Microbiome for Mental Health: Could Homovanillic Acid Treat Depression?

What if a simple combination of probiotics and amino acids could help relieve depression? I know it sounds far-fetched, but I wouldn't joke about this. Recent research reveals that a specific neurotransmitter produced by the gut microbiome and capable of crossing the blood-brain barrier is actually depleted in human patients with depression, along with the microbes that produce it. And remarkably, That same study, which we're going to talk about, shows in multiple animal models of depression that replenishing either the gut bacteria or the neurotransmitter can alleviate depressive symptoms with remarkable consistency across interventions and behavioral tests. We're going to get into the data and you are going to leave with hope and practical insights and direction to more resources for either you or a loved one who may be suffering with their mental health. So let's get into the paper. It was published in Cell Metabolism, and the researchers began by comparing the microbiome and metabolic profiles of individuals—patients with depression, 262 patients with depression—they compared them to healthy controls, 207 controls. And the differences were striking. The most pronounced change though was in a circulating hormone and neurotransmitter called homovanillic acid, or HVA for short. It's a metabolic cousin of dopamine, it's on the same synthesis pathway, and it's derived from the amino acid precursor tyrosine. Now, when I said most pronounced change, I'm actually referring to the effect size of the relationship between lower levels of homovanillic acid and worse depression symptoms as measured by the Hamilton Depression Rating Scale. Furthermore, and in parallel, levels of homovanillic acid-producing bacteria were also significantly reduced in individuals—patients with depression—as compared to controls. So in short, stepping back, in human patients with depression, homovanillic acid levels are lower, and so are the levels of the bacteria responsible for producing it. Of course, correlation doesn't equal causation. And if you've known me for a while, you've been following me on this channel and elsewhere, you already know what's coming next. Can you tell me? Mechanistic animal models. Just to take a step back and make an important point, we can't do certain mechanistic experiments on human patients. Those really needed to dissect the fine details. It's not ethical. So what do we do? We align what we can gather from the human data, like data we just discussed, with animal models to see if it all makes sense, to see if the puzzle fits together. And in this case, the researchers turned to two well-established mouse models of depression: the Chronic Unpredictable Mild Stress model and the Corticosterone-Induced Model, both named quite descriptively, I might add. And in both models, mice with depressive symptoms exhibited lower homovanillic acid levels, just like in human patients with depression, and reduced levels of homovanillic acid-associated bacteria, again, just like in the human patients. The team then confirmed which bacteria produced homovanillic acid and then tested whether supplementation either with these gut microbes or with homovanillic acid itself could reverse the signs and symptoms of depression. They found that the bacteria Bifidobacterium longum produced homovanillic acid, while another bacteria, Roseburia intestinalis, didn't produce homovanillic acid, but it significantly enhanced the ability of B. longum to do so. So you can think of R. intestinalis as B. longums neurotransmitter-producing assistant. But here's the hopeful part. Supplementation with either of these bacteria, either B. longum or R. intestinalis, which supports B. longum, or supplementation with homovanillic acid itself improved depressive symptoms across multiple behavioral tests. These results held true in both depressive models and across all three forms of intervention. Either bacteria or the neurotransmitter. The authors themselves write, now reading from the paper, "Our findings suggest that supplementation with a mixed cocktail of B. longum and R. intestinalis coupled with the consumption of tyrosine-rich, protein-rich foods could represent an effective strategy for treating depression." I'll let you evaluate these findings for yourself and decide whether they justify hope, For my part, I believe it's both reasonable and productive to consider that the microbiome influences mood directly and interacts with the external world. The microbiome, you can think of it, it's uniquely positioned as a conduit between the environment and mental state, a bridge between the outer world and our most internal experiences. So should we be hopeful that treating the microbiome could treat the mind. Yes. What's next? Now that we've reviewed the data, I know you're really curious about how can we support our microbiome or specifically increase levels and functionality of B. longum and R. intestinalis. I will address that now, but quickly, before I do so, let me offer a caveat. Nothing in this video is intended as medical