Probiotics: why the strain matters more than the CFU
Probiotic labels compete on one number: CFU, or colony-forming units. Fifty billion beats ten billion, so fifty billion must be better. That is not how the evidence works, and the number the research actually attaches to is usually printed in smaller type.
Why strain is the unit that matters
Probiotic effects don't generalize across the family tree. NIH puts it plainly: if a specific kind of Lactobacillus helps prevent an illness, that doesn't mean another kind of Lactobacillus — or any Bifidobacterium — would do the same thing.
So a trial showing benefit for Lactobacillus rhamnosus GG is evidence about GG. It is not evidence about "Lactobacillus," and it is certainly not evidence about a blend of eight species you've never seen tested together. A bacterial name has three parts — genus, species, and the strain code — and the evidence lives on the third one.
What the evidence actually supports
Less than the category implies, and NIH is candid about the gap: we still don't know how much people would need to take, or who is most likely to benefit. That's an honest description of a field that's promising rather than settled.
It's also why the CFU arms race is misleading. A large dose of a strain with no trial behind it is a large dose of nothing in particular.
Regulation, and the safety case that isn't hypothetical
Most probiotics are sold as dietary supplements, which don't require FDA approval before marketing. That's the same regime as the rest of the supplement aisle, and it means label claims aren't vetted.
Risk is also not evenly distributed. It's higher in people who are severely ill or immunocompromised. The FDA has warned about invasive disease in hospitalized preterm infants given probiotics, including one infant death in 2023, and no probiotic is FDA-approved as a drug for infants of any age.
What to actually look for
The full strain designation — genus, species and strain code, like Bifidobacterium lactis HN019. A stated use case that strain has been tested for, rather than "supports gut health." And if you're immunocompromised, pregnant, or buying for an infant, treat this as a conversation with your clinician rather than a shelf decision.
