Recently, I worked with a client experiencing persistent gastrointestinal inflammation, autonomic instability, neurological hypersensitivity, difficulty regulating attention and arousal, and an unusually complex response to foods and interventions. He had already investigated his condition extensively and had attempted to use lactulose to acidify the colon and alter parts of the microbial ecosystem. Despite multiple interventions, the underlying pattern persisted. During our consultation, we integrated his stool microbiome results, organic-acid profiles, bile-acid pattern, genetics, methylation status, mitochondrial findings, and microbial metabolite burden. The resulting picture was far more complex than simply having “dysbiosis.”
One of the most striking findings was a marked elevation of HPHPA—3-(3-hydroxyphenyl)-3-hydroxypropionic acid, an aromatic microbial metabolite associated with altered bacterial metabolism of phenylalanine, tyrosine, and related aromatic compounds. HPHPA has historically been associated with increased aromatic amino-acid fermentation by certain members of the Clostridia class, although it should not be considered a metabolite unique to any single bacterial species. Rather, it represents evidence that microbial aromatic amino-acid metabolism has shifted toward producing compounds capable of interacting with host physiology.
The client’s microbiome testing identified a selective elevation of Clostridium hathewayi, while Clostridium ramosum and Clostridium symbiosum remained within that laboratory’s reference limits. A later microbiome assessment also identified a modest increase in Clostridium perfringens, together with an elevated Desulfovibrio signal consistent with increased hydrogen-sulfide-producing capacity. Importantly, these findings did not indicate that the entire Clostridia class was pathogenic. Clostridia comprise one of the largest and most metabolically diverse bacterial groups within the human intestine. Many members are among our most important butyrate producers, support colonocyte bioenergetics, contribute to immune tolerance, and participate in normal bile-acid metabolism. Likewise, Clostridia cluster XIVa was not abnormally elevated in this case and contains numerous organisms that are generally considered beneficial. This distinction is critical because microbiome ecology is determined far more by metabolic function than by bacterial taxonomy.
The more important question therefore becomes:
Which organisms are expressing which metabolic pathways, what substrates are feeding those pathways, and why has the host lost the capacity to regulate their metabolic output?
When proteins and aromatic amino acids are incompletely digested or absorbed within the upper gastrointestinal tract, greater amounts of phenylalanine and tyrosine reach the colon. Certain anaerobic organisms possess enzymatic pathways capable of fermenting these aromatic amino acids into numerous biologically active compounds, including HPHPA, hydroxyphenyl acids, phenylpropionic acids, phenolic metabolites, and related aromatic fermentation products. The amount produced depends upon substrate availability, microbial gene expression, intestinal transit, luminal pH, hydrogen disposal, bile-acid composition, epithelial physiology, and host immune regulation.
The presence of a bacterial species alone does not prove production of any specific metabolite. However, when elevated aromatic metabolites, compatible microbial species, altered ecological conditions, and impaired host clearance converge within the same individual, they create a biologically coherent systems-level hypothesis that deserves careful investigation.
This became particularly interesting because some aromatic microbial metabolites have been proposed to interfere with catecholamine regulation, particularly the activity of dopamine β-hydroxylase (DBH), the copper- and ascorbate-dependent enzyme responsible for converting dopamine into norepinephrine. Although direct inhibition of human DBH by physiological concentrations of HPHPA has not yet been conclusively established, sustained exposure to aromatic microbial metabolites could plausibly increase biochemical pressure on an already vulnerable catecholamine system.
In this client, that vulnerability extended beyond microbial metabolism. Genetic analysis identified variants involving the DBH pathway, together with reduced catecholamine clearance through COMT. COMT depends upon adequate S-adenosylmethionine (SAM) availability, while regeneration of SAM depends upon ATP, intact one-carbon metabolism, folate, vitamin B12, magnesium, methionine cycling, and a favorable SAM-to-SAH ratio. When SAH accumulates, methyltransferase reactions become progressively inhibited, reducing the efficiency of catecholamine metabolism even when enzyme structure remains genetically intact.
Equally important, catecholamine regulation is fundamentally an energetic process.
