Alright, the brain post. As promised. Please note, this is not a deviation from our central line of inquiry — we’re dot connecting, always. So hang tight! We’re going somewhere with all this…
One topic we haven’t covered here yet is the microbiome and the brain. But not for lack of interest. Quite the opposite — it’s too big, too important. I just haven’t been ready for it. Excitement overload, you might say. And the other reason is that the research in this area is more limited. But fear not, there is plenty enough to make for another episode of You Can’t Be Serious? Theater. I’ll start with a flurry of the more commonly cited studies involving experiments with isolated bacterial species, and then end with what I would consider to be the most compelling and authoritative study yet done on the gut-brain link — and what it all means for the case we’ve built so far.
Infection with C. jejuni & C. rodentium
Over the past decade, researchers have shown that introducing non-invasive strains of gram-negative bacteria into mice can alone stimulate anxiety, apart from any inflammatory or infectious effect of the pathogens. A series of three studies introduced the bacteria Campylobacter jejuni into mice to induce “sub-clinical” infections, which did not stimulate any immune response. The experiments showed that the pathogen induced an anxiety response in mice directly through neural pathways.
Similar studies were done with gram-negative bacteria Citrobacter rodentium. One study showed that mice infected with C. rodentium elicited anxious behavior without any signs of immune stimulation, showing the effect being transmitted directly from the gut to the brain. In another experiment, mice exposed to C. rodentium showed impaired learning and memory. Pre-treatment with probiotics prevented these effects. The study concluded: “The intestinal microbiota influences the ability to form memory.” The study also looked at germ-free mice (mice without a microbiota) and found that they lacked non-spatial and working memory altogether, regardless of infection, “indicating the requirement for a commensal gut microbiota in memory.”
C. jejuni and C. rodentium belong to the Proteobacteria phylum. Proteobacteria go down when you eat plant fiber and stop smoking.
Probiotics: Lactobacilli and Bifidobacteria
There is a decent amount of research on specific strains of probiotic bacteria — specifically the lactic-acid producing bacterial groups Lactobacillus and Bifidobacterium. The vast majority of bacteria found in fermented foods and probiotic supplements belong to these groups. They are also normal commensal residents of the human gut.
In one study, supplementation with the probiotic bacteria Lactobacillus rhamnosus in mice was found to significantly reduce anxiety and depression. The effect was not found in mice whose vagus nerve was removed, proving a direct gut-to-brain mechanism. Another study found L. rhamnosus effective at attenuating Obsessive-Compulsive behavior in mice. The probiotic was found to be as effective as Fluoxetine (better known as Prozac, which is often used to treat OCD).
In a study using maternal separation as a standard inducement of depresson in mice, the administration of Bifidobacterium infantis was found to reverse depression and normalize motivation.
The combination of Lactobacillus helveticum and Bifidobacterium longum has shown anti-anxiety and anti-depression effects in mice, rats and humans. That same combination was shown to reduce post-myocardial infarction depression in rats.
And finally, a human study — perhaps the definitive one in this class of studies. 36 women with no gastrointestinal or psychiatric symptoms were split into three groups: one fed a fermented milk product (ie, yogurt with lots of lactobacilli and bifidobacteria), one fed a non-fermented milk product (no probiotic bacteria), and one with no product at all. Brain scans showed that in the women consuming the fermented yogurt, wide-ranging alterations in brain connectivity and regional stimulation were observed. They showed decreased reactivity in an emotional reactivity task (angry and frightened faces) and increased connections in cognition-associated areas of the prefrontal cortex.
Lactobacilli belong to the Firmicutes phylum. Bifidobacteria belong to the Actinobacteria phylum. Firmicutes and Actinobacteria go up when you eat plant fiber and stop smoking.
Diet, Microbial Diversity, Cognition
An interesting study. As opposed to the previous studies we just looked at, which tested the effect of introducing isolated species of bacteria into the microbiome, this study looked at the existing microbiome of mice as a whole — a lot more like the studies this blog has covered so far. Specifically, the researchers wanted to see if a diet-induced change in mice’s microbiota could have an effect on learning & memory.
The researchers designed two diets with nearly identical nutrient ratios — calories, amino acids, fatty acids. One diet, however, included lean beef, which was known to have the ability to modulate mouse microbiota. Results: the study found that in lean beef fed mice, microbial diversity rose significantly. And so did learning & memory ability. A decrease in anxiety was also observed. It should be noted that while they went to great lengths to equalize the diets, they were not perfectly identical (specifically, the lean beef diet contained more taurine and double the fat). As such, microbial causation was not proven, but a correlation between microbial diversity and better learning & memory ability was firmly established.
Microbial diversity goes up when you eat plant fiber and stop smoking.
