Does serotonin in your gut affect your mental health?
You may have heard that 95% of your body's serotonin is produced in your gut. While this is true, it's important to understand it doesn't mean gut serotonin directly affects your mood.
Here's the crucial point: Serotonin produced in the gut cannot cross the blood-brain barrier. Although gut-derived serotonin and brain-derived serotonin are chemically identical, they're produced in completely different areas of the body and serve entirely different purposes.
Gut serotonin is made by enterochromaffin cells and functions more like a hormone, helping regulate digestion, appetite, and the movement of your intestines.
Brain serotonin is produced by the raphe nuclei in the brainstem and acts as a neurotransmitter, affecting mood, sleep, behavior, and other cognitive functions.
So, while your gut has plenty of serotonin, it doesn't directly influence your mood the way brain serotonin does.
It's also important to clarify that low serotonin alone has been shown not to be the primary cause of depression. The previously popular monoamine theory of depression, suggesting depression arises from low serotonin levels, has largely been debunked. Therefore, if we keep emphasizing the gut serotonin-mood link, we might be oversimplifying or misunderstanding the actual mechanisms of mental health.
That said, this doesn't mean there's no connection between gut health and mental health. There's a very real, complex, and fascinating relationship known as the gut-brain axis—a two-way communication channel between your gastrointestinal system and brain. For instance, one study involving 410 people with gut-brain disorders like irritable bowel syndrome (IBS) found significant overlaps with mental health conditions: 55% had a psychiatric disorder, 35% had anxiety, and 28% had depression.
Additionally, research is exploring the role of your gut microbiome—the diverse community of bacteria in your digestive system—in mental health. Studies have consistently noted differences between the microbiomes of people with depression and those without, including an increase in inflammation-promoting bacteria and fewer anti-inflammatory bacteria. This aligns with the broader inflammatory hypothesis of depression. However, these results are inconsistent, and it's unclear whether microbiome changes cause depression or result from it.
Notably, one study even found that transferring gut bacteria from depressed people into mice induced depressive-like behaviors, hinting at a potential causal link—but human evidence for this is still lacking.
Surprisingly, probiotics are listed as potential adjunctive treatments in Canadian guidelines (CANMAT) for mood and anxiety disorders. However, reviewing these studies closely reveals several challenges:
There's no standard probiotic strain recommended.
Most studies combine probiotics with other treatments, making it hard to know how much probiotics alone actually help.
Despite promising results, probiotics aren't something I'd currently recommend for mental health on their own, unless there are other reasons—like digestive issues—where I might suggest them as an additional support.
Probiotic strains that show potential include Bifidobacterium longum, Lactobacillus rhamnosus, and Lactobacillus plantarum. But the current evidence isn't yet conclusive.
In summary, gut serotonin and brain serotonin differ fundamentally, serving entirely separate functions. While the gut-brain connection is undeniable, it's much more complicated than what is typically portrayed. There's potential for the microbiome to influence mental health, but we're still figuring out the extent of this relationship. Clinically, it's best to be cautious with exaggerated claims about probiotics or psychobiotics. A better-supported approach may be focusing on dietary strategies—such as increasing fiber intake and diversity in your diet—which could beneficially influence both gut and mental health.
