1. The world of 100 trillion microbes
The human gut has about 100 trillion microbes — roughly 3× the 30 trillion human cells. 1,000+ species; total weight 1.5–2 kg (similar to the brain). The microbiome is humans' "second genome":
- Human genes: ~25,000; microbial genes: ~3 million
- Crucial roles in digestion, immunity, and neural communication
- At birth, we receive "seed" from the mother (different in vaginal birth vs C-section)
2. 4 pathways of gut-brain communication
1. Vagus nerve (#272)
Carries 80% of gut → brain signals. The vagus is the "brain's cable". Vagal stimulation can lower depression.
2. Neurotransmitter synthesis
| Substance | % synthesized in gut |
|---|---|
| Serotonin | 90% |
| Dopamine | 50% |
| GABA | Partial |
| Acetylcholine | Partial |
3. Short-chain fatty acids (SCFAs)
Gut microbes ferment dietary fiber → produce SCFAs (butyrate, acetate, propionate) → cross the blood-brain barrier → reduce brain inflammation, raise BDNF.
4. Immunity (cytokines)
Microbiome imbalance → gut "leakiness" (leaky gut) → inflammatory cytokines → brain inflammation → depression. A major mechanism of chronic inflammatory depression.
3. The birth of psychobiotics
John Cryan and Ted Dinan (University College Cork, Ireland) coined "Psychobiotics" in 2013 Biological Psychiatry: "live microorganisms that, when ingested in adequate amounts, produce a mental-health benefit".
Representative strains:
- Lactobacillus rhamnosus JB-1 (GABA effect; lowers depression in animals)
- Bifidobacterium longum 1714 (stress ↓, memory ↑)
- Lactobacillus helveticus + Bifidobacterium longum combination (Messaoudi 2011, human depression ↓)
4. Liu 2019 meta-analysis
Liu et al. (2019) Neuroscience and Biobehavioral Reviews, 16-RCT meta-analysis:
| Metric | Effect size (SMD) |
|---|---|
| Depression | -0.27 |
| Anxiety | -0.42 |
| Stress | -0.30 |
Effect sizes are smaller than medication / exercise but valuable as a "side-effect-free adjunct". Effects begin with 4–8 weeks of consistent intake.
5. Microbiome-mental-illness associations
| Mental illness | Microbiome pattern |
|---|---|
| Depression | Bifidobacterium / Lactobacillus ↓ |
| Anxiety | Diversity ↓ |
| Autism | Specific-strain abnormalities |
| Dementia (#254) | Pro-inflammatory gut bacteria ↑ |
| IBS | Hypersensitive gut-brain communication |
| Obesity / diabetes | Firmicutes / Bacteroidetes ratio shift |
6. 5-axis recovery
1. Fiber (prebiotics)
"Prebiotics" = food for microbes. Fiber raises gut-microbe diversity and SCFA production:
- Vegetables (5–7 colors)
- Fruits (with skin)
- Whole grains (brown rice, oats, barley)
- Beans, lentils
- Nuts, seeds
- 30+ g/day (Korean average ~20g)
2. Fermented foods (probiotics)
Korea is a "treasure trove" of fermented foods:
- Kimchi: diverse Lactobacillus (especially kimchi-origin Leuconostoc / Weissella)
- Doenjang / gochujang: Bacillus / fungi
- Cheonggukjang: Bacillus subtilis
- Makgeolli: Saccharomyces (small amounts)
- Yogurt / kefir: Western probiotics
- Sauerkraut / miso / kombucha: global options
Note: kimchi fermented 1–2+ weeks has rich microbes; fresh kimchi does not. Commercial processed kimchi (low-salt, pasteurized) has fewer microbes.
3. Judicious antibiotic use
- Korea has one of the highest antibiotic-prescription rates in the OECD
- One course of antibiotics → 6 months – 1 year to recover the microbiome
- Use only when necessary; complete the course
- Probiotics 1–2 months after antibiotic use
4. Stress management
Chronic stress → cortisol → microbiome imbalance. Stress management is gut health. Mindfulness (#191), yoga (#272), nature (#232).
5. Exercise / sleep
- Exercise (#275): raises gut-microbe diversity and SCFAs
- Sleep (#225, #273): deprivation alters the gut microbiome
- Sleep + exercise = core of microbiome recovery
7. The "magic pill" trap
- Commercial probiotic capsules are less effective than diet
- Effects vary by strain; "genus" labeling is insufficient
- Stomach acid is hard to pass — some don't reach the gut
- No long-term effect — gone on discontinuation
- Some Korean commercial products list incorrect microbe counts
Diet first; capsules adjunct.
8. Strengths and weaknesses of Korean diet
Strengths
- Diverse fermented foods in daily diet
- Vegetable intake (quantitatively) adequate
- Traditional-diet fiber
Weaknesses
- Rising processed food / dining out (especially youth)
- Sugar / refined carbs ↑
- Decline in kimchi "fermentation" (processed kimchi)
- Stress / sleep deficits damage the gut
9. Clinical use
- Depression / anxiety: integrated medication + diet + probiotics
- IBS: psychobiotics + CBT
- Autistic children: some-strain (Bifidobacterium) research ongoing
- Dementia prevention: Mediterranean diet (raises microbiome diversity)
- After antibiotics: recovery probiotics
10. Korean resources
- "The Mind-Gut Connection" (Emeran Mayer, Korean edition)
- "The Psychobiotic Revolution" (Cryan / Dinan, Korean edition)
- Korean Microbiome Society
- "Gut-brain" clinics at university-hospital integrative medicine / gastroenterology / psychiatry
- Clinical-use research on Korean-style fermented foods
