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What Are Probiotics? (A Dietitian Explains Everything)

Researched and Written by:
Bailey Franzen MS, RDN Bailey Franzen MS, RDN

Probiotics are everywhere.  They're in your yogurt, kombucha, kefir and of course, supplements.  But what the heck are they and how do they help?  Let's find out.

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Table of Contents

    What Are Probiotics?

    The term “probiotic” comes from the Latin “pro,” which means “for” and the Greek “biotic” meaning “bios” or “life.”1 

    The definition of a probiotic as “live microorganisms which when administered in adequate amounts confer a health benefit on the host” was determined by the Food and Agriculture Organization of the United Nations (FAO) and the World Health Organization (WHO) in 2001 and confirmed in 2014 by the International Scientific Association for Probiotics and Prebiotics (ISAPP). and later remain unchanged being agreed in the broad expert communities.2

    What Qualifies As A Probiotic?

    Although there can be many characteristics of probiotic microbes, it’s useful to remember what the ISAPP consensus statement stipulates about probiotics: 
    • A probiotic must be alive when administered
    • Have sufficient evidence of a health benefit 
    • Be safe for the intended use
    • Be delivered at an effective dose at the time of administration (not time of manufacture) 
    • Must be a defined entity allowing for appropriate identification at the strain level. 

    What Is NOT A Probiotic?

    • The ISAPP panel suggests that consuming live microbes and cultures through fermented foods is likely a beneficial dietary recommendation, but given the often diverse and undefined nature of the microbes found in these foods, along with the challenge of distinguishing benefits derived from the food matrix instead of the microbes, these foods do not meet the threshold of being labeled as “probiotics.”3,4
    • In order for a food to meet the designation of “probiotic” the consortia of microbes must be defined, have been studied with demonstrable benefits, and must be delivered at a dose capable of achieving these benefits. For instance, traditional yogurts contain live bacteria which are defined entities and that have been shown to improve lactose intolerance. Therefore, yogurt cultures could potentially meet the benchmark to be considered a probiotic, whereas other fermented foods with an undefined microbial consortia may not.5,6
    • If looking to consume definitive probiotics through fermented foods, look for labeling on the product which distinguishes the type of microorganisms in the product along with the quantity of microbes in the product, often written as colony forming units (CFUs). Supplements or food products which state CFU “at the time of manufacture” do not account for decline of CFU during storage, so ideally, the probiotic CFU count should be good through the expiration date.3
    • The CFU count is usually a total count of the live cultures, though products which provide more specifics of CFUs at the strain or species level would be preferable. The CFUs listed on the product label should equal the amount shown of that strain to be beneficial in human studies.3

    Which Fermented Foods Actually Have Probiotics?

    Or, to be a little more accurate...

    Whether Meeting the Definition of a Probiotic or Not, Which Fermented Foods Supply Living Cultures? Which Do Not? 

    • Fermented foods may benefit human health by reducing risk for some acute and chronic diseases and helping maintain a healthy intestinal microbiota. Depending on the fermented food, a blend of certain bacteria, yeasts, and/or molds carry out the fermentation process. These microbes are still alive at the time of consumption for some foods, whereas for other fermented foods, they are not due to processing techniques (e.g. pasteurization, baking, or filtering).7  
    • Fermented foods which retain living cultures include: 7 
      • Fresh kimchi
      • Fresh sauerkraut
      • Fresh sour dill pickles 
      • Water or brine cured olives
      • Yogurt
      • Kefir
      • Some cheeses
      • Traditional Salami
    • Fermented foods without living cultures include: 7 
      • Tempeh
      • Most soy sauce
      • Most beer
      • Most wine
      • Sourdough bread
      • Chocolate

    Importantly, the microorganisms in fermented foods, both live and dead, can have an impact on the microbiota and immune system. Studies are currently limited, but the current data suggests fermented foods may be linked to better health.8

    What is Not Required of Probiotics?

    • There is no requirement for probiotics to exhibit properties such as colonization, the ability to survive the intestinal transit, adherence, anti-pathogenic properties, or the ability to balance the host microbiota. Further, the source of the probiotic strain – human or otherwise – is not stipulated in the definition.3,4
    • Given that probiotics have many applications ranging from common uses like ingestion for gastrointestinal health to less common uses such as oral, intravaginal, or topical applications, the defining characteristics of probiotics were left intentionally broad by ISAPP; a necessity so as to not place restrictions which work to the detriment of consumers, producers, and health care providers.1 (e.g. not requiring probiotics to be able to survive the intestinal tract, as this would be unnecessarily restrictive for oral or topical applications, etc.)

    Do They Have to Alter the Microbiota to Be Effective? 

    • While some may be under the misconception that ingested probiotics are colonizing the gut microbiota, the evidence suggests that colonization is a more elusive and challenging state to achieve. In fact, most studies show that ingested probiotics do not evoke detectable changes in the microbes that are normally present, and they also do not appear to become permanent members of the gut microbiota.3,6
    • As probiotics pass through the gut, the microbes and the substances they produce interact with immune cells, our gut cells, other microbes residing in our guts, and dietary compounds to exert benefits.6
    • Therefore, strict requirements regarding adherence, colonization, and so forth do not appear to be necessary for the sake of deriving clinical benefits from probiotics.10,6

    What Are The Different Types of Probiotics?

