What CFU means, and why a weight is not one
The definition of a probiotic has a quantity buried in the middle of it. Everything difficult about reading a probiotic label comes from the fact that the quantity on the label is often measuring something else.
A probiotic is defined as live microorganisms that, when administered in adequate amounts, confer a health benefit on the host. The unit that counts live microorganisms is the colony-forming unit — one particle able to grow into a visible colony. United States regulations, however, require only the total weight of microorganisms on a Supplement Facts panel, not a viable count.
So a milligram figure is legal and a count is optional. The National Institutes of Health’s Office of Dietary Supplements advises choosing products that state colony-forming units at the end of shelf life rather than at manufacture, and cautions that a higher count is not automatically better.
The definition has a quantity inside it
Read it slowly and the hard part is in the middle clause.
The Office of Dietary Supplements at the National Institutes of Health gives the standard definition of a probiotic: live microorganisms that, when administered in adequate amounts, confer a health benefit on the host. It is a good definition and it is doing three separate jobs at once.
The first word is live. A dead organism is not a probiotic under this definition, whatever else it may be. The last clause is confer a health benefit, which is the part every label gestures at. The middle clause is the one nobody reads: in adequate amounts.
That middle clause is a quantity, and it means the definition cannot be satisfied by a name. A bottle that tells you which organisms are inside has told you the first part. A bottle that tells you how many of them are alive has told you the part that makes the word apply at all.
So the question a label has to answer is not “what is in here” but “how much of what is in here is alive”, and those are answered by different kinds of number.
What a colony-forming unit actually counts
The name of the unit is a description of the measurement, which is rarer than it sounds.
A colony-forming unit is exactly what the phrase says: a unit capable of forming a colony. Spread a sample on a growth medium, leave it, and count the visible colonies that appear. Each colony grew from something that was alive and able to reproduce. The count of colonies is the count of colony-forming units.
That is why the unit is not called “cells”. It does not count cells; it counts things that did something. A cell that is present but dead forms no colony and is not counted. A clump of several cells that grows into one colony is counted once. The unit is deliberately conservative about what it credits.
- Colony-forming units (CFU)
- A count of viable, reproducing organisms. The number that matches the word live in the definition of a probiotic.
- Milligrams of a blend
- A weight of powder. It includes whatever is in the powder: live organisms, organisms that are no longer alive, and whatever the organisms were dried onto or mixed with.
- A species name
- Neither a count nor a weight. It identifies what kind of organism the powder was made from, and says nothing about quantity or viability.
- “At the end of shelf life”
- A statement about when the count applies. Without it, a count describes a moment in a factory rather than the moment you swallow the capsule.
Hold those four apart and most probiotic labels become readable in seconds. The question is only ever which of the four you were given, and the answer is frequently the third.
Why a weight is legal and a count is optional
This is not a loophole somebody found. It is what the labelling rules ask for.
ODS states it plainly: United States regulations require the total weight of microorganisms to be declared, rather than a count of viable organisms. A manufacturer that prints a milligram figure for a probiotic blend has done what the law asks. A manufacturer that prints a CFU count has done more.
FDA’s labelling guidance sets out what a Supplement Facts panel must carry, including the rule for a proprietary blend: the blend’s total weight appears on its own line, with the ingredients listed underneath it in descending order of predominance by weight. Weight is the currency the panel is built in, for every ingredient, and organisms are not treated as a special case.
The consequence is worth stating without indignation. A weight and a count are not two ways of writing the same measurement, and one cannot be converted into the other — the conversion depends on facts that appear on no supplement label anywhere.
The trap in a blend total
A proprietary blend gives you one weight for several organisms. Because the ingredients underneath must be in descending order by weight, the first-named organism can be at most the whole total, the second at most half of it, the third at most a third of it.
That is real arithmetic and it is worth doing. But notice what it produces: a ceiling in milligrams, in a unit that was never a count of living things. A tighter bound on the wrong quantity is still the wrong quantity.
At manufacture, or at the end of shelf life
Two labels can print the same number and mean entirely different things by it.
Live organisms die over time. They die faster when they are warm, and warehousing, transport and a kitchen shelf are all warmer than the conditions the organisms were stabilised in. Any count on a probiotic label is therefore a count at a moment, and the moment matters more than the number.
