If an alien came to Earth and randomly took a gene sample from one of us… they would most likely end up with a microbial, not human gene. Humans are outnumbered: we have 10 times more microbial cells and 100 times more microbial genes than our own. (February 2017 correction: apparently, that 10:1 ratio was based on one sample from the 70s… while actually, the ratio is more 1:1, so a human body has equivalent amounts of microbial and own cells 😉
Today I heard a fantastic talk by Dr. Alcock- a practicing physician and a PhD in evolutionary biology (great combination that more health professionals should/will be getting in the future!). The question he and many other researchers are now asking is: can microbioms manipulate us to behave in their favor? Perhaps we are not autonomous in our eating; perhaps some of our behaviors represent another genetic interest.
The talk included a lot if interesting outcomes from animal studies on the subject, but that is material for a longer post ( by somehow who is more qualified than I). I will simply summarize some interesting points:
- Microbiota and humans are not enemies- it is a marriage of convenience where the interests of one are also beneficial to the other. The bugs in our gut do need us to stay alive, as it is good for them also. However, microbiota might not always act in the best interest of the host as the interests of two might result in conflict. Even small differences in gene relatedness can result in conflict (pregnant mothers and fetus, for example, share 50% of genes). And there is zero relatedness in humans and bugs in our guts.A cool example of a conflict is mice infected by toxoplasmosis- such mice in fact seek out feline urine and are attracted to it (vs. having an innate aversion in non-infected mice); they thus look for cat urine, increasing the chances of being eaten by a cat and the parasite spreads further. Microbes can manipulate behavior in other ways- the cordyceps fungus causes its hosts (ants) to act completely out of “character” and leave their normal ground habitat to move onto the high leaves, helping the parasite disperse more spores [The Last of Us video game probably has you familiar with this fun fungus!]. Lastly, human organisms harbor known pathogens as well, yet they do not cause us harm (e.g. e-coli); it is interesting that these pathogens do not produce virulent factors when given simple carb solution (sugar!), a nutrient they require and would otherwise need to get aggressive for.
- We can surely manipulate our microbioms. The diet we eat has a huge effect on who’s in our guts- a bug profile of a vegetarian and meat eater are quite different (see photo above). There appears to be a witch-hunt for the bacteria that might be responsible for obesity in humans, but this search has been unproductive with the same suspects being either related or completely unrelated to weight gain.What seems to matter most is the diversity of the bacteria, not having a certain type of it. A Mediterranean style diet with high fiber, for example, produces a more diverse gut environment, while a fast food diet creates a limited one. Some other determinants of a diverse vs. limited environment are on the slide below. Key Point: Low diversity= Bad health outcomes. Some non-dietary ways to ensure a healthy diverse gut population are probiotics intake (and decreasing antibiotic use), vaginal birth, and breastfeeding; Babies not born vaginally in fact acquire a less complex microbiota that harbors more pathogens.
- Can microbioms manipulate…us?? That is the question. Can the gut bugs make us behave differently? Can they make us eat badly and become overweight? While we shouldn’t assume anything for humans yet, there are some examples of potential mechanisms through which bacteria could have an effect on the host.
- Bacteria has been shown to produce hormones (some are important in stimulating appetite) and neurotransmitters (e.g. serononin that makes us feel good). We need to ask though- do these things pass from the gut to the brain and are actually successful in influencing us? The human body does fight these hormones and neurotransmitters by producing enzymes and antibodies that degrade them and prevent them from crossing barriers (another mechanisms is encapsulating the gut in fat, which also protects the body from escaped bacteria “signals”). It is interesting to note that the microbes might have evolved the ability to produce these things before humans did (as environmental microbes do this too, not only ones that inhabit us), thus they are not primarily meant to affect us.
In conclusion, what we can take out of this knowledge is that we need to encourage people to have high diversity of bacteria in their gut! A whole food diverse diet is a great way to do so, for example. Should we blame our gut bacteria for making us eat junk? Probably not. We can’t pin the blame for obesity and other chronic conditions on a particular microbe, though an overgrowth of one specific group in the gut might be a conceivable issue with their signals interfering with the bodily signals (e.g. in terms of how to eat via hormones and neurotransmitters).
There is so much more fascinating material to cover on the subject but that is for a later date! Lastly, an interesting question was asked in the audience- obese individuals in fact have a low diversity of bacteria… would unhealthy weight gain then be a bad thing for the microbes? Wouldn’t it be a “win” for the human and loose for the bacteria? Interesting question to test 🙂
All material was obtained by a lecture “Allies or Enemies? Gut micribiota & the war on fat” by Dr. Joe Alcock, MD. 2014.