How Your Microbiome Shapes Your Appetite

It’s commonly believed that our dietary preferences are fairly static, in the sense that they don’t change much, irrespective of how we live our lives. This assumption is erroneous. A person whose favorite food is doughnuts won’t necessarily crave doughnuts his whole life. By altering his lifestyle and behavior, in particular his eating behavior, he can modify his physiology, as well as his microbiome, in such a way that his appetite changes.

Your dietary preferences are partly determined by what your gut microbes like to eat

Many years back I started to suspect that the microorganisms that inhabit our bodies exert control over our eating behavior and appetite. This suspicion emerged as a result of certain puzzling observations I’d made whilst I was studying human eating behavior. Over time, I grew increasingly more convinced that I was onto something, in part because research papers such as this one and this one appeared in the scientific literature.

For quite some time now, there’s been no doubt in my mind that microbes do indeed exert control over our eating behavior and appetite. Actually, I would go as far as to say that the microbiome is in some respects even more important than the brain in the context of human appetite regulation and control. I’ve now created a basic model that explains this phenomenon, so as to make the microbiome-host appetite connection easily understandable.

The key message I’m trying to get across with the model/infographic above is that our taste preferences and appetite are malleable. A person who’s “hooked on” certain unhealthy foods isn’t doomed to crave those foods his whole life. If he changes his diet, the reward and appetite centers in his brain will be affected in such a way that his taste preferences and eating behavior change. These effects will be partly mediated by his microbiota.

Conversely, a person who almost exclusively craves and eats healthy foods can end up developing an appetite for various inflammatory foods if he changes his diet to a less healthy one and/or otherwise stops nurturing the friendly bacteria that live in his bowels. This in turn can make him gain weight and get sick and depressed.

Besides informing us about how our appetite and eating behavior evolve over time, the model helps explain why work stress, disordered sleep, and other things that cause mild disturbances of the microbiota are often accompanied by cravings for foods rich in sugar, saturated fat, and/or salt. Obviously, microbiota disturbances are not the only factor at play here; however, it’s likely an important one.

Junk foods, in combination with other microbiota-disruptors, have altered our appetite and eating behavior

Today, a lot of humans are situated in the vicious cycle shown to the left in the infographic. This is an evolutionarily novel occurrence. Up until very recently in our evolutionary history, nobody on this planet had access to or ate highly processed junk food. Our primal ancestors ate exclusively nutrient-dense, whole foods. Moreover, they were very physically active and didn’t upset the microbial communities in their guts by taking antibiotics or other drugs. Hence, it goes without saying that they, unlike us, were “trapped” in the virtuous cycle shown to the right in the picture. That’s certainly not the worst situation to be trapped in.

The implications of these changes in the human condition are profound. The microbiome-host appetite model helps explain why hunter-gatherers are lean and fit, whereas many contemporary, industrialized humans are fat, sick, and metabolically deranged. By infusing massive quantities of sugary, salty, and fatty foods into our environment, we’ve unknowingly altered our microbiotas, taste preferences, and eating behavior in such a way that we’re now in a situation in which a substantial number of people find it very difficult to go even a single day without eating a bar chocolate, a doughnut, or some other type of highly processed food.

It’s difficult, if not impossible, to turn this around on a global scale, seeing as highly processed, rewarding foods are now a ubiquitous part of our environment. That said, on a somewhat smaller scale, it’s certainly possible to exert change. One of the most important things we can do is to get the information out there. If the average Joe knew how different foods affect his gut microbiota and brain, he would perhaps be more inclined to eat healthy.

Microbes exert control over the appetite and eating behavior of many multicellular organisms

It’s important to point out that microbes exert control over the eating behavior and appetite of many other multicellular organisms besides us humans. Chances are the appetite of all macroscopic organisms that harbor complex microbial communities are affected, to some extent, by microbial inputs. One of the main reasons why I think this is the case is that it makes complete sense from a Darwinian point of view.

A microbe that is capable of influencing the appetite and eating behavior of its host will have an obvious evolutionary advantage over a microbe that doesn’t possess this capability, seeing as it can influence its host in such a way that the host ingests compounds that somehow has a favorable impact on the microbe’s ability to grow and reproduce. Hence, it’s not surprising that recent research suggests that various types of microbes have evolved an ability to influence the eating behavior of larger, more complex organisms (1, 2, 3).