ODS’s advice is direct: choose products that state the colony-forming units at the end of shelf life rather than at the time of manufacture. A count at manufacture describes the best moment in the product’s life. A count at the end of shelf life is a promise about the worst moment you are likely to encounter.
| What the label says | What it tells you | What it leaves open |
|---|---|---|
| “10 billion CFU at time of manufacture” | A real count, taken at the factory. | How many survive shipping, storage and the months before you open the bottle. That figure is not on the label. |
| “10 billion CFU at end of shelf life” | A count the manufacturer is prepared to stand behind through to the expiry date. | Very little, on the count itself. This is the form ODS tells you to look for. |
| “10 billion CFU”, with no timing | That a count was measured at some point. | Which point. In practice an undated count is the earlier, kinder one. |
| “Probiotic blend — 36 mg” | The weight of the powder in a serving. | Everything about viability. A weight cannot be dated into a count, in either direction. |
Scroll this table sideways →
There is a second reason the timing matters, and it is why storage instructions appear on these products at all. A label asking for refrigeration and giving no count has told you the number is fragile and declined to say what it was.
A bigger number is not automatically a better one
The one part of this that runs against the buyer’s instinct.
Once a reader learns to look for a count, the next instinct is to compare counts and prefer the larger. ODS cautions specifically against that: a higher number of colony-forming units does not automatically mean a product is better, and the effective dose is a question about the particular organism and the particular effect being claimed.
The reason is the same reason a strain designation matters. What was studied was a specific organism at a specific amount for a specific outcome. A number that is larger than the studied amount is not more of the finding; it is a different amount, outside the range anyone measured.
Is there a count at all?
If the label declares only a weight, stop here. There is nothing to compare and no arithmetic that will produce one.
Is the count dated?
End of shelf life, or at manufacture. An undated count is the weaker of the two, and should be read as the kinder one.
Is the count attached to a named strain?
A count against a species is a quantity of something you cannot look up. A count against a strain designation can be checked against published work on that strain.
Only then, compare
And compare against what was studied for the effect you care about, rather than against the next bottle on the shelf. Most bottles fail at step one, which is fast to find out and saves the rest of the exercise.
This bottle: 36 mg, and no count in either form
Everything above, applied to the label the rest of this site is about.
The SlimTide panel declares a Probiotic Blend — 36 mg, with three organisms listed underneath it: Bifidobacterium infantis, Clostridium butyricum and Akkermansia muciniphila. Genus and species for all three, which is more than many blends print.
No colony-forming unit count appears anywhere on it. Not at manufacture, not at the end of shelf life, and not undated. The declaration is a weight, which is what United States regulations require and nothing more.
The descending-order rule gives one piece of arithmetic and only one. The first-named organism weighs at most 36 mg, the second at most 18 mg, the third at most 12 mg. Those are real ceilings, honestly derived, and they are ceilings in milligrams of powder rather than counts of living organisms.
The label also asks, twice, for refrigeration — and four lines below that it asks for storage in a cool, dry place. Whatever else that contradiction does, it confirms the count is temperature sensitive on a bottle where the count was never published.
None of this makes the panel dishonest. It is the best-documented label this series of sites has been given, and it declares three real per-serving amounts. It means the one number that would tell you whether the word probiotic applies to what is in the capsule when you swallow it is not on the bottle, and cannot be worked out from what is.
See the SlimTide packs
One, three or six bottles. The price is on the seller’s checkout page, because it is one of the few figures this site was never given — and a figure we were not given is not a figure we will print.
Shipping cost and delivery time were not supplied either, which matters more than usual on a product whose label asks for refrigeration. The guarantee is 60 days from purchase unless the sales page prints longer.
References
The sources behind the numbers on this page.
- Office of Dietary Supplements, National Institutes of HealthProbiotics — Health Professional Fact Sheet · sourceThe definition of a probiotic, what colony-forming units are, the fact that US regulations require total microorganism weight rather than viable CFU, the advice to prefer products stating CFU at end of shelf life, strain specificity, and the safety record in immunocompromised people.
- U.S. Food and Drug AdministrationDietary Supplement Labeling Guide, Chapter IV: Nutrition Labeling · sourceWhat a Supplement Facts panel must carry, and the proprietary-blend rule: the blend's total weight on its own line with the ingredients listed underneath in descending order of predominance by weight.