How to Read a Probiotic Label: CFU, Strains, and What Actually Matters
Disclaimer: This content is for informational purposes only and is not medical advice. Consult a healthcare professional before starting any supplement. Individual results vary.
You are standing in the supplement aisle (or scrolling a product page) looking at two probiotic bottles. One says “10 Billion CFU.” The other says “200 Billion CFU.” The second one is £20 more expensive. Your instinct says: more bacteria, better product.
That instinct is wrong — and understanding why is the single most useful thing you can learn about probiotics.
What CFU Actually Means
CFU (Colony Forming Units) is the count of bacteria in a product that are capable of dividing and forming a visible colony on a lab growth plate. It is a measure of viable bacterial quantity at the time of manufacture (or, ideally, at the expiry date if the label says “at time of expiry” rather than “at time of manufacture”).
CFU vs AFU: The Counting Method Debate
Some newer products (primarily Seed DS-01) measure potency in AFU (Active Fluorescent Units) using flow cytometry — a method that counts both colony-forming and dormant-but-viable bacteria. AFU typically reads higher than CFU for the same sample (sometimes 20-40% higher).
The practical question — does AFU translate to better clinical outcomes? — has not been directly tested in human trials. Both methods are valid measures of bacterial viability; neither substitutes for strain-indication evidence.
The Only Number That Matters: Strain Identity
A probiotic label should show three things for each bacterial strain:
- Genus: e.g., Lactobacillus
- Species: e.g., rhamnosus
- Strain code: e.g., GG
The full designation is Lactobacillus rhamnosus GG — commonly abbreviated LGG. This is not the same strain as Lactobacillus rhamnosus HN001, which is a different bacterium with a different evidence base and different clinical effects.
A product that lists “Lactobacillus rhamnosus” without a strain code cannot be matched to any specific clinical study. It is unverifiable.
Verified vs Unverifiable Label Claims
| Label Claim | What It Means | Verifiable? |
|---|---|---|
| “10B CFU at time of expiry” | Viable count guaranteed through expiry date | Yes — look for third-party testing |
| ”10B CFU at time of manufacture” | May be lower at time of use | Partially |
| ”Lactobacillus rhamnosus GG” | Specific strain, matchable to clinical trials | Yes |
| ”Lactobacillus blend” | Unknown strains, no verifiable evidence | No |
| ”Clinically studied strains” | Vague — which strains, which studies? | Requires follow-up |
| ”Supports digestive health” | Structure/function claim — must carry FDA disclaimer | Marketing |
Shelf Stability and Storage
Probiotic viability depends on temperature, moisture, and oxygen exposure. Two types of products:
Refrigerated: Garden of Life Raw Probiotics, Symprove. Bacteria are not heat-treated; require continuous cold chain. If the cold chain breaks during shipping, stated CFU count may not be accurate at delivery.
Shelf-stable: Culturelle, Bio-Kult, Seed DS-01. Either freeze-dried bacteria with moisture barriers, or enteric-coated capsules that protect against environmental conditions. Shelf-stable products are more practical for travel and daily use.
Key label phrase: Look for “contains desiccant” or “nitrogen-flushed” in shelf-stable products — these indicate oxygen exclusion, which significantly extends viable shelf life.
Delivery to the Colon
Most probiotic bacteria need to reach the colon alive to produce their effects. Stomach acid (pH 1.5-3.5) kills many bacteria. Delivery technologies:
- Standard capsule: Modest acid protection; most of a large-dose product survives transit due to mass effect
- Enteric coating: pH-sensitive coating that dissolves at higher pH (intestine not stomach). Standard for pharmaceutical grade.
- Nested capsule (Seed ViaCap): Outer capsule dissolves to release prebiotic; inner capsule is acid-resistant. Most sophisticated consumer design.
- Liquid (Symprove): Bacteria in a water-based medium; claimed to “pass through” stomach faster than capsules. Third-party tested for viability at colon delivery.
A Practical Label-Reading Checklist
When evaluating a probiotic:
- Can you identify the full strain designation (genus + species + strain code) for each listed strain?
- Does the stated CFU count apply at expiry or at manufacture?
- Is there third-party testing (NSF, USP, ConsumerLab) confirming the stated CFU count?
- Does the product require refrigeration — and was the cold chain maintained during shipping?
- Does any strain in the product have RCT evidence for your specific health goal?
- Are those RCTs for the specific strain listed, or for a related but different strain?
If you cannot answer questions 1 and 5 from the label, the product’s clinical evidence is unverifiable. That is not necessarily a disqualifier — but it means you are buying on trust, not evidence.
Bottom Line
The key rule: check the strain identity first, the CFU count last. A product with one named, well-evidenced strain at 10B CFU is more likely to produce the outcome you want than a product with 50 unnamed strains at 200B CFU. The label tells you which category you are buying from, if you know what to look for.
Note: probiotic effects are strain-specific. A study on one strain does not apply to other strains — even within the same species.