Supplement Guide | Probiotics 03 | The Number on the Label Is Only the Beginning

This is Part 3 of SuppCo's 4-part series on probiotics. We'll cover the biology of your gut microbiome, what probiotics actually are, and how the full family of "biotics" fits together.

To read part 1 in the series, click here.

To read part 2 in the series, click here.

You found a probiotic with the right strain for your goal. You checked the evidence. Then you looked at the label. One product says 10 billion CFU. Another says 10 billion AFU. A third says 50 billion CFU. None of them explain what any of it means.

The strain matters enormously, as we covered in Part 2. But whether those bacteria are alive and capable of surviving to your large intestine is a separate question entirely. And the number on the front of the label answers it less reliably than most people assume.

A probiotic label tells you how many bacteria were in the product. It does not always tell you how many will make it to your gut.

CFU: The Standard Everyone Uses

CFU stands for colony-forming units. It is the traditional way probiotics have been measured for decades. A sample is spread on a growth medium, incubated, and the resulting bacterial colonies are counted. Each colony is assumed to have grown from one viable cell. CFU represents the number of bacteria in a sample that are capable of dividing and forming colonies. 

CFU is the measurement behind virtually every published clinical trial. When the study in Part 2 showed that the probiotic LGG reduced ADD risk by 49%, the dose was 10 to 20 billion colony-forming units. It is inexpensive, well-established, and the standard regulatory agencies recognize.

The limitations are palpable. CFU only counts bacteria that grow under specific lab conditions. Some viable bacteria won't grow on a standard culture plate even though they are alive and functional in your gut. CFU counts may not reflect how many microbes are actually active and functional when you take them.

AFU: The More Precise Measurement

AFU stands for active fluorescent units. Instead of growing colonies on a plate, AFU uses flow cytometry, a laser-based process that counts viable microorganisms by detecting how they interact with fluorescent dyes. Cells with intact membranes light up differently than dead or damaged cells.

AFU can detect cells that are viable but not easily cultured, a common feature of spore-forming probiotics, and provides a more consistent view of what is actually inside a capsule. AFU counts are consistently higher than CFU counts from the same sample, not because AFU inflates numbers, but because CFU misses bacteria that AFU catches.

The caveat: CFU remains the more useful standard for label comparison because it is what many clinical trials use. A product measured in AFU cannot be directly compared to a clinical trial dosed in CFU. Both are legitimate. They are just measuring different things. Part 4 will show you how to use either number intelligently at the shelf.

What Actually Reaches Your Gut

Counting bacteria in a lab and counting bacteria that survive the trip through your stomach are two different things. Gastric acid at a pH of 1.5 to 3.5 kills a significant portion of unprotected bacteria before they reach the large intestine. The CFU or AFU count on a label does not tell you what survives.

Three approaches can address this:

• Strain selection. Some strains are inherently resilient. Simulations show that LGG tolerates stomach acid at pH 2.5 for up to four hours. Spore-forming strains like Bacillus coagulans enter a dormant state that survives acid, heat, and moisture, germinating only once they reach the intestine. Strain-level survival data is a reliable indicator. It is also the hardest thing to find on a standard label.

• Enteric coating. Capsules designed to pass through the stomach intact and dissolve in the small intestine. Works in theory, but evidence on whether coatings dissolve in the right location is mixed.

• Microencapsulation. Entraps probiotic bacteria within a protective material to improve their survival and functionality.

Inherently resilient strains tend to outperform delivery technology. A well-researched spore-forming strain appears to outperform a fragile strain in an elaborate capsule.

The Number That Matters

The label may state that the count is "at time of manufacture," a number that is likely inadequate if you purchase the product close to the end of its shelf life. Most probiotic strains suffer cell count decline over the course of shelf life.

International Scientific Association for Probiotics and Prebiotics (ISAPP) advises manufacturers to list the total CFU through the expiration date, and suggests that consumers avoid products that list CFU only at time of manufacture. A product guaranteeing 10 billion CFU at expiry under proper storage conditions is a more reliable label than one showing 50 billion CFU at manufacture with no expiry guarantee.

Heat, moisture, and light are the enemies. Refrigeration can meaningfully preserve  bacterial viability. Freeze-dried, shelf-stable products can maintain viability without refrigeration, but quality varies widely by manufacturer.

The number that matters is not the one on the front of the label. It is the one guaranteed to survive until the expiry date.

Three Frontiers Worth Watching

Psychobiotics. Probiotic strains studied for their effects on mood, stress, and anxiety through the gut-brain axis. Possible mechanisms include the role of bacteria in the production of neurotransmitters, including serotonin and GABA, as well as reductions in systemic inflammation. Specific strains with emerging evidence include Lacticaseibacillus casei Shirota, Lactobacillus gasseri CP2305, Lactiplantibacillus plantarum PS128, and Bifidobacterium longum R0175. The science is real. The commercial overclaiming is already beginning. Watch this space carefully.

Gerobiotics. An emerging concept in geroscience involving probiotics to target key aging processes, including inflammation, cognitive decline, and metabolic function. The biological rationale is grounded in well-documented changes to the microbiome associated with age, including declining microbial diversity, falling Bifidobacterium populations, and increased pro-inflammatory bacteria. Clinical evidence in humans is early, but the mechanistic case is compelling.

The estrobolome. A subset of the gut microbiome with direct relevance to women's hormonal health. The estrobolome is defined as the aggregate of enteric bacteria capable of metabolizing estrogen, playing a role in regulating estrogen levels and influencing hormone-related conditions. Enzymes involved in the mechanism include beta-glucuronidase, which reactivates estrogen for recirculation. Dysbiosis (an imbalance in types and amounts of microorganisms) of the estrobolome has been associated with PCOS and certain menopausal symptoms. Targeted probiotic interventions here are at an early stage, but represent one of the most clinically interesting frontiers in the field.

What's Coming in Part 4

Parts 1 through 3 have given you the biology, the evidence, and the measurement science. Part 4 is where all of it becomes actionable. We will walk through how to read a probiotic label from top to bottom, how to match a strain to a specific goal, what to look for and what to avoid when you are standing at the shelf.