CFUs vs. Strains: The Ultimate Probiotic Explainer

Certain probiotic strains may help with antibiotic-associated diarrhea, eczema and IBS.
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You've been hearing about probiotics for years now, and may have already added them to your supplement shelf. But do you really know what's going on inside your probiotic bottle? It's official: You're not the only one a bit confused on this subject.


Potency guarantee, clinically studied strains and CFUs are all important factors that go into choosing the right probiotic for your body. So, if you don't know what these terms mean, how are you supposed to reap all the gut-friendly benefits?

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Starting with the basics, let's break it down and start from square one.

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What Are Probiotics, Exactly?

Probiotics actually have a scientific definition. The International Scientific Association for Probiotics and Prebiotics explains that they are "live microorganisms, that when administered in adequate amounts, confer a health benefit on the host." In this case, you're the host.


Probiotics may be more beneficial than just getting you to add more yogurt to your diet. They help your gut by protecting your body from bad bacteria that can make you sick, according to Harvard Health Publishing. If you get the good bacteria to outnumber the bad, you've won — it's sort of like gut warfare.

Take yourself back to high school biology for a second and remember how organisms are classified. Probiotics work in the same manner — they are identified by genus, species and then strain. The specific type of strain will determine the health benefit from the probiotic.


It will probably be easier to just forget about the species for now and focus on the genus and strain. Two of the seven most studied genera of probiotics are lactobacillus and bifidobacterium, and these are the ones you have probably heard of, especially if you're eating foods like yogurt for their probiotic benefit.

Probiotic Strains

Within each genera (that's the plural of genus), there are multiple strains, each responsible for specific health benefits. This means if you're taking probiotics to help with a certain condition, you need to make sure you're taking the right strain.


"To break this down," Kathleen Tabb, RD, LD, a dietitian with Rebecca Bitzer & Associates, tells LIVESTRONG, "a common probiotic on the market is Lactobacillus Rhamnosus GG. Lactobacillus is the genus, rhamnosus is the species and GG is the strain. The GG strain is what differentiates this from another product containing Lactobacillus Rhamnosus."


It's all about matching the specific strain to the condition you are hoping to target, according to December 2018 research in PLOS One. Tabb agrees: "Every strain works differently, some with completely different benefits from the other."


There is research on certain strains of probiotics that show promise for some individuals with the following conditions, according to the National Institutes of Health:

  • Antibiotic-associated diarrhea:​ Lactobacillus rhamnosus GC and Saccharomyces boulardii may reduce the risk of developing antibiotic-associated diarrhea.
  • Atopic dermatitis:​ In children ages 1 to 18, Lactobacillus fermentum was found to reduce the severity of eczema.
  • Irritable bowel syndrome:​ Probiotics that contain the strains Bifidobacterium breve, Bifidobacterium longum, or Lactobacillus acidophilus may help reduce symptoms.


CFUs Explained

CFU stands for colony forming unit, and is used interchangeably with the phrases "live cultures" and "culture count." Translation: This refers to the "good bacteria" probiotics are famous for. The NIH indicates that CFUs are the viable cells that will be available for you, the host, to use. You see, if these bacteria are not alive, you gain no benefit.


CFU amounts are usually in the billions, but the NIH cautions that just because there are more CFUs, this doesn't mean you are getting an added health benefit. You must look for the number of CFUs that are going to still be alive at the end of the shelf life, that's what matters.

Do not confuse CFUs with microorganism weight, which is required on the bottle. CFU labeling is voluntary. Microorganism weight lists the total weight of bacteria in your probiotics — alive or dead. This is not helpful to you when choosing a probiotic.



Follow Tabb's advice when looking at probiotics: "If you do not know much about probiotics and are just looking to supplement your diet, you may think that whatever has the highest amount of CFU's and the most amount of probiotics in it is the best, but don't let this trick you. Always look at the strain."

If you're not too confused on the difference between strains and CFUs, follow these steps when choosing a probiotic:

  1. Keep in mind which condition you are targeting.
  2. Ask your health care provider or registered dietitian which strain of probiotic would be most helpful.
  3. Look for a probiotic type that has the strain you desire with the source of probiotics that works with your lifestyle.

Choosing the Right Source of Probiotics

The right type of probiotic is the one that works well with your lifestyle. Probiotics are available in food or supplement form.

Probiotics are naturally found in fermented foods, such as kimchi, miso, sauerkraut and yogurt. When buying a product with probiotics, ensure that it is labeled as containing live cultures. Many foods, such as pickles and sauerkraut, while considered fermented foods, are often heated during processing, which has the potential to kill the bacteria.

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Are Probiotics Harmful?

When you're looking to improve gut health, you might choose probiotics to "see if it works". The experts at the Cleveland Clinic indicate that probiotics are generally safe, although if you're not taking the right strain, you might just be wasting a few bucks.

Also, dietary supplements aren't tightly regulated by the Food and Drug Administration, so there's actually no telling if you're getting what you are paying for.

Probiotics have been known to cause the following symptoms:

  • Upset stomach
  • Excessive gas
  • Allergic reactions
  • Bloating
  • Diarrhea

If you have a compromised immune system or were recently in the hospital for an illness or surgery, it's best to talk with your doctor before taking a probiotic supplement.




Is this an emergency? If you are experiencing serious medical symptoms, please see the National Library of Medicine’s list of signs you need emergency medical attention or call 911.

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