• TDO is primarily expressed in the liver and upregulated by glucocorticoids (stress hormones, e.g., cortisol) and TRP availability. It plays a central role in baseline tryptophan regulation and is often elevated during prolonged stress or high metabolic load. 
• IDO is expressed in immune and epithelial cells, and is strongly induced by immune messengers (cytokines). It reflects immune activation and is a hallmark of immune stress-driven tryptophan use. 

In a normal physiological state, most dietary tryptophan is catabolized through the kynurenine pathway, with the primary purpose of generating NAD+, a vital coenzyme for mitochondrial and cellular energy metabolism. Only a small fraction is used for protein synthesis, the serotonin pathway, and the indole pathway.   

While some activity through the kynurenine pathway is essential, what matters is how and where it’s regulated – and whether this pathway is appropriately balanced or upregulated in response to sustained stress, immune strain, or microbial disruption. 

IPA itself may also contribute to indirect suppression of IDO, by supporting gut barrier integrity, reducing systemic LPS exposure, and helping to balance immune signaling. In this way, the microbiota doesn’t just reflect the body’s status – it actively shapes the regulatory enzymes that determine tryptophan’s fate.

Conversely, when the gut microbiota is depleted of beneficial microbes (as seen in microbial imbalance) – due to low fiber intake, highly processed diets, sedentary behavior, or prolonged stress – this protective system is weakened. Barrier function weakens, immune stress rises, and tryptophan is increasingly diverted into the kynurenine pathway. This results in the buildup of metabolites like KYN and quinolinic acid (QA), which are associated with heightened oxidative stress, shifts in immune balance, and metabolic strain. 

• High-fiber diets increase SCFA production and microbial diversity – supporting the indole pathway and helping to balance immune tone. . 
• Exercise has been shown to raise IPA levels and lower KYN/TRP ratios, reflecting both improved microbial activity and reduced immune strain. 
• Stress management and quality sleep help regulate cortisol levels and TDO activity, supporting more efficient use of tryptophan. 
• Intermittent fasting supports microbial renewal, enhances autophagy, and may lower KYN levels by reducing immune stress. 

These are not just generic health tips – they are targeted strategies that act on a molecular level to rebalance tryptophan metabolism, enhance resilience, and support whole-body balance across the gut-brain-immune metabolic axis.   

By measuring key metabolites and ratios – such as IPA, KYN, and TRP – our test helps reveal where you are on this spectrum and how your lifestyle is influencing one of the body’s most dynamic and interconnected systems. 

Quick summary

• Lifestyle habits like stress, sleep, and diet determine how tryptophan is used in the body. 
• Beneficial microbes and SCFAs help suppress rate-limiting enzymes that initiate tryptophan breakdown through the kynurenine pathway. 
• The gut microbiota not only reflects but also regulates key enzymes that govern immune tone and resilience.