think of our gut largely as somewhere that we digest our food. And obviously that that's a key part of it, but there's so much more that goes on beyond that in the way that our gut can communicate with our immune system, our nervous system and our hormones. And so one aspect to do with this is that we think that our gut microbes may actually have some effect on our appetite control and hunger, through interacting with our body's own hormones and also nervous system, especially the vagus nerve.

Katie - Now, last time we spoke, this was actually for another Naked Neuroscience episode about gut feelings. We talked a little bit about perhaps if what you eat is relevant in the question of how eating and gut microbes are related. But from what you said last time, I got the sense that we don't really know that much about the specifics in terms of specific dietary choices. Is that fair enough?

Katerina - Yes. The one definite is the fibre is really beneficial for our gut and gut health. And that's true also when it comes to potentially affecting our feelings of hunger and satiety. When our gut microbes break down our food, which we call microbial fermentation, they release lots of different chemicals. And in particular, when microbes break down fiber, they produce short chain fatty acids. And we actually know that these short chain fatty acids can affect the levels of hormones that are involved in appetite regulation. So that might actually be one reason why we feel fuller for longer when we eat a lot of fibre. Because actually, when this fibre is broken down, it produces these short chain fatty acids that regulate our gut hormones such as peptide tyrosine tyrosine, glucagon like peptide. And we actually know for example, that the short chain fatty acid butyrate promotes the feelings of satiety. These short chain fatty acids actually have a really diverse range of effects on the body and our physiology. So not only do they affect our appetite control, but they can also affect our behaviour and cognition. And they're actually small enough that they can, in some cases, cross the blood brain barrier so they can get directly from our blood circulation into our brain.

Katie - Now, there are a couple of different categories of fibre that we can eat. The type that Katerina is referring to in terms of fermentation is soluble fibre. Think soft, moist fibre - fruit, pulses that blend with water to make a gel. Perhaps less well known than its famous counterpart, insoluble fibre - think roughage, the stringy nature of celery, sweetcorn, things that don't match up. And fibre isn't the only context in which gut microbes get involved with our food.

Katerina - We do think that they play a role in laying down fat deposits and helping us to metabolise food. And it's quite a controversial area, but we think that some types of bacteria are better at extracting nutrients from food than others, other types of bacteria. And this might actually be one reason why some people are prone to putting on weight and some people tend to be quite slim. It might actually be that the slim people have less efficient microbes. So they extract less nutrients from their food.

Katie - Katerina explained to me that in general, more diversity in the microbes in your gut tends to be linked to good health. And more diversity in the diet tends to be linked to diversity in the gut microbes. After all different microbes like to break down different types of food. So, I wondered, could a problem with your gut microbiome lead to your eating behaviour changing for the worse?

Katerina - We don't know this area in detail, but we know that the gut microbiome interacts with lots of hormones and neuropeptides. And so for example, our hormones that control appetite, particularly, ghrelin, which is associated with the feeling of hunger and leptin, which is associated with us feeling full. So if we disrupt our gut microbiome, it may well influence these hormones and might make us, for example, more hungry.

Although on the other hand, we have to be kind of careful that we don't put too much agency on the bacteria. So there's this kind of temptation to think that they might manipulate our eating behaviour, you know, perhaps for their own means to make us eat certain foods. But if we consider this scenario using evolutionary theory, this kind of manipulation is unlikely to be the case because we have such an immense diversity of microbial species and strains that live in our gut. So for example, imagine if there was a bacteria that went to a bit of an extra effort to produce a signaling chemical term manipulate our eating behaviour, this bacteria wouldn't actually last very long in the gut, because it would be out competed by all the other bacteria that weren't making this additional energetic investment. So for sure our gut microbiome influences our appetite control, but probably not in a way that is kind of purposeful or to manipulate the behaviour of the host.

Katie - I see. And I guess you've got to be careful that you're not equating potential correlation to potential causation right? In this relationship?

Katerina - Sure. Yeah, exactly.

Katerina - Katerina shared a couple of other interesting research highlights from the world of gut microbes and appetite.

Katerina - We tend to think that when we feel full, it's kind of our stomach or intestines that, you know, have stretched. But there's some research suggesting that [gut microbes] might also be involved in, you know, effecting how full we feel. So basically the researchers looked at proteins that are produced by the common bacteria in our gut, E-coli. And they found that about 20 minutes after feeding, the study was done in animals, mice and rats, but the E-coli basically started producing a different set of proteins. And it was interesting that it was 20 minutes after feeding because that tends to be the amount of time that it takes someone to feel full. So they wondered whether these proteins produced by the bacteria were contributing to this feeling of fullness. So basically they took these proteins and injected them into rats and mice, and they actually found that the rodents reduced their food intake, independent of whether they were hungry or whether they'd just eaten. The proteins that are produced by the bacteria actually stimulated the release of hormones that we know are implicated in regulating our satiety. Another finding from their research was that the animals' bloodstream had a chemical in that derived from the bacteria, which actually increased the firing of brain neurones that helped to diminish their appetite. Yeah, it's quite interesting because these proteins that are produced by E coli can actually be involved in the same molecular pathways that are used by our own body to signal when we're full up.