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Media from Belgium put out a far reaching article a couple of days ago claiming

Depression does not start in the brain, but in the intestines

Source: "Depressie ontstaat niet in het brein, maar in de darmen": zo groot is de invloed van uw darmen op uw leven

The article where the claim is presented is an interview with Michael Mosley in response to him writing a book called Clever Gut Diet.

A lot of pages on the internet seem to make similar claims. Did some research myself, so might self answer at some point, but I wasn't able to find the original research which kicked of this idea, so I feel somewhat uncomfortable quoting (higher quality) popular science in an answer.

Some excerpts from the interview to make it more accessible for non-Dutch speakers and help in researching the actual claim:

The brain in our head is one lump of brain cells. The brain in our intestines contains about as many brain cells as a cat's brain and lies like a thin layer along the entire length of the intestine. It consists of exactly the same brain cells as our other brain and produces the same neurotransmitters. It communicates with our brain via the nervus vagus, the nerve that connects the brain to the intestines and one of the most important nerves we have. It is also super fast, because our intestines and brains need to be able to communicate very quickly with each other. The bacteria in our intestines can hack that system and change the signals. That way they can give our main brain instructions.

[...]

A psychiatrist told me that people with depression often suffer from severe constipation. This has been known for a long time, but no one has ever come up with the idea to remedy the constipation and see if the depression then improved. And that turns out to be the case. This is not because people feel better because they are no longer constipated, but because the constipation has a direct effect on the brain. We have long thought that depression would originate in the brain, but there is increasing evidence that it happens in the intestines. You have small inflammations in your body all the time and the intestinal bacteria play an important role in this.

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David Mulder
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  • Since the brief page name-drops Michael Mosley, why not use a source in English? There are possibly many e.g. https://www.radiotimes.com/news/tv/2017-11-01/dr-michael-mosley-how-diet-could-conquer-depression/ The actual study seems to be about the effects of a Mediterranean diet on depression. – Fizz Jul 10 '18 at 14:54
  • @Fizz It's apparently a claim from this book: https://www.amazon.com/Clever-Guts-Diet-Revolutionise-Inside/dp/1780723040 but I have no idea how notable that book is... De Morgen however I know is a major national news outlet. – David Mulder Jul 10 '18 at 15:16
  • Give the other articles of Moseley, my bet is a mistranslation of some kind. – Fizz Jul 10 '18 at 15:20
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    What is the claim? That **any** depression is a result of not eating properly? Or that the organic **effects** of depression can be measured in the intestines earlier than in the brain? Something else entirely? I cannot tell from the question, and I should be able to... I'm interested, because I *know* that there are kinds of depression that have *nothing* to do with what you eat or not eat... :-\ – DevSolar Jul 10 '18 at 15:27
  • @DevSolar Will translate some more useful/juicy portions in a bit to give context, wanted to ensure to have just a single claim though. Anyway, the claim in a more nuanced version down in the article is "often" not "always" – David Mulder Jul 10 '18 at 15:37
  • @DevSolar Took me awhile to translate, but here you go. – David Mulder Jul 10 '18 at 15:49
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    Oh boy... that will not be an easy one to tackle. As with so many health questions, *especially* those about mental health... what means "originating"? Bacteria and intestines did work together a certain way all your life. Then you became depressive. Did you become depressive because your diet changed? Because the bacteria changed how they behaved? Or did one, the other, or both happen because of some yet other factors? -- Note that this is all completely unrelated to "would you benefit from a healthy diet", to which the answer is "of course" regardless of whether you are depressive or not. :) – DevSolar Jul 10 '18 at 15:58
  • @DevSolar I just take the claim as "is there any scientific consensus (or at least a serious scientific indication that it's likely) that bad gut flora causes depression?" as that's what this guy is claiming. – David Mulder Jul 10 '18 at 16:09
  • That's easily answered as "no", but then so is almost any question on the origin of depression, including the "chemical imbalance" and so forth, i.e. I think [no theory has consensus](http://www.psychiatrictimes.com/couch-crisis/nuances-narratives-and-chemical-imbalance-debate)... except something trivial like "it originates in the brain".perhaps, but that's usually not even put to the consensus test. But even that is problematic. If someone [drinks alcohol and gets depressed](https://www.rcpsych.ac.uk/healthadvice/problemsdisorders/alcoholdepression.aspx), where did his depression originate? – Fizz Jul 10 '18 at 18:48
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    Anyway, your actual question seems to be about constipation and depression. Perhaps you should change the title. There's definitely an association: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4806284/ – Fizz Jul 10 '18 at 19:05
  • Nah, depression originates from hanging out on the Internet. – Daniel R Hicks Jul 10 '18 at 22:18
  • @fizz: the stuff I added at the bottom is just to contextualize. The claim I am asking about doesn't mention constipation at all. – David Mulder Jul 11 '18 at 06:06
  • This is often called the "gut brain connection". I don't know how good the science is on it, but it's been a semi popular health topic for about 5 years now. –  Jul 11 '18 at 06:37
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    As with everything related with depression, I would like to add a **health warning** at this point. *Anything* that even *sounds* like "but it's just X, do Y and you will be fine" is *positively harmful* to someone actually afflicted with depression. That **is** what a psychologist will tell you. For comparison, and I am not exaggerating here, it's not unlike to telling a rape victim "it's just sex, lay back and try to enjoy". So... explore the "gut brain connection" any way you like, but be careful about how you're wording conclusions and what you take away from the discussion. – DevSolar Jul 11 '18 at 07:53
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    @devsolar: Totally agree, it's part of the reason I posted this here – David Mulder Jul 11 '18 at 07:54
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    I wouldn't be at all surprised if your intestinal flora and your brain chemistry affect each other (and depression is at least in part driven by brain chemistry), but I've correlation does not make causation. It is just as likely that changes in brain chemistry cause changes in your metabolism than the other way around, or maybe there's some feedback mechanism where they influence each other. – jwenting Jul 11 '18 at 12:27
  • ...or something that effects changes in *both* brain chemistry *and* metabolism. – DevSolar Jul 11 '18 at 14:19
  • https://www.mayoclinicproceedings.org/article/S0025-6196(20)30785-0/fulltext -- Retrospective studies suggest that antibiotics administered to infants younger than 2 years old with an increased risk of several disorders, from asthma to ADHD. This is apparently due to the effect of the drugs on the "microbiome". – Daniel R Hicks Nov 17 '20 at 22:22

