“Hello, is that you Brain? Coprococcus calling! Just checking to be sure you’re doing fine today.”
To which Brain might respond: “Thanks for checking in, and thanks for sending good thoughts my way.”
Aside from the wording, this scenario isn’t that far fetched. I think of it as a chat line, but the scientific term is the gut-brain axis . The term refers to the two-way communication between the digestive system and the brain, which involves different pathways and many different molecules. It’s not well understood right now, but it’s a very hot topic in research, particularly the impact of gut microbes.
According to a new study of gut microbes, some conversations along this gut-brain chat line might be linked to risk for depression. The study looked at gut microbe populations in over 1000 people. Subjects who had been diagnosed with depression, or showed signs of depression on a survey, were actually missing two types of microbes — Coprococcus and Dialister — that were present in non-depressed people.
This result was intriguing, but the researchers wanted to see if the finding held up elsewhere. They looked at data from a separate study of over 1000 people and had the same result: those two microbe groups were missing from subjects who had been diagnosed with depression.
So you might be thinking: This is great. People with depression should just take probiotic supplements with these two microbes and all will be well.
Not. So. Fast.
Association Does Not Equal Causation
It helps to keep that phrase in mind whenever analyzing study results. With this particular finding, there are several distinct possibilities:
- Coprococcus and Dialister have some direct effect on risk for depression, whether by releasing chemicals that directly impact brain function or that affect other metabolic pathways that impact brain function
- The state of depression exerts some influence on what gut microbes thrive in the intestines
- Coprococcus and Dialister discourage growth of other microbes that are the real culprits in depression
- Depressed people eat in a fundamentally different way from non-depressed people, which impacts gut microbe populations. Perhaps more sugar or fewer vegetables or more alcohol or caffeine?
- Depressed people experienced some life event prior to diagnosis that set off changes in gut microbe populations. Perhaps being exposed to different bacterial contaminants on food, or exposure to something in the water supply.
- Depressed people may have a genetic tendency to harbor certain gut microbes but not others.
- Even if you did take a probiotic with these allegedly beneficial probiotics, they might not survive in your gut if you don’t eat foods that encourage their growth. What are those foods? Are there foods that discourage them? We don’t know.
This list could go on, but hopefully you get the point. The existence of specific bacteria does not necessarily mean those bacteria prevent depression. Additional research can help answer that question. But the researchers are taking a “Where there’s smoke, there’s fire” approach, pointing out that Coprococcus has a metabolic system related to dopamine and another related to a known anti-inflammatory molecule. Perhaps with Coprococcus in the house, so to speak, the brain receives signals that brighten your mood.
There’s another wrinkle in this topic for older adults: as we age, gut microbe populations change, sometimes dramatically compared to younger adults. According to one study, older adults have on average more of one group of microbes (Bacteroidetes) and less of another (Firmicutes). Ratios of other microbe populations were also altered. Further complicating the picture, the study found wide variations in the proportions of these bacteria between people. The study authors described it as “dramatic interindividual variability,” not found in younger people. What does any of this mean for the gut-brain axis? And can you do anything about it?
The short answer is We don’t know yet. But that doesn’t mean you can’t take some common sense steps to help yourself. The sensible strategy would be a diet that’s loaded with foods that encourage healthy gut bacteria: vegetables, fruit, whole grains legumes, nuts. As well as natural probiotic foods like yogurt, kefir, kombucha, cheese, kimchi and fermented soy foods like tofu.
I do not recommend probiotic supplements. I recommend food. At the moment, since we don’t actually know how to define what a beneficial gut microbe population is, how can we define a beneficial probiotic supplement? It’s all speculation. Until we have more good information from studies, stick to a plant-based diet that includes probiotic-containing foods, such as those listed above. They have plenty of other benefits besides sending encouraging messages to your brain.