    • Probiotics comprise many different types of nonpathogenic, beneficial microbes. The most commonly used probiotics are Lactobacillus, Bifidobacterium, Bacillus coagulans, Escherichia coli strain Nissle 1917, certain enterococci, especially Enterococcus faeciumSF68, and the yeast Saccharomyces boulardii.11,12
    • Lactobacillus includes several individual species, the most notable of which include L acidophilus, L rhamnosus, L bulgaricus, L reuteri, and L casei. Similarly, the Bifidobacterium species that are most commonly used in probiotics include B animalis, B infanti, B lactis, and B longum.12
    • Bacterial spore formers, mostly of the genus Bacillus dominate the scene, too. These probiotics are added to foods, particularly fermented milk products, either singly or in combinations.11
    • Probiotic products may contain either a single strain or a mixture of two or more strains. A single strain may exhibit different benefits when used individually and in combination.11
    • Notably,  not all species of probiotics are part of the normal human gut flora and the beneficial effects attributed to one strain cannot always be generalized to other strains.12
    • It is important, therefore, to remember that probiotic microbes are described by their genus, species and strain designations. Using the example of one well-studied probiotic, Lactobacillus rhamnosus GG – Lactobacillus is the genus, rhamnosus is the species and GG is the strain designation. All three components are necessary to identify a probiotic.3
    • The full name enables the reader to link the specific strain to studies describing health benefits and safety assessments. Further, health benefits shown for one strain may not be established for another strain, even of the same species, although at times common mechanisms among different strains may result in similar clinical outcomes.3,12
    • Many different probiotic strains have been shown to be beneficial. If your goal is simply to bring beneficial microbes on board through diet or supplements, then choosing high-dose, multi-strain probiotics, and/or fermented foods with live and active cultures might be suitable for your needs.6
    • If a clinical benefit for a specific medical concern is desired, then a more targeted and strain-specific probiotic or probiotic blend is likely necessary. If you are seeking a particular effect from probiotics, consult a health care provider to help in choosing the appropriate probiotic for your needs.

    Is a Higher Number of Strains Better? 

    It really depends on your goals. Some studies show benefits from a single-strain probiotic product, while others find that specific probiotic blends have a positive outcome. It’s important to note that having a lot of strains in a probiotic is not a guarantee of a more beneficial product.6

    What Are The Benefits of Probiotics?

    • Among the main effects of probiotics at the intestinal level, the following are noteworthy: Balancing and restoration of the gut microbiota, protection against pathogens, immunomodulation, and maintenance of intestinal barrier integrity.13
    • Probiotics are widely used in dietary supplements, food, infant formula formulations, and medical devices. They have demonstrated significant potential as therapeutic options for a variety of diseases, mainly gastrointestinal diseases (including acute infectious diarrhea, antibiotic-associated diarrhea, ulcerative colitis, irritable bowel syndrome, functional gastrointestinal disorders, or necrotizing enterocolitis), but also extra-intestinal disorders, such as hepatic encephalopathy.13
    • Likewise, they’ve been used to help with allergic disorders such as eczema and allergic rhinitis in infants.In addition, probiotics may be helpful when combined with traditional treatments for metabolic disorders, including obesity, metabolic syndrome, nonalcoholic fatty liver disease, and type 2 diabetes.14
  • Widespread Benefits Among Studied Probiotics

      • Commonly studied probiotics confer the following benefits:4
        • Resistance of
        •  colonization and competitive exclusion of pathogenic microbes.  
        • Normalizing the perturbed microbiota 
        • Increased turnover of the intestinal absorptive cells (Enterocytes)
        • Regulating intestinal transit 
        • Production of acid and beneficial short chain fatty acids (SCFAs).
  • Frequently Observed Benefits at the Species Level:

      • Creation of vitamins
      • Neutralization of cancer-causing compounds
      • Bile salt metabolism 
        • Alterations of which have been found in certain disease states such as IBD15 
      • Gut barrier reinforcement 
      • Enzymatic activity 
        • For example, production of digestive enzymes capable of breaking down lactose. 
      • Direct antagonism of pathogens 
        • For instance, by production of antimicrobial compounds against pathogenic microbes.4
  • Rare Strain-Specific Benefits4

      • Neurological effects
      • Immune effects
      • Endocrine effects
      • Production of specific bioactive compounds

      Evidence Based

      An evidence hierarchy is followed to ensure conclusions are formed off of the most up-to-date and well-designed studies available. We aim to reference studies conducted within the past five years when possible.

      • Systematic review or meta-analysis of randomized controlled trials
      • Randomized controlled trials
      • Controlled trials without randomization
      • Case-control (retrospective) and cohort (prospective) studies
      • A systematic review of descriptive, qualitative, or mixed-method studies
      • A single descriptive, qualitative, or mixed-method study
      • Studies without controls, case reports, and case series
      • Animal research
      • In vitro research

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