The Big One
Finally: the mouse study to rule them all. One reason I like this study is that, as in the previous study, it looks at the mouse microbiome as a whole. But the other is the methodology — germ-free controls and microbiota transplants galore. Solid isolation of microbial causation. As such, I’ve found it to be the most oft-cited study in gut-brain research.
The study had a familiar goal: to see if changes in the mouse microbiome had a direct, causal effect on mouse mood and behavior. To cause a shift in microbiota, researchers applied antimicrobials to one group of mice, but not to the control group. Also, just to be sure the antimicrobials weren’t having some other non-microbial effect, they also gave antimicrobials to germ-free mice — mice without a microbiota. Mice given the antimicrobials, but not the control or germ-free group, experienced a marked shift towards more exploratory behavior and less anxiety. The mice — which were a strain bred to be overly timid and shy — all of a sudden became “bold and adventurous,” in the words of one of the study’s authors. But that’s not all. Researchers then implanted the microbiota of the mice that were given antimicrobials into the germ-free mice. The germ-free mice, after the transplant, experienced the same shift toward more exploratory behavior and less anxiety. Thus, it was shown that the shift in microbiota was the cause for the change in mood and behavior.
Oh, and what exactly about this microbial shift caused this change? Let’s let the study speak for itself:
A 7-day course of ATM resulted in a significant increase in Firmicutes and Actinobacteria, and a decrease in γ-proteobacteria and Bacteroidetes. We believe that these changes in bacterial composition of the colon were responsible for the documented changes in brain BDNF levels and in behavior.
Ahem. Anxiety gone when Bacteroides and Proteobacteria go down, and Firmicutes and Actinobacteria go up. The same exact thing that happens when you eat plant fiber and stop smoking.
Once again, it’s our “golden ratio” of microbiota. But this time implicated in a completely different area of health — the brain. We now see that this exact same microbial shift can impact brain functioning. As I noted in my last post, the ability to show similar mechanisms in different factors of disease — or different areas of health, in this case — is a very good way to build a broader case. That we now see this microbial shift affecting both the brain and the heart further solidifies a link between it and general human health.
Neat, huh? Thanks for reading. See you next time…
Oh, wait. One more thing…
The fact that we’ve established a firm case for a causal link between microbiota and heart disease, as well as the possibility for microbiota to influence brain functioning, puts us in a position to do something interesting: it allows us to to test that second brain/microbiota link for specific cases. We’re now in the position to use these two findings to put other specific brain-related factors/attributes under suspicion of having a microbial link. How? By using the microbial-heart link as our anchor.
Assuming an established microbial origin for heart disease, and assuming an established ability for microbiota to influence the brain, it is possible that a brain-related factor with a strong correlation to heart disease is, itself, strongly and independently influenced by microbiota.
Here’s what I mean.
In the case of plants and smoking, we’ve essentially created two separate 3-pointed triangles of causality.
These two separate triangles have the same direction of causality. We can be confident of this direction because both factors have been shown to directly cause this microbial shift. And these two factors are separately correlated to heart disease. The fact that they are so different, yet both cause the same shift in microbiota, reinforces the microbiota -> heart disease causation theory.
Since we’ve now established the possibility that this microbial shift can also affect the brain, we can do something interesting: we can test for other brain/microbiota links by replacing plant fiber and smoking with a brain-related factor — but only one that has been shown to have a correlation with heart disease:
You’ll notice that there’s one thing different about this triangle: the direction of causality. That’s because, in this case, our data shows that microbes cause the brain change, rather than the other way around. In the case of plants and smoking, we have proof of the opposite: plants and smoking cause the microbial shift. But it also makes intuitive sense — it’s hard to see how microbiota could force you to smoke or eat plants. But in the case of the brain, it’s a little less clear cut. Yes, our studies above show a microbial -> brain change direction, so we know that’s possible. But we also know that things like stress may affect our microbiome — the gut-brain is a two-way street. But since we’re specifically interested right now in a microbial -> brain change effect, we’re going to structure our theoretical triangle this way and let our reasoning ability decide if the brain factor we find makes sense or not. If it does, it’s correlation with heart disease will mean it is a side effect of the microbial mechanism, rather than a root factor like plant fiber and smoking.
Which brings us to the final, crucial task: finding a good brain-related factor correlated to heart disease.
To really make this worth our while, we’d have to find some kind of brain-related factor that has a really high correlation with heart disease. Something strong enough that would lower the probability of it being just a side effect of, or a precondition for, some other stronger correlation. And as I said, it would also have to make intuitive sense in terms of direction of causation . But most of all, it has to be a pretty strong link — something that comes at least close to smoking’s place as the NUMBER ONE predictor of heart disease. In other words, we’d have to get pretty lucky. And I just don’t see that happening any time soo…