Neurons require continuous mitochondrial ATP production to maintain ionic gradients, package neurotransmitters into vesicles, recycle catecholamines following release, maintain membrane potentials, and support synaptic transmission. DBH function depends upon healthy vesicular physiology, while methylation reactions ultimately depend upon ATP generation. Magnesium participates throughout these ATP-dependent pathways. Consequently, catecholamine physiology should not be viewed as being controlled by a single gene or enzyme. It represents the integrated output of mitochondrial bioenergetics, methylation, micronutrient availability, redox regulation, vesicular biology, and neurotransmitter recycling.
This client also demonstrated evidence of a hydrogen-sulfide-producing microbial ecology through elevated Desulfovibrio. At physiological concentrations hydrogen sulfide functions as an endogenous signaling molecule. However, excessive microbial H₂S production may inhibit cytochrome-c oxidase (Complex IV), impair oxidative phosphorylation, disturb iron-sulfur proteins, alter cellular redox balance, and increase energetic stress within both colonocytes and neurons. Under these conditions, microbial aromatic metabolites may increase pressure on catecholamine regulation while hydrogen sulfide simultaneously reduces the mitochondrial ATP required to sustain those same pathways.
Another important question is why HPHPA accumulates in some individuals.
Following absorption from the colon, HPHPA and related aromatic microbial metabolites undergo hepatic biotransformation before renal excretion. Clearance depends upon multiple host systems including sulfation, glucuronidation, methylation, glutathione availability, transporter activity, renal elimination, and preservation of intestinal barrier integrity. Sulfation is particularly important because sulfotransferase enzymes require 3’-phosphoadenosine-5’-phosphosulfate (PAPS) as the activated sulfate donor, and PAPS synthesis itself requires ATP. Consequently, impaired mitochondrial function may simultaneously reduce detoxification capacity while microbial production of aromatic metabolites continues to increase.
The client’s bile-acid profile added another interesting layer. Cholic acid represented only a very small fraction of the measured bile-acid pool, while secondary bile acids predominated. This pattern suggested extensive microbial transformation of primary bile acids but did not fully explain the neurological phenotype. I also considered whether inflammatory activation of indoleamine-2,3-dioxygenase (IDO) and diversion of tryptophan toward the kynurenine pathway could account for many of the neurological findings. While this pathway likely contributed to the overall inflammatory environment, integrating the complete dataset suggested that aromatic microbial metabolites provided a stronger mechanistic explanation for many of the observed catecholamine-related features.
Beyond HPHPA, my research has increasingly focused on other bacterial metabolites including D-lactate, ammonia, hydrogen sulfide, phenolic compounds, hydroxyphenyl acids, and additional aromatic fermentation products. Humans metabolize D-lactate considerably less efficiently than L-lactate, making excessive microbial production particularly interesting from a neurological perspective. Likewise, chronic ammonia production from amino-acid fermentation may alter astrocyte metabolism, glutamate-glutamine cycling, mitochondrial function, and overall neuroenergetics.
Taken together, these observations suggest that many individuals with SIBO, IBS, MCAS, POTS, ADHD-like symptoms, autism-spectrum features, and chronic neuroinflammatory presentations may not simply be living with altered microbial composition. They may be living with continuous exposure to microbial metabolites interacting with genetics, mitochondrial function, methylation, autonomic physiology, immune regulation, bile-acid metabolism, and neurotransmitter biology.
The microbiome is not merely a collection of organisms.
It is a metabolically active biochemical organ whose products may profoundly influence human physiology.
The clinically important question is therefore not simply which bacteria are present, but which metabolic pathways they are expressing—and whether the host still possesses the physiological capacity to regulate and eliminate their metabolic output.
In a future article, I will examine HPHPA, aromatic amino-acid fermentation, Clostridial metabolic pathways, hydrogen sulfide, D-lactate, ammonia, dopamine β-hydroxylase, COMT, mitochondrial bioenergetics, astrocyte metabolism, glutamate regulation, autism, ADHD, MCAS, SIBO, and the broader Host Capacity Model in much greater mechanistic detail.