What Works Skin — Independent · Evidence-First · Ad-FreeIssue 014 · 20 April 2026 · Next: 04 Maywhatworksskin.com
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Supplement · Barrier & inflammation · Probiotics

P. 19 · BRIEF

Probiotics.

Strain matters. Adult skin data thinner than the marketing.

The most exciting supplement category by trend volume; the most disappointing by adult skin endpoints. The strain-specific paediatric eczema literature is genuine; the adult skin literature, particularly for acne and rosacea, is far less developed than the marketing implies.

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— § 02

What the literature shows.

Paediatric eczema prevention
RCTs, L. rhamnosus GG

Real, strain-specific. Most consistent in high-risk infants with family history of atopy.

65%
Adult atopic dermatitis
Mixed RCTs

Smaller, less consistent than paediatric data. Reasonable trial; manage expectations.

40%
Inflammatory acne (adult)
Small early trials

Promising mechanistic story (gut-skin axis); the clinical trial evidence has not yet caught up.

35%
Rosacea
Tiny pilot trials

Early signals on flushing and inflammation; not yet a treatment recommendation.

30%
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— § 03

Forms and bioavailability.

L. rhamnosus GG 10⁹ CFU

Absorption · Strain-specific

The best-studied strain in paediatric eczema prevention. Adults: reasonable trial.

Multi-strain blends 10–25 billion CFU

Absorption · Variable

Most commercial 'skin probiotic' products. Strain blends rarely match published trial protocols.

L. paracasei ST11

Absorption · Strain-specific

Adult atopic skin data; small trials show modest barrier endpoints.

Bifidobacterium-led blends

Absorption · Strain-specific

Different mechanism; some constipation-and-skin overlap. Limited skin-specific evidence.

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Bottom line

The category where strain discipline matters more than dose. Reasonable trial in atopic skin; weak evidence in adult acne. Match the strain to the trial.

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— § 04

Frequently asked.

Does it matter which strain?

Yes, dramatically. Probiotic effects are strain-specific, not species-specific, and almost never genus-specific. A study showing benefit from L. rhamnosus GG does not generalise to L. rhamnosus more broadly, let alone to other Lactobacillus species. Match the strain on the label to the strain in the published trial; if unmatched, the published data does not apply.

How long should I trial?

12 weeks at 10⁹–10¹⁰ CFU/d. If no measurable effect by then, the strain is not your bottleneck. Switch strain or pivot strategy rather than escalating dose indefinitely.

Does fermented food substitute?

Partly. Yogurt, kefir, kimchi, and dosa-batter ferments deliver live cultures, but at variable strain composition and CFU count. They are reasonable as part of a broader gut-health strategy; they are not equivalent to a strain-matched supplement when treating a specific endpoint.

Should I refrigerate?

Most live-culture probiotics genuinely require refrigeration to maintain CFU count to expiry. Spore-based or shelf-stable formulations are exceptions. Buy from a vendor with appropriate cold-chain practice.