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CFU and strains: What do the numbers on the tub mean?

By Published 3 min read

"50 billion CFU!" "14 strains!" The back of a probiotic supplement is full of big numbers. But what do they actually tell you, and is more always better?

Short answer: No. The research suggests that what matters is not how big the number is, but whether that strain at that dose has been tested for your purpose.

What is CFU?

CFU stands for colony forming units. It is a measure of how many live bacteria can grow in a laboratory from a given amount of product. So the number tells you something about the number of live bacteria, but nothing about which bacteria they are, or whether they do anything.

Is a higher dose better?

This has been studied, and the answer is: sometimes.

  • For antibiotic-associated diarrhoea there are signs of a link between dose and effect. A Cochrane review of 33 trials in children found that doses of at least 5 billion CFU a day did better than lower doses [1].
  • For most other purposes no such link has been shown. A review of dose studies found no dose effect in meta-analyses of, among other things, IBS, eczema prevention and slow gut transit [2].
  • An instructive counterexample: In a trial with 362 women with IBS, a Bifidobacterium strain only worked at the middle dose (100 million CFU). Neither the lowest nor the highest dose (10 billion) was better than placebo, and the highest dose caused problems with the capsule itself [3].

The point: a CFU number only means something if the product has actually been tested at that dose and in that form.

Does the number on the label hold up?

Live bacteria slowly die during storage, especially in heat and humidity [4]. That is why it matters whether the CFU number is guaranteed "at manufacture" or "until the expiry date". Only the latter tells you what you actually get.

Independent checks of products on the market show a mixed picture. In an American study, only 4 of 13 products matched their label [5], while a small Italian study found that all five capsules and powders tested did [6]. In Slovenia, 3 out of 26 pharmacy products did not have enough live bacteria [7]. Several studies have also found species on the label that were in fact a different species [6][7].

Many strains or one?

It sounds logical that 14 strains cover more ground than one. But when the researcher Lynne McFarland reviewed 65 trials with over 10,000 participants, the conclusion was that single strains were in most cases just as good as mixtures. So the choice should not be based on the number of strains, but on the evidence [8].

An earlier review of 228 trials also showed that the effect is both strain-specific and disease-specific: one Lactobacillus strain may work in a trial, while another from the same genus does not [9].

Read the name right down to the strain code

A bacterial name has three parts:

Part Example
Genus Lacticaseibacillus
Species rhamnosus
Strain GG

Research results apply to the strain, not to the species in general. Two products with "L. rhamnosus" can therefore be completely different.

Checklist: How to read the tub

  1. Is the strain code given? (e.g. "GG", "BB-12", "299v"). If not, you cannot look up the research.
  2. Is the CFU guaranteed until the expiry date? Not just at manufacture.
  3. Are there trials with that exact strain, at that dose, for your purpose? The manufacturer should be able to point you to them.
  4. Store the product as directed, because humidity and heat cost you live bacteria.

And remember: if you have persistent stomach problems, your doctor should be your first stop, not the supplement shelf.

Sources

  1. Guo Q et al. Probiotics for the prevention of pediatric antibiotic-associated diarrhea. Cochrane Database Syst Rev. 2019. PMID 31039287
  2. Ouwehand AC. A review of dose-responses of probiotics in human studies. Benef Microbes. 2017. PMID 28008787
  3. Whorwell PJ et al. Efficacy of an encapsulated probiotic Bifidobacterium infantis 35624 in women with irritable bowel syndrome. Am J Gastroenterol. 2006. PMID 16863564
  4. Dianawati D, Mishra V, Shah NP. Viability, Acid and Bile Tolerance of Spray Dried Probiotic Bacteria and Some Commercial Probiotic Supplement Products Kept at Room Temperature. J Food Sci. 2016. PMID 27145163
  5. Drago L et al. Microbiological evaluation of commercial probiotic products available in the USA in 2009. J Chemother. 2010. PMID 21303743
  6. Blandino G et al. Labeling quality and molecular characterization studies of products containing Lactobacillus spp. strains. Int J Immunopathol Pharmacol. 2016. PMID 26667227
  7. Mohar Lorbeg P et al. Evaluation of Dietary Supplements Containing Viable Bacteria by Cultivation/MALDI-TOF Mass Spectrometry and PCR Identification. Front Microbiol. 2021. PMID 34349743
  8. McFarland LV. Efficacy of single-strain probiotics versus multi-strain mixtures. Dig Dis Sci. 2021. PMID 32274669
  9. McFarland LV et al. Strain-specificity and disease-specificity of probiotic efficacy. Front Med. 2018. PMID 29868585

This article is general information and not medical advice.