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I can't find a scientific article with this claim or anything like it made explicit, but a quick search of the scientific literature turns up research that suggests this may be the case.

Melancholic microbes: a link between gut microbiota and depression? investigates possible links between bacteria in the guts and depression. The title ends in a question mark, because the paper shows an interesting correlation, but stops short of proving causality.

The interplay between the intestinal microbiota and the brain investigates the chemical and neural communication pathways between the gut bacteria and the brain. They "review how this extended communication system might influence a broad spectrum of diseases, including ... psychiatric disorders... [emphasis mine]" Once again, scientists are stopping short of making conclusive statements.

A Possible Link between Food and Mood: Dietary Impact on Gut Microbiota and Behavior in BALB/c Mice, shows that a diet can affect mood in mice. This suggests that the same is true for people. They also showed that the different diets had an impact on the mice's gut bacteria.

Correlation between the human fecal microbiota and depression found that people with depression are more likely to have certain types of bacteria in their guts.

Gut brain axis: diet microbiota interactions and implications for modulation of anxiety and depression reviews the literature about the interaction between the guts and anxiety and depression. Their final section contains the following passage:

We now know that microbiota influence behaviour, in particular, stress-related behaviours such as anxiety and depression. Attention to the importance of microbiota and behaviour is rapidly expanding. Clinically, the link between obesity and anxiety is suggested [43], however studies have not yet considered the role of microbiota in this overlap. Understanding the link between mood and metabolism is a necessary direction for research studies to pursue.

This further suggests that we know there is some kind of link, but scientists are being cautious about making firm statements about what that link is in peer reviewed papers.

Disclaimer I did not do a thorough literature search reading all of the latest literature; those can take hundreds of hours. It is possible that a paper I did not read has a definite answer to this question.

Summary: I found a decent amount of research suggesting that this might be the case. This same research does not rule out other possibilities, depression may cause changes in the microbiome instead of the other way around. The review article seemed hopeful that future science will find a more conclusive answer.

BobTheAverage
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  • The fact that that review was published in *Current Opinion in Biotechnology* should be a good hint this brain-gut-depression issue is nowhere near mainstream in psychiatry. – Fizz Jul 11 '18 at 20:12
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    @Fizz are you a biologist who is familiar with that particular journal? Would you like to back up that claim with some evidence? Decisions about where to publish can be more complicated than simply the appropriateness of the journal's title for the particular paper. – BobTheAverage Jul 12 '18 at 00:06
  • It's the first psychiatry paper I've come across being published in a bio*technology* journal. One has to wonder WTF does biotechnology have to do with any of the topics touched in that paper. There are plenty of low-ranked psychiatry journals (and some largely opinion-based, e.g. there is a Current Opinion in Psychiatry), but my guess is they couldn't even get it into one of those. – Fizz Jul 12 '18 at 20:38
  • @Fizz The 2 authors list 4 institutional affiliations, only one of which sounds like a traditional psychiatry department. Much of the literature they review is not psychiatric. – BobTheAverage Jul 12 '18 at 20:48
  • I see no reason to doubt that article simply because it is written in a biotech review journal - the microbiome connection is very much biotech; the sources cited are from a broad range of journals that are very well regarded. I'd instead argue that if one wants to learn about new findings in psychiatric disorders one should stay far away from *psychiatry* journals, as these are focused on *clinical practice* which is often decades behind. I still see the more psychiatry-focused folk at psych.SE throw around Freud as something useful. – Bryan Krause Jul 12 '18 at 21:34
  • I also think your "There are plenty of low-ranked psychiatry journals" comment is really off-base: I see no evidence that these authors are reviewing this topic from the perspective of psychiatry, they are reviewing it from the perspective of more basic immunology, endocrinology, and neuroscience. They aren't saying "the gut causes all depression," either, they are saying "we should consider whether the gut could be a target for therapy for depression." – Bryan Krause Jul 12 '18 at 21:37
  • @BryanKrause: ...which is pretty much exactly what I expected, really hoped for would be behind this all, and have zero problems with. (The gut being one more *target for therapy*.) – DevSolar Jul 13 '18 at 08:43