Probiotics: How to Choose the Right Strain for Your Gut

What probiotics actually do in your gut

You've probably taken probiotics at some point — without being entirely sure which ones to choose. Capsules, yoghurt, kefir, supplements from the pharmacy — the options are endless, and the promises even more so. Yet one golden rule stands above the rest: a probiotic's effectiveness depends above all on the specific strain, not simply the bacterial genus or the billions of CFUs advertised on the label.

According to the FAO/WHO definition, probiotics are live micro-organisms which, when administered in adequate amounts, confer a health benefit on the host. That word "adequate" conceals a complex reality: a strain that works well for preventing antibiotic-associated diarrhoea won't necessarily relieve irritable bowel syndrome (IBS).

Three key mechanisms to understand

To make an informed choice, it helps to understand how probiotics actually work:

• Competition and protection: Certain strains such as Lactobacillus rhamnosus GG adhere to the intestinal mucosa and block pathogens by competing for their attachment sites, whilst also producing natural antimicrobial substances.
• Immune modulation: Bifidobacterium longum BB536 stimulates the production of IL-10, an anti-inflammatory cytokine, and reduces TNF-α, supporting a more balanced immune response.
• Strengthening the intestinal barrier: Lactobacillus reuteri acts on the tight junction proteins between intestinal cells, limiting excessive permeability — sometimes referred to as "leaky gut".

Another particularly interesting player is Akkermansia muciniphila, which produces short-chain fatty acids (SCFAs) such as butyrate — the primary fuel for colon cells and a key regulator of local inflammation.

Which strain for which situation?

Here's where things get practical. This is what recent studies in human populations show:

• Acute diarrhoea (particularly in children): Lactobacillus rhamnosus GG is the best-documented strain — a meta-analysis covering more than 12,000 participants demonstrated a reduction in symptom duration of approximately one day.
• Antibiotic-associated diarrhoea: Saccharomyces boulardii (a yeast, not a bacterium!) reduces the risk by 42%, according to a meta-analysis of 21 clinical trials published in 2023.
• Irritable bowel syndrome (IBS): Bifidobacterium longum 35624 showed a 30% reduction in symptom scores versus placebo in a phase III trial. Lactobacillus plantarum 299v is also recommended as a second-line option by the World Gastroenterology Organisation (WGO, 2023).
• Metabolic health: Pasteurised Akkermansia muciniphila demonstrated significant weight loss (-2.3 kg versus placebo) and improved blood glucose levels in a 2024 trial, via SCFA production and GLP-1 stimulation.

An important reminder: approximately 10–15% of the global population lives with IBS, and 70% of patients respond at least partially to targeted probiotics. But "targeted" is the operative word.

What to check on the label

When faced with a shelf full of supplements, here is what you should always look for:

• The full strain designation: genus + species + strain code (e.g. Lactobacillus rhamnosus GG). Without this code, there is no way to link the product to clinical studies.
• The dose: a minimum of 1 billion CFU (colony-forming units) per serving; ideally 10 to 50 billion per day for a systemic effect.
• Viability at expiry: the CFU count should be guaranteed up to the best-before date, not just at the point of manufacture.
• Gastric resistance: only 10–50% of probiotics survive the acidity of the stomach (pH < 2). Opt for microencapsulated formulations or strains specifically selected for their resistance.

Where do fermented foods fit in?

Yoghurt, kefir, raw sauerkraut, kimchi — these foods do contain natural probiotics. However, their strain composition is variable and not guaranteed, and pasteurisation frequently eliminates all living micro-organisms.

Milk kefir can reach 10⁸ to 10⁹ CFU/ml, and unpasteurised sauerkraut up to 10⁹ CFU/g — which are genuinely interesting levels. But for a specific indication (IBS, diarrhoea, and so on), supplements offer greater reliability in terms of both strain and dose.

One effective tip: combine probiotics with prebiotics. Prebiotics (inulin and FOS, found in asparagus, artichokes, and unripe bananas) feed the beneficial bacteria and can increase their survival in the colon tenfold.

Key takeaways

• There is no universal probiotic: choose according to your specific needs.
• Allow 4 to 8 weeks: this is the recommended timeframe for assessing any real effect.
• Consult a healthcare professional if you are immunocompromised — probiotics are contraindicated in such cases.
• More billions does not mean better: it's the right strain, at the right dose, that makes the difference.

Your microbiome is unique. Choosing wisely means giving it exactly what it genuinely needs — nothing more, nothing less.