advice, obviously. Don't get your medical advice off YouTube. Additionally, When it comes to microbiome-targeted intervention, response variability is the rule, not the exception. What works for one person, say me, may not work for another, say you. With that said, let me try to give you some practical advice. B. longum is one of the better characterized strains available in commercial probiotic formulations. So you can actually look on probiotics for B. longum, in particular B. longum BB 536, I think it was, has been quite extensively studied. More details and specific probiotic options, or at least one option, can be found at staycuriousmetabolism.com in the associated newsletter that goes with this video. Additionally, we can talk about prebiotic fibers. Now, I will say, as much as Google and AI wanted to convince me that fiber is the way to enhance B. longitrum levels, I really struggled to find literature that provides clear and convincing evidence that supplementation or intake of specific fibers alters B. longum populations and functionality in any clinically meaningful way. That said, I'm not going to discredit this concept. It's reasonable speculation at the very least. So, with that heavy caveat, I will report that B. longum feeds off prebiotic substrates like inulin and resistant starches. These can be found in foods like chicory root, garlic, onion, leeks, and asparagus. Including these regularly in your diet may, big caveat here, but it may create the internal environment that supports B. longum growth and function. So, you can do your own risk-benefit analysis for these sort of things, but if you enjoy and tolerate these foods, I see little downside and potential upside to including them in your diet. Also, let's talk about supporting Rosiburria intestinalis levels. Well, R. intestinalis is a bit trickier. You can't directly supplement with it as a probiotic. It's just not currently available in commercial probiotics because it's highly oxygen sensitive and difficult to manufacture. So again, the best I can reasonably offer is, I guess I'll call them reinforcement platitudes about treating your microbiome well. It is possible that dietary prebiotics like those I just mentioned in addition to dietary polyphenols could enhance R. intestinalis levels. And at the very least, I'd suggest avoiding highly processed foods and high-sugar Western dietary patterns. And you can consider starting a low-dose fermented foods protocol with a gradual uptitration in your fermented food intake to approximately 6 servings per day or as tolerated. For more on fermented foods, see this video. And then finally, tyrosine-rich foods. This provides the raw material for homovanillic acid generation since homovanillic acid is derived from the amino acid tyrosine. So, consuming these foods can provide the substrate for its production. The authors themselves actually stipulate that sufficient substrate—tyrosine from protein-rich foods—is required to produce homovanillic acid. That said, I'm just gonna add my two cents here, I don't think this would be a huge factor. I think likely most of us get enough tyrosine in our diets such that substrate availability isn't a primary concern, it's not a limiting factor. But I don't know for certain. And foods rich in tyrosine like turkey, chicken, meat, fish, cheese, and especially aged cheeses, they can be healthy, protein-packed, and delicious. So at the very least, you'll support your muscle growth by including these foods and have a fun fact to toss out at the Thanksgiving table when you're with family eating a turkey. The bottom line is this is a really hopeful frontier worth exploring. The science is early, but signal is clear: our guts and our brains, they're not separate domains. Hey, that rhymes. They actually— that wasn't intentional, they did rhyme. They are deeply intertwined, the gut and the brain, chemically, immunologically, and emotionally. What this study offers isn't a miracle cure or a finished protocol, but a new framework that specific bacteria paired with the right substrates like tyrosine may influence mood in a measurable, even treatable way. The promise here is not perfection, it's progress. A reminder that biology is plastic, responsive, and sometimes somewhat simple. Give the right bacteria and the right food and you may shift the chemistry of your mind. Finally, I promised to point you to some more resources. Here are a few ways to continue your learning. First, for a broad overview of metabolic psychiatry and why it is the future of psychiatry, please check out this newsletter. I've linked it below. It reviews the limitation of current evidence-based medicine, how it can actually lead us astray, and what needs to change. Also check out these prior videos on inflammation and anxiety and on the underlying metabolic contributor to depression, a process called autophagy, both based on breaking 2025 literature. So let's be bold enough to bridge metabolic health and mental health. Stay curious and stay compassionate too. I hope you learned something. Please give this a share with someone if you think it might help them.