Probiotics 101: A Beginner's Guide to Probiotics

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PSA: There’s way more to the gut than probiotics. Here’s what to know about prebiotics and postbiotics.

Fun fact: The human gastrointestinal tract houses trillions and trillions of microbial cells—not millions, not billions, trillions. With that in mind, it probably comes as no surprise that it’s also home to several different kinds of microorganisms, from bacteria and viruses to fungi, archaea, and protozoa. In other words, there’s a lot going on—and a delicate equilibrium to maintain. Not-so-fun fact? Keeping that balance isn’t quite as simple as drinking kombucha or noshing on kimchi and sauerkraut. (A common misconception, FWIW.) (1,2,3,4,5,6)

Gut health is complex—and when it comes to supporting the gut, scientists are still elucidating the exact mechanics behind “how the sausage is made.” The formula isn’t as cut-and-dried as you might assume. Take fermented foods, for instance. While some, like yogurt, have been associated with health benefits in studies, scientists point out it’s not always possible to pinpoint whether those benefits are a direct result of the live cultures in the yogurt, some other ingredient in the yogurt, or the yogurt in its entirety. See where we’re going with this? (5)

All that said, for certain inquiries, clarity does exist: How might people feel more comfortable with their digestive health, for instance? Are there actions anyone, no matter their diet or lifestyle, can take to increase their confidence in this arena? Which supplements can best support their journey? Pose these questions to any number of dietitians, doctors, biologists, and scientists, and you’ll likely encounter a recurring answer: probiotics. Get up to speed on some of the most common Qs below.*

What are probiotics?

The World Health Organization (WHO) defines probiotics as live microorganisms, or “good bacteria”, which, when administered in adequate amounts, provide a health benefit to the person taking them. They can be found in fermented foods and probiotic supplements—the latter being the focus of this piece (more on why in a sec). (7,8)

Before we dive into the benefits of probiotics, let’s touch on the microbiome, which plays an important role in human health. When we talk about the gut microbiome, we’re referring to the trillions of microorganisms that live in the gastrointestinal tract. Some people may think of these gut microbes as inherently harmful “germs”, but reality tells us otherwise: Many bacteria are actually helpful, and work hard behind the scenes to support gut health, digestive health, and immune health.* (1,2,3)

What are the benefits of probiotics?

Each of us has a unique gut microbiome, a signature bacterial fingerprint shaped by our environment, diet, and lifestyle. But the daily disturbances of modern life—poor diets, stress, travel, the use of certain medications (we’re looking at you, antibiotics), not to mention plenty of other factors—can all throw our gut microbiota off kilter. That’s where ingested probiotics, or beneficial bacteria, come in: to help support gut health, especially in the face of modern life’s inevitable hurdles.* (2,4,9)


What types of bacteria are in probiotics?

Probiotics can contain many different types of microorganisms. They’re identified by their specific strain, which includes the genus, species, subspecies (where applicable), and an alphanumeric strain designation. (1,4)

Current evidence indicates that probiotic effects are strain-specific (in other words, just because one type of Lactobacillus might produce a specific effect, it doesn’t necessarily mean another Lactobacillus strain will, too—although it’s possible). That’s why it’s really important to look for probiotic supplements that include strains supported by evidence from human clinical studies.* (1,4,7)

Fermented foods vs. probiotic supplements

Many people looking to increase their probiotic intake turn to fermented foods and drinks for their fill of good bugs. Miso, kimchi, kefir, kombucha, sauerkraut, pickles, sourdough bread, wine—there’s no shortage of options. (4)

What there is a shortage of, however, is sound scientific evidence: While many fermented foods do include live and active cultures—and these cultures can survive well during a product’s shelf life—studies show that once they’re actually ingested, they typically don’t make it through the harsh conditions of the stomach. In other words, fermented foods may play a role in gut health, but by relying solely on food, there’s no guarantee the intended bacterial benefits will be reaped.* (4,10)

Fermented foods are not the same as probiotics. To be clear: Live and active cultures are not the same thing as proven probiotic microorganisms. According to the World Health Organization’s definition, fermented foods do not qualify as probiotic because:

  • They often do not have proven health benefits. (Published research on the impact of fermented foods on gastrointestinal health is sorely lacking, and many of the studies that do exist are of subpar quality.)
  • They may not survive transit through the stomach and small intestines.
  • The product may not include an adequate amount of strain to be considered probiotic.

