Gut Dysbiosis

Definition and Overview

• Guyton and Hall Textbook of Medical Physiology, p. 785

Normal Microbiome Functions (What Dysbiosis Disrupts)

• Harrison's Principles of Internal Medicine 22E, p. 3884

Causes of Dysbiosis

• Antibiotics - most common iatrogenic cause; reduces diversity for weeks after stopping
• High-calorie, Western-style diet - high fat, low fiber; promotes Bacteroides dominance and inflammatory strains
• C-section delivery - alters initial colonization (vs. vaginal delivery)
• Formula feeding (vs. breastfeeding) - delays microbiome diversification
• Stress - alters microbiota composition via the gut-brain axis
• Medications - PPIs, NSAIDs, immunosuppressants
• Infections - C. difficile, enteropathogens

Diseases Associated with Dysbiosis

Gastrointestinal

• Inflammatory Bowel Disease (IBD) - Akkermansia muciniphila overgrowth degrades intestinal mucin; Prevotellaceae upregulates chemokine-mediated inflammation; FMT shows promising results in IBD
• C. difficile colitis - classic antibiotic-associated dysbiosis allowing C. difficile to proliferate and express enterotoxins
• Colorectal cancer - Fusobacterium nucleatum, enterotoxigenic Bacteroides fragilis, and E. coli-derived colibactin have been implicated in CRC pathogenesis
• NASH/NAFLD - high-fat diet-driven dysbiosis leads to leaky gut, bacterial translocation, and liver inflammation
• IBS - gut dysbiosis, particularly SIBO, is a frequent finding

Metabolic

• Obesity - certain microbiota more efficiently extract energy from complex carbohydrates, predisposing to weight gain
• Type 2 Diabetes - dysbiosis contributes to insulin resistance and impaired glucose metabolism
• Metabolic syndrome

Rheumatologic / Immune

• Psoriatic arthritis - decreased bacterial diversity resembling IBD-type dysbiosis
• Ankylosing spondylitis - specific intestinal dysbiosis pattern; HLA-B27 influences microbiota
• Celiac disease - absence of gluten-digesting bacteria may unmask disease in genetically predisposed individuals

Neuropsychiatric (Gut-Brain Axis)

• Depression - reduced Faecalibacterium prausnitzii (anti-inflammatory) correlates with symptom severity; FMT from depressed humans induces depressive behavior in rodents
• Schizophrenia - reduced Lactobacillus and Bifidobacterium species
• Anxiety and stress-related disorders - "leaky gut" activates systemic inflammatory circuits
• Medical Microbiology 9e; Kaplan & Sadock's Comprehensive Textbook of Psychiatry, p. 694; Firestein & Kelley's Textbook of Rheumatology

Pathophysiology: Leaky Gut Hypothesis

1. Loss of protective commensal bacteria -> reduced mucus layer and tight junction proteins
2. Bacterial products (LPS, peptidoglycans) translocate across a compromised barrier
3. Systemic TLR activation triggers low-grade chronic inflammation
4. Inflammatory mediators disseminate to liver (NASH), joints (arthritis), brain (neuroinflammation)

Therapeutic Approaches

Dietary Modifications

1. Increase Dietary Fiber (Prebiotics)

• Promote growth of Prevotella and Bacteroides while reducing pro-inflammatory Firmicutes
• Increase SCFA-producing bacteria (Bifidobacteria, Lactobacilli, Faecalibacterium prausnitzii)
• Butyrate produced = colonocyte energy source, Treg induction, anti-inflammatory

2. Adopt a Mediterranean-Style Diet

• Olive oil (polyphenols, oleic acid)
• Fatty fish (omega-3s reduce inflammatory dysbiosis)
• Abundant vegetables and legumes
• Moderate whole grains
• Limited red meat and processed foods

3. Include Fermented Foods (Natural Probiotics)

4. Reduce Ultra-Processed and High-Fat Foods

• Western high-fat, high-sugar diet promotes gut dysbiosis and systemic inflammation
• Ultra-processed foods reduce microbial diversity and SCFA production
• Trans fats and refined sugar particularly harmful to the Firmicutes/Bacteroidetes ratio

• Refined sugars and high-fructose corn syrup
• Ultra-processed snacks and ready meals
• Excessive red meat / processed meats
• Artificial sweeteners (may negatively alter microbiota - saccharin, sucralose)

5. Polyphenol-Rich Foods

6. Omega-3 Fatty Acids

• Promote anti-inflammatory microbiota profiles
• Increase production of SCFA and reduce LPS-producing bacteria
• Sources: fatty fish (salmon, mackerel, sardines) 2-3x/week, walnuts, flaxseed, chia seeds

7. Foods to Limit or Avoid

8. Practical Dietary Pattern Summary

EMPHASIZE                      LIMIT
--------                       -----
Leafy greens, colorful veg     White bread, pastries
Legumes (daily)                Sugary drinks
Whole grains                   Ultra-processed snacks
Fermented foods (1-2/day)      Processed meats
Fruits (with skin)             Artificial sweeteners
Fatty fish (2-3x/week)         Excessive alcohol
Nuts and seeds                 Antibiotic use (non-essential)
Olive oil (primary fat)        
Prebiotic foods: garlic,       
onion, leeks, asparagus        

Diagnostic Approach

• Clinical history: antibiotic use, dietary pattern, stool changes, bloating, mood
• Stool microbiome analysis: 16S rRNA sequencing (limited clinical standardization)
• Breath tests: for SIBO (hydrogen/methane breath test)
• Fecal calprotectin: mucosal inflammation marker
• Fecal short-chain fatty acid levels: research tool

Key Takeaway

Recent evidence note: A 2025 PMC review (Pandit et al., PMID: 40893913) confirmed that dietary fiber, prebiotics, fermented foods, and polyphenols represent the most evidence-backed dietary tools for harmonizing gut dysbiosis, with precision nutrition approaches integrating host, microbial, and dietary factors being an emerging frontier.