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  • Alex Cloherty

Junk food for bacteria

Last week on Microbial Mondays, we took a look at how the microbes living in your gut are one of the reasons why eating lots of plants, which are high in dietary fibres, is a healthy habit to get into. Today, we’ll take a look at the flip side. What about foods we think of as less healthy, or downright unhealthy? Do the microbes living in our guts play a role in how those foods affect our health, too?


In short, yes, they do, whether that be highly processed foods, foods with lots of additives to make them last longer or taste better, or even red meat.


In the case of processed foods, a lot of the story seems to come down largely to how easy those foods are for not only us, but also our microbes, to digest. When we eat plants, our own digestive systems, as well as the microbes living within us, have to work pretty hard to break down these fibre-rich foods. This is partly because plants are made up of a unique type of cell that has a tough outer layer. While our cells are sort of squishy (like us!), plant cells have rigid walls that keep them stiffly standing up, and also make them more difficult to break down and digest. You can think of it like biting down on a chocolate truffle, versus an M&M. The truffle is soft and squishy all the way through, like animal cells or highly processed foods, while the M&Ms have a hard coating that requires some extra energy to crunch down on and break through, like plant cells.


All of this means that the nutrients in plant based foods are contained within tough cell walls, and are therefore harder to get at and digest. In addition, plant-based foods usually have a lower energy density - in other words, a handful of chopped celery has way fewer calories than a handful of french fries. Taken together, this means there is not only a lot less work involved in digesting processed foods, but also a higher energy payoff after their digestion.


You might think that that is a good thing, but in a perhaps surprising twist of fate, this extra energy availability is not so great for our guts after all. It turns out that the easily digestible nutrients in highly processed foods can allow bacteria to overgrow in the small intestine - including the bacteria that we really don’t want to overgrow. This bacterial overgrowth can lead to an unbalanced community of microbes in the gut, which can have a bunch of negative downstream effects. For instance, you might remember from last week’s article that the microbes living in our intestines can impact our immune system and hormones. When the community of microbes living in our guts is nice and balanced, our gut microbes can have a beneficial, moderating effect on these processes, but an imbalanced microbial make-up can in turn mess with human immune functioning and hormone regulation.


Something similar can happen with food additives. For instance, eating excess amounts of salt can also mess with the microbiome. Research has shown that eating a high-salt diet can leave you with less lactobacilli, which are some of those really good bugs in yoghurt and sauerkraut that you might have heard about. In mouse guts, lower lactobacilli numbers have been linked to worse immune functioning, higher blood pressure, inflammation, and colitis development. Mice are often used as a proxy for humans in microbiome studies - partially because it’s a lot easier to force mice to stick to a strict diet than humans! We don’t have the full story in humans yet, but seeing these effects in mice is a good hint that low lactobacilli numbers could also mess with the human immune system and other bodily functions.


And then finally, there is the question of red and processed meat. You probably remember that it was pretty big news when the World Health Organization classified processed meat as a class one carcinogen - and there’s still a lot of debate about this. As the authors of the review article on which this blog is based put it, “The gut microbiome provides an additional perspective in this controversy”. In the last decade or so, a series of research articles have shown that when gut microbes digest the protein in red and processed meats, they poop out a plethora of different by-products. And this type of bacterial poop is some literal bad shit.


Unlike the bacterial poop that results from bacteria digesting plant foods, this post-meat bacterial poop includes molecules like ammonia and hydrogen sulfide, which can mess with the functioning of our intestines, activate so-called “oncogenes” (i.e., genes that can trigger the development of cancer given the right circumstances), and deactivate tumour-suppressor genes (the opposite of oncogenes: these genes protect against the development of cancer). Convincingly, one study in mice showed that when mice were treated with antibiotics, animal protein no longer had such a strong impact on the functioning of these cancer genes, showing that microbes really do have a key role in this pathway. Hold up there, though - this doesn’t mean that you should start taking antibiotics before you eat red or processed meat. You need your gut bacteria to carry out lots of good, healthy actions for you, as we covered last week, so it’s not a good idea to cull your bugs with antibiotics. Rather, this is just a bit more evidence for why it’s not considered healthy to over-eat meat.


But what about plant-based proteins, you might ask? How do our microbes digest plant-based proteins? And could plant-based bacterial protein poop also have any harmful effects on us?


Well, we don’t actually know the answer to that yet. It might be that, because plant-based foods are harder to digest thanks to the plants’ cell walls, any negative effects of microbes digesting our plant-based proteins for us could be limited. The actual molecules that make up plant based proteins are also different, and in different ratios, which could also mean that the downstream effects of digesting them differ from the effects of digesting animal-derived proteins. But for now, we don’t have enough data to say exactly how our gut microbes interact with plant-based proteins. I hope that with the building popularity of plant-based diets, this will develop into an exciting new niche for future scientists to explore!


For now, we do know one thing for sure, though. For better or for worse, the food that we give ourselves, and in turn our gut bacteria, has a big impact on our health! Hippocrates had it figured out in that regard.

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