Gut microbiome · your second genome肠道菌群 · 比基因还个性化的第二基因组

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30 万亿菌 · 多样性 = 健康指标 · 纤维 + 发酵食 + 多样植物 · 益生菌不是万能丸 · 粪菌移植 (FMT) 限适应症

Gut microbiome · basics

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肠道菌群 · 基础解剖学

Gut microbiome = the ~ 30-100 trillion microbes in the human gut + their collective genome, with ~ 100-150× more total genes than the human genome (Valdes 2018 BMJ).

Key facts:

Count: humans have ~ 30 trillion cells (new estimate; older 100 trillion was high), and microbe count is on the same orderSpecies: a healthy adult gut hosts ~ 500-1000 species, 95% in the colonMass: total ~ 200-500 g (about a bag of rice)Metabolic capability: microbes encode enzymes humans lack → digest fiber / synthesize vitamins / detoxifyIndividual variation: any two people share < 50% microbiome similarity (vs > 99% for human genome)
Main functions:

Digest undigested food: mainly insoluble + soluble fiber → short-chain fatty acids (: acetate / propionate / butyrate)Synthesize vitamins: B-vitamins (especially partial B12 / K2 / folate)Train the immune system: ~ 70% of immune cells around the gut; diverse childhood microbiome → lifelong immune balanceMetabolic regulation: SCFAs affect satiety hormones ( / PYY) + insulin sensitivityGut-brain axis: microbial metabolites (tryptophan derivatives / SCFAs / LPS) → vagus nerve + blood → brainBarrier protection: healthy microbiome occupies niches → suppresses pathogens
Measures of a 'healthy microbiome':

Diversity (alpha diversity): species richness. Healthy > chronic-disease patients > post-antibiotic. Reduced diversity is an early signal for IBD / diabetes / CVD / depression (Valdes 2018 review)Stability: same person's microbiome similar over timeKey species abundance: Akkermansia muciniphila (mucus layer) / Faecalibacterium prausnitzii (butyrate producer) — low abundance of these 'beneficials' correlates with many diseases
'Good bug' / 'bad bug' is an oversimplification:

The same species can be beneficial or harmful depending on host and environmentThe microbiome is an ecosystem, not a 'friend-or-foe' listWhat matters is overall balance + diversity, not any single star species
Microbiome through life stages:

Fetus: once thought sterile; modern evidence suggests small in-utero presenceBirth mode: vaginal → mother's vaginal flora; C-section → skin flora (mild upward link with allergy / autoimmune risk)Infancy (0-3 y): breast vs formula microbiome differs, introduction of solids triggers diversity leapAdulthood: relatively stable (unless antibiotics / major illness / relocation)Old age: diversity declines, correlates with frailty / dementia / mortality risk

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