The Gut Microbiome and Ageing: What Changes and Why It Matters

How the microbiome changes with age

In healthy young adults, the gut microbiome is characterised by high diversity — many different species filling different ecological niches — and a composition dominated by beneficial genera including Bifidobacterium and Lactobacillus. With age, several consistent changes occur:

Declining diversity

Alpha diversity (species richness and evenness) typically declines with age. Studies of centenarians — people who live to 100+ in good health — consistently find unusually high gut microbiome diversity, suggesting that maintaining diversity is associated with healthy ageing.

Reduced beneficial bacteria

Bifidobacterium species, which produce short-chain fatty acids (SCFAs) that nourish the gut lining and reduce inflammation, decline significantly with age in most Western populations.

Increased inflammatory bacteria

Certain pro-inflammatory bacterial species increase with age, contributing to 'inflammageing' — the chronic low-level inflammation associated with most age-related diseases.

🔬 The gut-brain axis

The gut produces approximately 90% of the body's serotonin and communicates bidirectionally with the brain via the vagus nerve. Changes in microbiome composition are increasingly linked to mood, cognitive function and neurodegenerative disease risk.

What drives microbiome ageing?

Diet — the dominant factor

Dietary fibre is the primary food source for beneficial gut bacteria. The typical Western diet — low in fibre, high in processed food and added sugar — is significantly associated with reduced microbiome diversity independent of age. Conversely, high-fibre diets (30g+ per day) consistently support microbiome diversity.

Exercise

Regular physical activity increases gut microbiome diversity independently of diet — athletes show measurably more diverse microbiomes than sedentary individuals with comparable diets.

Antibiotics

Each antibiotic course causes significant microbiome disruption, typically recovering over weeks to months but rarely returning completely to baseline. Cumulative antibiotic exposure over a lifetime contributes to microbiome ageing.

✅ The 30-plant rule

Consuming 30 different plant foods per week — across all categories including vegetables, fruits, legumes, wholegrains, nuts, seeds and herbs — is associated with significantly higher microbiome diversity. Variety, not just quantity, is key.

Dietary factors and gut microbiome diversity

Factor	Effect on Diversity	Evidence Level
High dietary fibre (30g+/day)	Significantly increases	Strong
30+ plant foods/week	Significantly increases	Strong (American Gut Project)
Regular exercise	Increases independently	Moderate–strong
Fermented foods (yoghurt, kefir)	Increases diversity markers	Moderate (Stanford RCT 2021)
Ultra-processed food diet	Significantly reduces	Strong
Antibiotics	Disrupts — recovery variable	Strong
Heavy alcohol use	Reduces diversity, increases permeability	Strong

Frequently asked questions

Do probiotic supplements work?

The evidence for commercial probiotic supplements is mixed and highly strain-specific. They can be useful for specific conditions (antibiotic-associated diarrhoea, IBS) but there is limited evidence for general gut health improvement in healthy people. Fermented foods (live yoghurt, kefir, kimchi, sauerkraut) show stronger evidence for increasing microbiome diversity than most supplement products.

Can you 'reset' your gut microbiome?

No single intervention resets the microbiome. But sustained dietary change — particularly increasing fibre and diversity of plant foods — produces measurable microbiome changes within 2–4 weeks. The microbiome is highly responsive to diet but also returns toward its baseline when diet reverts.

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The Gut Microbiome and Ageing: Why Your Gut Bacteria Matter