What if your mood was not entirely determined by your brain?

That question might have sounded radical a decade ago. Today, it is one of the most active frontiers in neuroscience and gastroenterology. And a groundbreaking 2024 study published in Cell Reports Medicine has identified one of the clearest mechanisms yet: a specific compound produced by gut bacteria that directly alleviates depressive symptoms through brain signaling pathways (Qian et al., 2024).

The compound is called indole-3-lactic acid (ILA), and it represents a new chapter in our understanding of the gut-brain axis.

What Is Indole-3-Lactic Acid?

Indole-3-lactic acid is a postbiotic metabolite, a compound produced by certain gut bacteria when they metabolize the amino acid tryptophan. Tryptophan is an essential amino acid found in protein-rich foods like turkey, eggs, cheese, nuts, and seeds.

Your body uses tryptophan through several metabolic pathways. The most well-known produces serotonin, the "feel-good" neurotransmitter. But gut bacteria can metabolize tryptophan through alternative pathways to produce indole compounds, including ILA.

These indole derivatives are part of the larger family of postbiotic metabolites that your gut microbiome produces daily. And as the Qian et al. research demonstrates, they have far more influence on your brain than anyone previously appreciated.

The Study: ILA, AhR Signaling, and Depression

Qian and colleagues discovered that ILA alleviates depressive symptoms through a specific molecular pathway: the aryl hydrocarbon receptor (AhR) signaling pathway.

Here is the mechanism in simplified terms:

1. Gut bacteria produce ILA from dietary tryptophan
2. ILA activates AhR receptors, which are present throughout the gut lining and in the brain
3. AhR activation triggers downstream signaling that reduces neuroinflammation and supports neuroprotective pathways
4. The net result is measurable improvement in depressive behaviors and markers of brain inflammation

The researchers found that subjects with lower ILA levels exhibited more depressive symptoms, while increasing ILA availability improved mood markers. This establishes a direct molecular link between a gut-produced compound and brain function.

According to PubMed-indexed research, the AhR pathway is increasingly recognized as a key mediator of gut-brain communication, with implications for mood disorders, neuroinflammation, and cognitive function.

What This Means for Understanding Mood

This study reshapes several assumptions about mood and depression:

Mood is not just a brain problem. For decades, depression and mood disorders have been understood primarily as brain chemistry problems, specifically, imbalances in serotonin, dopamine, or norepinephrine. While brain chemistry is involved, research like the Qian et al. study shows that the source of those imbalances may originate in the gut.

The microbiome is a drug factory. Your gut bacteria are producing psychoactive compounds every day. ILA is just one of many. Other postbiotic metabolites influence the same neurotransmitter pathways. When your microbiome is healthy and diverse, it produces a balanced array of these compounds. When it is disrupted, the production shifts, and mood can suffer.

Diet affects mood through gut metabolism. Your gut bacteria can only produce ILA if they have tryptophan to work with. This creates a direct chain: what you eat feeds your gut bacteria, which produce metabolites that influence your brain chemistry. According to Harvard Health, the brain-gut connection is among the most significant medical insights of the past two decades.

The Postbiotic Connection

ILA is a postbiotic. It is a metabolic product of bacterial activity, not a live bacterium. This is important because it further validates the postbiotic approach to gut-brain health.

If mood-regulating compounds like ILA are the output of microbial metabolism, then the logical approach to supporting mood through the gut is to:

1. Support your microbiome's ability to produce these compounds through diet and lifestyle
2. Supplement with postbiotic compounds directly to ensure your body has adequate levels regardless of microbiome variability

This is the principle behind SecondKind's Mood Balance. Rather than hoping that live probiotic bacteria will colonize, survive, and eventually produce mood-supporting metabolites, Mood Balance delivers clinically studied postbiotic compounds that support the gut-brain axis from day one.

The Inflammation-Mood Connection

One of the most important aspects of the Qian et al. findings is the role of neuroinflammation. ILA did not simply increase serotonin levels. It reduced inflammation in the brain through the AhR pathway.

This aligns with a growing body of evidence linking chronic inflammation to mood disorders:

• People with depression consistently show elevated inflammatory markers (CRP, IL-6, TNF-alpha)
• Anti-inflammatory treatments have shown antidepressant effects in clinical trials
• Conditions that cause systemic inflammation (like IBS, autoimmune disorders, and chronic stress) are strongly associated with mood disturbances

A compromised gut barrier allows bacterial fragments (lipopolysaccharides) to enter the bloodstream and trigger systemic inflammation that reaches the brain. Postbiotics strengthen this barrier, reducing the inflammatory cascade at its source.

The Cleveland Clinic emphasizes that gut barrier health is fundamental to both digestive wellness and systemic health, including brain function.

Practical Implications: Supporting Your Gut for Better Mood

Based on the Qian et al. research and the broader gut-brain literature, here is how to apply this science:

Eat tryptophan-rich foods. Your gut bacteria need tryptophan to produce ILA and other mood-supporting metabolites. Good sources include eggs, turkey, salmon, nuts, seeds, tofu, and cheese. Aim to include a tryptophan source at each meal.

Feed your microbiome diversity. The bacteria that produce ILA need a healthy gut environment to thrive. Dietary fiber from vegetables, legumes, whole grains, and fermented foods supports the microbial diversity that underlies robust postbiotic production. The NIDDK recommends a fiber-rich, diverse diet as a cornerstone of digestive and overall health.

Supplement with targeted postbiotics. Even with a perfect diet, microbiome composition varies between individuals. Some people's gut bacteria may not produce adequate ILA or other mood-supporting metabolites. Postbiotic supplementation helps bridge that gap.

SecondKind's Mood Balance provides gut-brain postbiotic support, while Gut Balance ensures the underlying digestive environment is optimized for both comfort and metabolite production.

Reduce inflammatory triggers. Ultra-processed foods, excess sugar, alcohol, and chronic stress all increase gut inflammation and disrupt the microbial balance needed for healthy metabolite production. Reducing these inputs amplifies the benefits of postbiotic support.

Address gut and mood together. If you experience both digestive discomfort and mood challenges, they are likely connected. Treating them as separate issues leads to incomplete solutions. The Gut + Mood Bundle addresses both sides of the gut-brain axis.

Beyond ILA: The Bigger Picture

Indole-3-lactic acid is one compound in a vast metabolic landscape. Your gut bacteria produce thousands of metabolites, many of which have not yet been fully characterized. What the Qian et al. study demonstrates is a clear, mechanistic proof of concept: gut bacteria produce specific compounds that directly affect brain chemistry and mood.

As this field advances, we will likely discover dozens more such pathways. But you do not need to wait for every pathway to be mapped. The practical implications are already clear: support your gut, support your mood.

Your gut bacteria are already producing the chemistry of your emotions. The question is whether you are giving them what they need to do it well.

Start with the Gut + Mood Bundle.

Reference:
Qian, Y. et al. (2024). Gut microbiota-derived indole-3-lactic acid alleviates depression via AhR signaling. Cell Reports Medicine, 5(7), 100545. View Study