Just a reminder that these answers are for educational and informational purposes only, and aren’t a substitute for medical or mental-health advice. Although I am a registered dietitian, I’m not your dietitian (unless you happen to be one of my 1:1 clients—hi!—but even then, this email isn’t a session).
The first question today is available to all subscribers (about the link between the gut microbiome and mental health), and there’s a bonus one for paid subscribers (about whether it’s really necessary to take supplements for gut health, especially probiotics and digestive enzymes).
I am wondering whether you’ve written about this new body of research linking the gut microbiome to various mental health conditions. As a mental health practitioner, I keep hearing people mention it, but I also wonder if it could set people up for disordered eating as well as another reason to feel shameful about food choices. Your thoughts are greatly appreciated.
Thank you.
—Julia
I definitely share your concern that the increasing hype about the gut microbiome1 can set people up for disordered eating and shame.
In many ways this hype is understandable, given how many of us struggle with digestive issues and other chronic conditions (myself included). There’s a lot of excitement in particular about the gut-brain axis—the two-way communication network between the gut and the brain—because it’s seen as a potential key to unlocking new treatments for common conditions such as depression, anxiety, and irritable bowel syndrome, among others. And I get it: as someone with a number of those conditions myself, it’s exciting to think there might possibly be some better treatments on the horizon. I understand why we want to believe the hype—part of me really does, too.
But I think for many people living in diet and wellness culture—which praise restriction and thinness and instill a fraught relationship with food—it’s almost inevitable that gut health diets can create a sense of restriction and anxiety around eating. For people who already have disordered eating (or a history of it), these kinds of diets can be especially damaging.
That’s ironic, given that gut health is often positioned as a sort of psychiatric panacea. In my view, if trying to follow a gut-health diet triggers disordered eating, it’s not actually helpful for your mental health.
But maybe some people would be willing to take that risk, if the potential payoff meant addressing other mental-health issues that feel more urgent. Almost every day there are new articles and posts about how healing the gut supposedly can heal the brain. So is there any truth to those claims? What does the science really say?
These are complicated questions, so I’m only going to scratch the surface of an answer here (though I’ll likely return to this topic in the future) and have limited my search to some of the most-studied conditions (namely: depression, anxiety, and bipolar disorder). That said, based on what I’ve seen of the evidence in this area, I think it’s safe to say this: The evidence for a link between gut health and mental health is a lot less compelling than the breathless headlines and viral TikToks make it seem.
First of all, much of the research on the gut-brain axis comes from animal studies, which are nothing to hang your hat on. Those kinds of studies need to be replicated in high-quality human trials before they can be used to guide any sort of clinical decision-making.
The research that has been conducted in humans is largely observational, which means it can only tell us about correlation, not causation—though you wouldn’t know that from most media reporting. Even reading the abstracts (free summaries) of the studies themselves can sometimes make it seem like the evidence is more robust than it really is.
Take this oft-cited 2021 systematic review of observational studies on the gut microbiome in anxiety and depression. It reported that certain classes of microbes may be correlated with those mental-health conditions.
But digging into the study results reveals that these associations are very inconsistent: even the most common correlations are only present in less than one-third of the reviewed studies. Faecalibacterium, for example, was found to be lower among people with depression in just 5 out of 19 studies (a little over 26 percent). One study found it was actually higher, and the rest (68 percent) found no association. And yet the supposed correlation was reported in the review’s abstract, giving the impression that it was much more of a sure thing.
That same review also found that there were no consistent differences in microbial diversity between people with and without anxiety/depression. In the gut-health world, microbial diversity is typically seen as an unequivocally good thing, and some previous studies have found that it’s correlated with better health outcomes. Yet that may not be the case for every condition.
In a similar vein, a 2023 systematic review found that overall gut microbiota composition was different between people with and without bipolar disorder, but the differences weren’t consistent across studies. A few classes of bacteria appeared to have some sort of relationship with the disorder—but in some studies they were associated with an increased risk, and in others with a decreased risk.
This variability is common in microbiome research, and one reason may be because it’s incredibly difficult to collect and interpret microbiome data. Bias can enter the equation at any number of points: in the selection and number of study participants (there’s evidence that studies need to include at least hundreds of people to be accurate); confounding factors like age, sex, diet, and medication use; study length (our microbiomes can change rapidly); whether the microbiome is analyzed from a stool sample or a biopsy, and where in the gut the biopsy is taken; the methods of sample storage and culturing; the methods of analysis; and more.
