Dr Emeran Mayer, University of California, Los Angeles
It's well known that chocolate has the power to make you happy, but how does food affect your mood? New research shows that stomach-brain communications are complicated, and involve the billions of microbes that live in your stomach. Dr Emeran Mayer from the University of California spoke to Kat Arney about the microbes in our bodies...
Emeran - The microbes are really present all over our body. It just happens to be that the highest concentration of them is in our gastrointestinal tract with a gradient from the oesophagus, stomach, small intestine, to the colon where we have the highest concentrations. The number of these microbes is staggering. Itís up to 100 trillion of these microbes that live with us. Which is about 10 times more microbial cells than we have human cells and as I say, the majority of them lives in our large intestine.
Kat - What are they doing there? Are they just helping us to digest our food or are they doing other things too?
Emeran - Itís a difficult question. They certainly do help us digest our food and harvest metabolites that we would normally lose because we donít have the enzymes to breakdown certain aspects of food like all the fibre components of food. The bacteria, they thrive on those things that our human intestine cannot digest properly. So, they harvest a certain percentage of the food that we take in and change it into metabolites thatís in our colon can absorb and we can take advantage of these extra calories. So, thatís the simplest way to explain what they're doing. That's probably the reason why they have been living with the host for millions of years. You can even go back to grasshoppers, bees, and other more primitive organisms that have their own gut microbiome.
Kat - We sort of hear a lot about the gut microbiome nowadays. Itís very trendy. Itís very sexy almost - if your gut bacteria can be sexy. But thinking about how they may have more roles in our overall health and perhaps in our mood as well, what do we know about that?
Emeran - Their main function has to do with our metabolism and harvesting calories, people have speculated and this is clearly at a stage a speculation, that they have developed capabilities of hacking into our own communication systems that are very elaborate that link our gastrointestinal tract with the brain and other organs, with the liver, with the immune system. And that they have developed this ability to hack into those human programmes and modify them according to their own needs.
Kat - So, our gut bacteria could basically be controlling us.
Emeran - So, some people have said, you know, weíre basically just a vessel for the microbes to move around and optimise their own survival and thriving. So, one example is that they may have the ability to hijack our dopamine system, the reward system that drives us to do a lot of things, our motivation. But particularly, it plays a role in food intake. That may be one of the reasons that they play a role in the obesity epidemic. There are studies that the microbes not only contribute to obesisty because of the mechanism of rescuing 10% of what we eat but normally not be absorbed. But also, that they have the ability of changing the food preference and ingestive behaviour. One example is, in a so-called knockout mouse, the genetically engineered mouse that lacks a certain receptor in the gut. That mouse is obese and itís characterised by having an increased food intake. So, these mice are hyperphagic Ė they always want to eat. And if you take the faeces, the microbes in the large intestine and transplant them into normal mice, these normal mice will now eat a lot more foods. So somehow, we donít know the mechanism, but it points towards the possibility that they can really connect into some very profound systems in the brain.
Kat - I mean, as well as being a warning that you probably shouldnít eat pooh from fat people, is this basically, our gut microbes could be telling us what we like, what comforts us, what food that makes us feel good? Could this explain why for example, people with sort of chronic conditions like IBS, could that be affecting their mood through their gut bacteria?
Emeran - The experimental data in humans is very sparce, so we really have Ė weíre forced to speculate. However, there's very intriguing observations in the variety of animal models that would suggest that, for example, the weight loss after bariatric surgery for example that is pretty sudden...
Kat - Itís stomach stapling basically.
Emeran - Yeah, various forms of stomach stapling and modifications that initial substantial drop in food intake is already too had change in the food preferences. We know that gastric operations are associated with major changes in the gut microbiome. So again, an indirect but intriguing speculationÖ