Do Gut Bacteria Really Cause Depression? Here’s what the science says

In 1898, the American professor of psychiatry, Daniel Brower, put forward a strange idea, which he called “self-poisoning.” In the paper he sent Journal of the American Medical Associationhe suggested that an excess of bacteria in the gut could cause “melancholy”—what we now know as depression—due to the production of bacterial waste. The body literally poisoned itself, producing chemicals in its own intestines that affected organs far beyond the intestines.

This was the start of what has become an ongoing craze for medical professionals: the idea that the microorganisms in our digestive system can influence our minds and our mental health in particular.

And in the last ten years, scientists have been in an active phase. Just take a look at the current medical literature and you’ll find claims about the potential impact of the “microbiome” – the set of microbes we all carry inside us – on our brains. Depression, anxiety, schizophrenia, bipolar disorder, autism, whatever, a scientist has written a paper on how this is affected by the “gut-brain axis”, a putative series of connections between what is related to what happens to our microbiome and what , what’s happening. to our thoughts.

However, many of these articles are just the opinions of scientists. One of the strange things you notice when you read about the gut-brain axis is that there are a lot of opinions and “opinion” articles on the subject – far more than the average on a scientific topic. It has even been suggested that there are more opinions than empirical studies providing new data on the subject. This is a warning sign: when there are more theories than data to test them, scientific research tends to be on shaky ground.

But the data we have is still intriguing. While this seems like a pretty specific research question, “does the gut affect the brain?” “There are so many different ways to study it. You can do research on humans or animals. You can do an “observational study” by taking stool samples from people with or without certain medical conditions. Or you can take some kind of intervention – maybe give your subjects probiotics (live bacteria in pills or drinks) or antibiotics to change their microbiome and watch for any impact on their mental health. There’s also stool transplantation, which is what it’s like, and we’ll get to that later.

One thing we could hope to learn from observational studies is what the microbiome of a depressed patient or a person with schizophrenia looks like, or even a rat with an anxiety problem. Is it different from the microbiome of a person who does not have the condition in question? Of course, you can often see studies reporting these types of microbiome differences. The problem is that there seems to be very little subsequence in these differences: we know little about which specific bacteria characterize which mental illness. One study focuses on one type of bacteria, another on a completely different species. As the 2021 review puts it, “there were more differences in the results … than similarities.”

However, there are a few similarities. For example, attempts to aggregate all available data result in lower amounts for patients with depression and anxiety. Faecalibacterium prausnitzii in their stomachs. It is a common bacterium that produces chemicals that reduce inflammation in the body. Indeed, experiments on mice that gave them more of this bacterium revealed lower levels of depressive and anxious behavior in response to laboratory methods designed to irritate them, such as exposure to high or low temperatures and tail-pinching.

But it’s still in its infancy: as you might think, the “pattern” of anxiety or depression that a lab mouse exhibits doesn’t necessarily match the disorder that humans experience. And because animal experiments are often very expensive to run, they are also often very small, making them more prone to false positives. Microbiome laboratory research is no exception.

Antibiotic research in rodents, which attempts to kill bacteria in the gut and reveal behavioral consequences, also suffers from a lack of consistency: A review study published last month found that scientists have developed many different antibiotics and studied many different ones. gut microbes, which prevented us from drawing general conclusions. The authors suggested that greater agreement in study design may lead to greater agreement in results.

what about people There have been quite a few studies of people with depression taking probiotics, although again there is no consistency. which In these studies, a specific probiotic is administered. Gathering together the research, a 2021 review found that probiotics alone don’t do much. However, if they were given in combination with antidepressants, they seemed to relieve symptoms of depression, at least moderately. But that only adds to the confusion: In animal studies, probiotics seemed to work on their own, so why not in humans? The answer is unknown, but it may be due to the large differences between “depression” in mice and humans mentioned above.

There are hints for other data types as well. We saw an example of the latter last month — a study found that older people who use laxatives more often have a slightly higher risk of some types of dementia, suggesting (but by no means confirming, as it was just a correlation) that regular blushing face all the bacteria in your gut can be unhealthy.

After all, we can’t stop talking about stool transplantation. There is growing evidence that feces are taken from a healthy donor and transplanted into the intestines of a person with C. Difficulty infection can greatly reduce symptoms; and this method may be useful for other gastrointestinal problems as well. This makes a lot of sense – it’s a (relatively) simple case of “replacing bad bacteria with good bacteria”.

It is much less clear how such transference would work in a mental or psychiatric state. Nevertheless, they tried it: in some cases, mice were transplanted with stool samples from people with a mental disorder, and scientists argued that after that the mice behaved differently – in a similar way (at least for the mouse). the condition in which the human donor was diagnosed. As always, these are mostly small studies, and the relationship between depression in humans (say) and mice can often be a bit tenuous.

There are very few examples of high-quality research on human-to-human stool transplantation – we have a few potentially promising but very limited case studies. No doubt we’ll get better studies soon, but the results should be pretty impressive for many people to overcome their innate aversion to this type of procedure.

As we have seen, there are far more opinions than hard evidence in the world of the microbiome, suggesting that the field of research is still at a very early stage. But the lack of conclusive evidence has done little to stop people from capitalizing on the microbiome hype: If you google the names of the bacteria that are often discussed in the microbiome literature, you get a list of websites, not scientific sources, offering diet foods. additives. almost everyone mentions the “gut-brain axis” along with other scientific-sounding buzzwords.

Very rarely, if ever, they point to all the problems we’ve seen in science: a lack of a clear understanding of which bacteria are most important for each condition, a lack of large, high-quality studies, and inconsistencies. and lack of consistency in their results. The problem is exacerbated by the fact that articles in the media tend to jump on the bandwagon rather than report nuanced evidence. Don’t take my word for it: A 2021 analysis of microbiome articles in U.S. and Canadian tabloids found that only 19% mentioned anything about the science being unproven or requiring more work.

And we really need to work harder. We are still far from a good, clear understanding of how the microbiome is connected to what goes on in our minds, and we are far from being able to use it to develop reliable treatments for mental health problems. As long as the balance does not shift from opinion to data and evidence, an idea remains the scientific equivalent of intuition.