In short: research on the gut microbiome is kind of all over the place. As a recent systematic review on the gut microbiome in anorexia nervosa put it, “the methodological limitations, lack of standardisation, and conceptual ambiguity hinder the analysis of microbiota as a key explanatory factor” for this eating disorder. With a few exceptions, I think the same can probably be said for most other diseases—and for the gut-microbiome field as a whole.
What about randomized controlled trials? These theoretically should be able to determine if changes to the gut microbiome have any causal connection to mental-health outcomes. Unfortunately, there are still a lot of issues with these studies: small sample sizes, the above-mentioned issues with analysis methods—and widespread industry funding.
Many studies funded by probiotic manufacturers show that (surprise, surprise) probiotics seem to have benefits for mental health. But in addition to their conflicts of interest, most of these are also small pilot studies.
For example, this one had 45 people and found that taking a probiotic significantly reduced perceived stress and improved subjective sleep quality compared to a placebo—and it was conducted by employees of Nestlé, which makes probiotic foods and supplements. And this study had 49 people and found that the probiotic group experienced greater improvements in some measures of depression and anxiety than the placebo group. It was funded in part by grants from ADM Protexin, a UK probiotics manufacturer.
A larger study, published last April without any financial support from probiotics companies (other than providing the pills used in the trial), found that probiotics had no effect on overall well-being, quality of life, emotional regulation, anxiety, mindfulness, or interoceptive awareness in a group of 135 participants without health conditions. The subgroup who scored higher in “healthy behaviors” did experience some improvements in anxiety, emotion regulation, and mindfulness after taking probiotics, though it’s unclear why (and people with lower health-behavior scores actually appeared to fare worse on probiotics, if my reading of this slightly confusing table is correct).
What’s the overall consensus from randomized controlled trials (RCTs)? A 2022 systematic review and meta-analysis found that most probiotics did not affect mood, stress, anxiety, depression, or psychiatric distress. Probiotics did slightly improve the latter two measures in a small subgroup of studies, but the researchers report that “the clinical relevance of these findings is questionable,” and 70 percent of studies in that subgroup were at high or moderate risk of bias. More than two-thirds of all the trials in the systematic review were sponsored or conducted by probiotics manufacturers.
For bipolar disorder, the aforementioned 2023 systematic review claims that probiotics improved outcomes. But the review only includes 2 RCTs, and one of them actually shows the exact opposite! This study from 2020, conducted in a small sample of 38 patients in Iran, found that both the probiotic and placebo groups had improvements in mania and depression scores over time, and there were no significant differences between the groups—meaning that the improvement was simply due to the placebo effect. If an intervention is no better than a placebo, you can’t claim it’s effective.
In contrast, this relatively small study from 2018 did find that among people who’d been hospitalized for an acute manic episode, those who took probiotics were significantly less likely to be rehospitalized than those who took a placebo. Of the patients who were rehospitalized, the probiotic group also had significantly shorter hospital stays. But there were no significant differences between the placebo and probiotic groups at any of the monthly study visits, which raises the possibility that there could have been some residual confounding factors between the groups—or at least that it’s not totally clear what’s going on here without further studies.
There’s a lot more I could say about the gut-brain axis (and likely will in a future piece), but I’ll leave it there for now, and I hope that’s a helpful start in thinking critically about this research.
Thanks so much for the great question, Julia, and thanks to all the free subscribers for reading! Paid subscribers can stick around for the bonus Q&A, and everyone can ask their own questions for a chance to have them answered in an upcoming newsletter.
I am a Health At Every Size nutritional therapist in the UK, and as of last year I also became a Certified intuitive Eating Counselor. I really love working with clients to heal from dieting and make peace with food. In my own life, I have worked hard to cultivate my now peaceful relationship with food, but there is one lasting issue from my “health”-obsessed days: supplements.
I saw a nutritional therapist years ago when I was suffering from what I now see was a very disordered relationship with food. She recommended supplements to help support my gut health, including digestive enzymes and probiotics. The former I feel the most fearful about giving up, as I was told that because I experience anxiety, I would have depleted digestive enzymes. I have been taking a similar cocktail of pricey supplements every day for years now, and it costs a lot of money and energy.
I would love to stop taking them, but I feel I need reassurance from a nutrition professional (who is not rooted in wellness culture) that my gut will cope on its own! I would be so grateful if you could tell me your thoughts about supplements, whether you recommend using them, and whether they are worth the money and energy for my gut health.
I also want to say a huge thank you for all you do in the anti-diet field, as it was your podcast that played a major role in my recovery and my change in career path from straight nutrition to weight-inclusive healthcare and intuitive eating. I am currently devouring your book and it is just awesome!
—Jessie
