Probiotic labels: strains, CFU, and what the big number means

A probiotic name has three parts

The NIH Office of Dietary Supplements explains that probiotics are identified by their specific strain, which includes the genus, the species, a subspecies where one applies, and an alphanumeric strain designation. Lacticaseibacillus rhamnosus GG, often shortened to LGG, is one of the most studied examples.

Many labels stop after the first two parts. "L. rhamnosus" tells you which kind of bacteria is in the capsule. It does not tell you which strain, and that is the part research results attach to.

Why the strain matters

According to the NIH, the effects of probiotics can be specific to particular strains. Some mechanisms are shared across many strains; others are unique to one species or strain. That is why clinical recommendations name strains rather than whole groups.

A practical consequence: a study of one strain of L. rhamnosus is not evidence for every product that lists L. rhamnosus. When a label does not give strain designations, the honest reading is that you cannot match it to specific studies.

What a CFU count measures

CFU stands for colony-forming units, an estimate of how many live, viable microorganisms are present. U.S. regulations require probiotic supplements to list the weight of the microorganisms; that weight can include dead cells, so it does not tell you how many are alive.

In 2018 the FDA issued draft guidance saying it intends to allow CFU to be declared in addition to the required weight.

Read the footnote next to the CFU figure. "At time of manufacture" means the count when the product was made. Live microorganisms decline over time, especially with heat and moisture, so the NIH suggests looking for products that state the CFU count at the end of shelf life.

A bigger number is not automatically better

Because effects depend on the strain and the amount used in the study of that strain, a CFU total on its own is not a useful way to compare products. Two labels with the same number can contain entirely different organisms.

Storage and safety

Check the storage instructions. Some probiotics need refrigeration and others are made to be kept at room temperature.

The NIH notes that probiotics are unlikely to cause harm in healthy people and that side effects, such as gas, are usually mild. Rare serious infections have been reported, mostly in people who were severely ill or had weakened immune systems, and the FDA has warned that probiotics can cause serious infections in preterm infants. If you are immunocompromised, seriously ill or in the hospital, talk with your clinician before taking one.

How we describe our own product

DECLARED Probiotic contains a 317 mg blend of 13 bacteria per capsule, declared as 20 billion CFU at the time of manufacture. The label lists each organism by genus and species, for example L. acidophilus and B. lactis.

The label does not give strain designations, and we do not have them on file. For that reason we do not connect this product to the results of any specific probiotic study. The batch report we reviewed recorded a total probiotic count of 30 billion CFU per gram of blend, and its microbial safety tests were within specification. That report did not include a heavy-metals test.

What a label cannot settle

A probiotic label can tell you which organisms were added, how many were live at a stated point in time, and how to store the product. It cannot tell you how any particular person will respond. Research on probiotics is active, results vary by strain and by condition, and many questions remain open.