In a fascinating example of the kind of thing that co-evolution can produce, scientist at Pittsburg University believe they have shown how the gut biota of mice could be controlling the mouse's cravings for particular foods.
If this holds true for humans, it could mean that our gut biota could be telling us what they want us to eat, based not on our nutritional needs, but on theirs. We have, of course, been evolving along with our gut organisms since the first multicellular organisms evolved the first alimentary cannal, and possibly even earlier. More recently, it has proved possible to track the dispersion out of Africa and across the globe by subtle differences in the strains of organisms in our gut biota.
In other words, evolution has had plenty of time to evolve suptle relationships between us and our internal biota and, biologically speaking, we have not been evolving as a single species but as a collony, so any species in the colony could be influencing the evolution of the colony as a whole.
That evolutionary relationship was behind the research by the Pittsburgh Scientists, Assistant professor, Kevin Kohl and Doctor Brian Trevelline, as the Pittsburgh University new release explains:
Eggs or yogurt, veggies or potato chips? We make decisions about what to eat every day, but those choices may not be fully our own. New University of Pittsburgh research on mice shows for the first time that the microbes in animals’ guts influence what they choose to eat, making substances that prompt cravings for different kinds of foods.The authors give more detail in the open access abstract to their paper published in PNAS. Sadly, the rest of the paper is behind a paywall.
[…]We all have those urges — like if you ever you just feel like you need to eat a salad or you really need to eat meat. Our work shows that animals with different compositions of gut microbes choose different kinds of diets.
Assistant professor Kevin Kohl, lead author
Department of Biology
Kenneth P. Dietrich School of Arts and Sciences
Pittsburgh University, Pittsburgh, PA, USA
Despite decades of speculation by scientists about whether microbes could influence our preferred diets, the idea has never been directly tested in animals bigger than a fruit fly. To explore the question, Kohl and his postdoc Brian Trevelline (A&S ’08), now at Cornell University, gave 30 mice that lacked gut microbes a cocktail of microorganisms from three species of wild rodents with very different natural diets.
The duo found that mice in each group chose food rich in different nutrients, showing that their microbiome changed their preferred diet. The researchers published their work today in the journal Proceedings of the National Academy of Sciences.
While the idea of the microbiome affecting your behavior may sound far-fetched, it’s no surprise for scientists. Your gut and your brain are in constant conversation, with certain kinds of molecules acting as go-betweens. These byproducts of digestion signal that you’ve eaten enough food or maybe that you need certain kinds of nutrients. But microbes in the gut can produce some of those same molecules, potentially hijacking that line of communication and changing the meaning of the message to benefit themselves.Tryptophan is an essential amino acid that’s common in turkey but is also produced by gut microbes. When it makes its way to the brain, it’s transformed into serotonin, which is a signal that’s important for feeling satiated after a meal. Eventually that gets converted into melatonin, and then you feel sleepy.
There are likely dozens of signals that are influencing feeding behavior on a day-to-day basis. Tryptophan produced by microbes could just be one aspect of that,
Dr Brian Trevelline, co-author
Formerly, Department of Biological Science
Pittsburgh University, Pittsburgh, PA, USA
Now, The Department of Ecology & Evolutionary Biology
Cornell University, Ithaca, NY, USA
One such messenger will be familiar to anyone who’s had to take a nap after a turkey dinner: tryptophan.
In their study, Trevelline and Kohl also showed that mice with different microbiomes had different levels of tryptophan in their blood, even before they were given the option to choose different diets — and those with more of the molecule in their blood also had more bacteria that can produce it in their gut.
It’s a convincing smoking gun, but tryptophan is just one thread of a complicated web of chemical communication, according to Trevelline.It could be that what you’ve eaten the day before is more important than just the microbes you have. Humans have way more going on that we ignore in our experiment. But it’s an interesting idea to think about.
Assistant professor Kevin Kohl
It does, however, establish a plausible way that microscopic organisms could alter what we want to eat — it’s one of just a few rigorous experiments to show such a link between the gut and the brain despite years of theorizing by scientists.
There’s still more science to do before you should start distrusting your food cravings, though. Along with not having a way to test the idea in humans, the team didn’t measure the importance of microbes in determining diet compared to anything else.
SignificanceJust another example of co-evolution, but from an intelligent [sic] design perspective, yet another example of the needless complexity that belies any idea of intelligence being involved in the design of these systems. And why would any intelligent, benevolent designer of its special creation - humans - give control of their nutrition to the microorganisms that live in their gut.
The behavior of diet selection or diet choice can have wide-reaching implications, scaling from individual animals to ecological and evolutionary processes. Previous work in this area has largely ignored the potential for intestinal microbiota to modulate host foraging decisions. The notion that the gut microbiome may influence host foraging behavior has been highly speculated for years but has not yet been explicitly tested. Here, we show that germ-free mice colonized by differential microbiomes from wild rodents with varying natural feeding strategies exhibited significant differences in their voluntary dietary selection. Specifically, colonized mice differed in voluntary carbohydrate selection, and divergent feeding preferences were associated with differences in circulating essential amino acids, bacterial tryptophan metabolism, and intestinal morphology. Together, these results demonstrate a role for the microbiome in host nutritional physiology and foraging behavior.
Abstract
Diet selection is a fundamental aspect of animal behavior with numerous ecological and evolutionary implications. While the underlying mechanisms are complex, the availability of essential dietary nutrients can strongly influence diet selection behavior. The gut microbiome has been shown to metabolize many of these same nutrients, leading to the untested hypothesis that intestinal microbiota may influence diet selection. Here, we show that germ-free mice colonized by gut microbiota from three rodent species with distinct foraging strategies differentially selected diets that varied in macronutrient composition. Specifically, we found that herbivore-conventionalized mice voluntarily selected a higher protein:carbohydrate (P:C) ratio diet, while omnivore- and carnivore-conventionalized mice selected a lower P:C ratio diet. In support of the long-standing hypothesis that tryptophan—the essential amino acid precursor of serotonin—serves as a peripheral signal regulating diet selection, bacterial genes involved in tryptophan metabolism and plasma tryptophan availability prior to the selection trial were significantly correlated with subsequent voluntary carbohydrate intake. Finally, herbivore-conventionalized mice exhibited larger intestinal compartments associated with microbial fermentation, broadly reflecting the intestinal morphology of their donor species. Together, these results demonstrate that gut microbiome can influence host diet selection behavior, perhaps by mediating the availability of essential amino acids, thereby revealing a mechanism by which the gut microbiota can influence host foraging behavior.
Trevelline, Brian K.; Kohl, Kevin D. (2022)
The gut microbiome influences host diet selection behavior
Proceedings of the National Academy of Sciences 119(17); e2117537119 DOI: 10.1073/pnas.2117537119
Copyright: © 2022 The authors. Published by The National Academy of Science of the United States of America
Open access
Reprinted under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International license (CC BY-NC-ND 4.0)
Time and again, research scientists looking to explain how the world works, come up with these examples that refute any notion of design, intelligent, stupid, benevolent or malevolent. Examples that can only realistically be seen as the product of a mindless, utilitarian, natural process - evolution by natural selection, in which all components of a system can influence the evolution of that system.
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