Supplements
Probiotics Are Strain Specific and the Label Rarely Is
Evidence for one bacterial strain does not transfer to another strain of the same species. That single fact explains most of the gap between what probiotic research shows and what probiotic products claim.
The options around probiotic strains, doses and label accuracy are set out side by side below, with the conditions that genuinely favour one over the other.
The difference in one place
- Genus, species and strain are three levels, and the evidence attaches to the strain.
- Counts are usually stated at manufacture rather than at the end of shelf life.
- Serious infections from probiotics are rare and concentrate in very unwell patients.
Three levels of a name
A bacterial name has a genus, a species and then a strain designation that usually looks like a short code of letters and numbers. Two strains of the same species can differ substantially in their genes, in what they produce and in whether they survive the gut. This is not a technicality, because the trials that produced any given result used one specific strain.
A product listing only the genus and species is therefore not telling you which of the evidence applies to it. Comparing this to medicines makes the point plainly, since nobody would accept a painkiller labelled only with its chemical family.
What the number on the front means
Colony forming units count the organisms capable of growing, and the figure quoted is often the count at the time of manufacture. Live organisms die over time, faster at higher temperatures, so the count at the end of shelf life can be considerably lower. A label stating the count at expiry under stated storage conditions is meaningfully more informative than one stating it at production.
A larger number is also not automatically better, since the trials that found effects used particular doses rather than the highest available. Stomach acid destroys a proportion of what you swallow, which is why delivery format is part of the product rather than a detail.
Where the evidence is comparatively strong
The clearest signals concern a few specific situations rather than general wellbeing, and even there the results differ between strains. Diarrhoea associated with antibiotic courses is the most studied of these, with a body of trials that is reasonably large by supplement standards. Certain infant conditions and some outcomes after particular bowel surgery also have a more developed literature behind them.
Where the claim is carefully worded, these are narrow, defined uses with a measurable endpoint, which is exactly why usable evidence accumulated there and not elsewhere. Claims about mood, immunity, skin and general energy sit far ahead of anything that has been established.
Colonisation is not what happens
The gut already contains an enormous established population, and a swallowed strain arrives as a small minority into an occupied space. Studies that track a supplemented strain generally find it passes through, appearing in stool while the product is taken and disappearing after it stops.
Against the trial data, any effect is therefore more likely to come from transit than from permanent residence, which changes what continuous use means. It also means benefits, where they exist, tend to stop when the product stops, which sellers rarely state.
Rebuilding a microbiome is a phrase without an agreed technical meaning, and no product has demonstrated it.
Foods and the same question
Fermented foods contain living organisms, usually in quantities and combinations that vary between batches and producers. They are food, they are generally enjoyable, and there is no reason to discourage them for most people.
Whether the organisms in a given jar match the strains studied for any effect is almost never established. Many pasteurised versions on shelves contain no living organisms at all, which is a labelling question worth checking. Eating fermented food because you like it is a sound reason and does not require an evidential claim.
Where safety is a real question
For healthy people, probiotics are among the lower-risk supplement categories, with wind and bloating the usual complaints. The picture is different for the severely ill, for people with weakened immune systems, and for those with central venous lines.
Bloodstream and other invasive infections traced to probiotic organisms have been reported in those groups, which is a serious if uncommon outcome. Very premature infants are also a group where use is a clinical decision made in a hospital, not a purchase. Anyone in those categories should ask their medical team rather than judging from the shelf.
Side by side
| Consideration | What it means in practice |
|---|---|
| Three levels of a name | Genus, species and strain are three levels, and the evidence attaches to the strain. |
| What the number on the front means | Counts are usually stated at manufacture rather than at the end of shelf life. |
| Where the evidence is comparatively strong | Serious infections from probiotics are rare and concentrate in very unwell patients. |
The takeaway
Ask which strain, ask what the count means at expiry, and ask which trial used that exact organism. Most labels cannot answer the first question.
An extraordinary mechanism needs better evidence than testimonials, and usually has less.
Questions readers ask
Should I take a probiotic with antibiotics?
This is one of the better-studied uses, and it depends on the strain, the antibiotic and the person. Ask the prescriber or pharmacist, particularly if you have a weakened immune system.
Do I need to refrigerate them?
It depends on the organism and the formulation, and the label should state it. Storage above the stated temperature reduces the live count, which is one reason the count at expiry matters more than the count at manufacture.
