Gut Microbiome: What Supplements Actually Help (Probiotics, Prebiotics, and More)

The gut microbiome — the ecosystem of 100 trillion bacteria living in your digestive tract — has been called "the second brain." Its influence extends far beyond digestion: 70–80% of your immune cells live in the gut, and 90% of your body's serotonin is produced there.

Supplement companies have exploited this growing interest aggressively. Here's what the evidence actually supports.

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Probiotics: specific strains for specific problems

Probiotics are live bacterial cultures. The mistake most people make is buying a "high CFU" probiotic with 10 random strains and expecting general health benefits. The evidence doesn't work that way.

What matters: Strain specificity. "Lactobacillus acidophilus" covers thousands of strains — their effects differ. Look for documented strain codes (e.g., Lactobacillus rhamnosus GG, Bifidobacterium longum 35624).

Evidence-backed applications:

| Condition | Strains with RCT evidence | |-----------|--------------------------| | Antibiotic-associated diarrhoea | L. rhamnosus GG, S. boulardii | | IBS (diarrhoea-predominant) | B. infantis 35624, VSL#3 | | IBS (bloating/general) | Multi-strain products — inconsistent | | H. pylori eradication support | L. acidophilus + B. lactis as adjunct | | Eczema prevention in infants | L. rhamnosus GG | | Traveller's diarrhoea prevention | S. boulardii |

For general gut health: Evidence is less clear. Some benefit for people after antibiotics or with poor diet, but "general wellness" probiotics are oversold.

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Prebiotics: feeding your existing bacteria

Prebiotics are non-digestible food components (mainly fibres) that selectively feed beneficial bacteria.

Effective prebiotics: - Inulin/FOS: Found in chicory, onions, and garlic. Also available as supplement. Feeds Bifidobacterium and Lactobacillus. - GOS (galacto-oligosaccharides): Strong evidence for Bifidobacterium growth. - Psyllium husk: Soluble fibre that feeds beneficial bacteria and improves bowel regularity. 5–10g/day. - Resistant starch: Found in cooked-and-cooled potatoes and rice. Available as raw potato starch.

Start low and slow: Prebiotics cause gas and bloating if you increase dose too quickly. Begin with 3–5g/day and increase over 2–3 weeks.

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L-Glutamine: gut lining repair

L-glutamine is the primary fuel for enterocytes (intestinal lining cells). Under stress — physical, dietary, or psychological — the gut lining becomes more permeable.

Evidence: Studies in critical care patients show glutamine supplementation reduces gut permeability. Evidence in healthy people and IBS is promising but mixed.

Dose: 5–10g daily, typically first thing in the morning on an empty stomach.

Relevant for: Post-antibiotic recovery, IBS, leaky gut syndrome, intense athletic training.

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Digestive enzymes: targeted use only

Digestive enzyme supplements (lipase, amylase, protease, lactase) are often sold as a general wellness product. The evidence supports specific applications only:

- Lactase: Works for lactose intolerance — take immediately before dairy. - Alpha-galactosidase: Helps with gas from legumes (Beano/similar). Works. - Papain/bromelain: Some evidence for protein digestion support. Modest benefit. - General "broad spectrum" enzymes: Minimal evidence for people with normal pancreatic function.

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Fibre supplements

Inadequate fibre is the most underappreciated driver of poor gut health in the UK. Average intake is 19g/day — the target is 30g/day.

Best supplement options: - Psyllium husk: 5–10g in water daily. Most evidence for cholesterol reduction and bowel regularity. - Partially hydrolysed guar gum (PHGG): Dissolves clear, no texture. Excellent for IBS. - Acacia fibre: Gentle, low-gas, good for sensitive guts.

Not ideal: Wheat bran — can worsen IBS symptoms in some people.

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What doesn't have good evidence

- Charcoal supplements for detox: Activated charcoal binds things in the gut — including medications and nutrients. No evidence for "detox" in healthy people. - "Gut cleanse" programmes: No evidence. Your liver and kidneys detoxify continuously. - Colonic irrigation: Not evidence-based; can cause harm.

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The evidence-based approach to gut health

1. Increase dietary fibre to 30g/day through whole foods first, supplements second 2. Take a strain-specific probiotic if you have a condition with documented benefit 3. Add a prebiotic (inulin, GOS, or psyllium) to feed beneficial bacteria 4. Consider L-glutamine if you've had antibiotics or have gut permeability issues 5. Avoid antibiotic overuse — the single biggest driver of microbiome disruption

The gut microbiome takes 3–6 months of consistent intervention to shift meaningfully. Short-term supplement courses won't rebuild a damaged microbiome.