1. Western-Style Diet (high-fat, low-fiber, high-sugar)

• Why it’s harmful: Lacks fermentable substrates, deficient in essential micronutrients, high in emulsifiers and additives that impair the gut barrier. 
• Microbial effect: Reduced diversity, increased endotoxin-producing bacteria.  
• Metabolite profile: Decreased SCFAs and indole derivatives like IPA; elevated kynurenine (KYN), a pro-inflammatory TRP metabolite.  
• Consequences risks: 
• Weakened barrier function 
• Elevated systemic immune activation
• Greater metabolic strain and reduced resilience

2. Low-Fiber, Ultra-Processed Diets 

• Why it’s harmful: Starves keystone microbes and promotes mucus degradation. These diets are also low in essential nutrients, which impairs both microbial metabolism and host immune function.   
• Microbial effect: Loss of keystone species and reduced microbial diversity. 
• Consequences: 
  • Weakened gut barrier function 
  • Higher susceptibility to immune stress 
  • Impaired microbial conversion of tryptophan to IPA 
  • Reduced resilience to dietary stressors and antibiotics 
     

3. Excess Alcohol and Chronic Stress

• Why it’s harmful: Alcohol directly damages the intestinal lining and alters microbial balance; stress increases cortisol, activating TDO and promoting heightened KYN production. Both can deplete key nutrients, further compromising gut and immune health.
• Consequences:
  • Elevated KYN:TRP ratio (a marker of immune stress) 
  • Altered mood, sleep, metabolism