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Probiotics · Moderate evidence
What probiotics are for and how to choose them
The short answer. Probiotics are the supplement where thinking in categories works worst: the effect does not belong to "probiotics" or even to a species, it belongs to a specific strain studied at a specific dose. The ISSN position stand reports effects on digestion, gut health and the frequency of respiratory infections in athletes, always tied to identified strains. A tub that does not name the strain can promise nothing.
There are studies in favour, but few, with small samples or measuring markers instead of outcomes. No figure on this page is attributed to a specific product: the effect belongs to the ingredient and every number carries its source next to it.
01 What it does and how much
Each row is a measured effect, with the figure the study gives and the source number. Where the evidence gives no figure, the column says so instead of making up a percentage.
| Effect | How much | Detail |
|---|---|---|
| What matters | the strain, not the genus | Lactobacillus rhamnosus GG and any other Lactobacillus rhamnosus are not the same. An honest label carries genus, species and strain designation. [1] |
| Respiratory infections in athletes | fewer and shorter episodes | It is one of the effects the position stand reports, with specific strains and in populations with high training loads. [1] |
| Digestive discomfort in endurance | improves in some trials | The long-distance runner’s gut is a real problem, and there are trials with specific strains there. [1] |
| Usual dose in the studies | 10^9 to 10^11 CFU a day | One billion to one hundred billion. More CFU of a strain that has not been studied does not make up for it not having been studied. [1] |
02 How to take it
- Dose. The one from that strain’s trial. In general, between one billion and one hundred billion CFU a day.
- When. As the manufacturer states; many strains are recommended with food or on an empty stomach depending on their acid resistance.
- How long before you notice it. Weeks. And the effect disappears when you stop: they do not colonise permanently.
Details that change the result
- Look on the label for the full strain designation. If it only says "Lactobacillus, 10 billion", you do not know what you are taking.
- Check that the CFU are guaranteed until the expiry date and not only at manufacture.
- Yoghurt, kefir, sauerkraut or kimchi provide live bacteria every day and cost less than a tub.
- With immunosuppression or serious illness, the decision is medical: "just to try" does not apply here.
03 When it makes sense and when it does not
Worth it if
- Very high training loads with repeated respiratory infections.
- Digestive discomfort in long endurance sports.
- After a course of antibiotics, with the appropriate strain.
Not worth it if
- As a generic wellbeing supplement with no specific goal.
- Buying by the number of CFU or number of strains, which is how they are sold, not how they work.
- With immunosuppression, without medical judgement.
04 Sources
Every figure above comes from here. They are position stands from scientific societies, meta-analyses and peer-reviewed reviews: if a claim cannot hang from one of these lines, it is not on the page. References are cited in their original form.
- Jager R et al. ISSN position stand: Probiotics. J Int Soc Sports Nutr. 2019;16:62. Los efectos son especificos de cepa; dosis habituales de 10^9 a 10^11 UFC al dia. doi:10.1186/s12970-019-0329-0
This is not medical advice. It is what some studies say about an ingredient, not a protocol for you. With an illness, medication or pregnancy involved, the decision belongs to a healthcare professional who knows you.
Questions about probiotic
Answered with the data on this page
- Do probiotics do anything?
- It depends on the strain and the goal. The ISSN position stand reports effects on digestive health, the frequency of respiratory infections in athletes and some immune markers, always with specific strains at specific doses. What has no support is "probiotics" as a generic category.
- How many CFU should a probiotic have?
- Studies use one billion to one hundred billion CFU a day depending on the strain. But the number of CFU is the wrong criterion for comparing: one billion of a strain with trials is worth more than one hundred billion of one without. Check that the CFU are guaranteed until expiry, not only at manufacture.
- Do they need to be refrigerated?
- It depends on the product. Some strains are freeze-dried and stable at room temperature, and others need cold to keep their CFU. The label says so, and it is important information: a badly stored probiotic is a tub of dead bacteria.
- Is it better to eat yoghurt or take a tub?
- For general wellbeing, the traditional fermented foods (yoghurt, kefir, sauerkraut, kimchi) provide live bacteria every day and cost much less. The tub makes sense when you are after a specific strain for a specific reason, which is when probiotics stop being a trendy product and become a tool.
And now, which to buy
Prices, kept apart from the evidence
This page explains what probiotic does and deliberately names no brand. The comparison is the other half: 107 products from 8 stores from €0.044 per capsule, sorted by price and by how verifiable their certification is.
Other guides
What each supplement does, with its sources