When it comes to consuming probiotics, going the supplement route may be the easiest way to ensure the beneficial bacteria makes it to the intestines alive (aka the whole purpose). It’s even more true once you consider the wide variation in live culture composition between fermented food products and brands.

Picking a high-quality probiotic supplement

In order to meet the definition of a probiotic, government agencies like the World Health Organization (WHO) and Food and Agriculture Organization (FAO) recommend a potential strain meet the following criteria: (7)

  • ✓ The strain must be alive. A probiotic strain has to be alive at the time of ingestion. It also has to survive the harsh conditions in the stomach and small intestines (and reach the large intestine still alive).*
  • ✓ The dosage must be adequate. Probiotic strains must be ingested at a high enough dose to have an impact. The specific dosage should be based on documentation from human clinical trials.*
  • ✓ The benefits should be science-backed. In order to qualify as a probiotic, a strain should be supported by evidence from human clinical trials that show the strain supports a health benefit. (If you’re taking a probiotic with multiple strains, you’ll want to ensure each individual strain has been clinically studied.)*

Related → “Double Blind” and “Peer-Reviewed”: A Beginner’s Guide to Clinical Terminology

CFUs: Does quantity matter? Probiotics are typically measured in colony forming units (CFU). Colony forming units represent the number of probiotic cells in a sample that are capable of dividing and forming colonies—so if we’re trying to support the gut, the higher the better… right? (4)

Actually, when it comes to probiotics, it’s less about the quantity of CFUs and more about the quality of the evidence. According to the National Institutes of Health, “Higher CFU counts do not necessarily improve the product’s health effects.” Put another way, the effects of probiotics are not always contingent upon a higher CFU count—there’s more to the picture (which strains of probiotics are present, for example?).* (1,4)

Why shelf-stable formulas are key

Many types of probiotics need to be protected in some way from heat, oxygen, light, and humidity in order to remain effective. Some brands require refrigeration, while others offer shelf-stable options that use intelligent packaging choices to preserve the formula.

While the route you choose is ultimately up to you, it’s important to keep in mind that reaping the benefits of probiotic supplementation requires consistency. (The organisms can’t stick around for very long, so for them to help support the gut, you’ll need to take them daily.) That’s why we recommend reaching for a shelf-stable probiotic you can keep in plain sight, instead of lost in the back of the fridge. Bonus points if it’s backed by decades of human clinical research. That way, instead of wondering, “Is this working?” or “How did my probiotic end up behind the pickles?” you can get on with what’s really important: living life with confidence.* (4,7)

On that note... Here's when to take a probiotic, according to experts.


  1. Guarner F, Sanders ME, Eliakim R, et al. World Gastroenterology Organization. World Gastroenterology Organisation Global Guidelines: Probiotics and Prebiotics. 2017.
  2. Lynch SV, Pedersen O. The Human Intestinal Microbiome in Health... N Engl J Med. 2016 Dec 15;375(24):2369-2379.
  3. Robles Alonso V, Guarner F. Linking the gut microbiota… Br J Nutr. 2013 Jan;109 Suppl 2:S21-6.
  4. Office of Dietary Supplements. Probiotics: Fact Sheet for Health Professionals. National Institutes of Health, Department of Health & Human Services. 2020.
  5. Hill, C., Guarner, F., Reid, G., Gibson, G.R., Merenstein, D.J., Pot, B., Morelli, L., Canani, R.B., Flint, H.J., Salminen, S., Calder, P.C., & Sanders, M.E. (2014). The International Scientific Association for Probiotics and Prebiotics consensus statement on the scope and appropriate use of the term probiotic. Nature Reviews Gastroenterology & Hepatology, 11(8), 506-514.
  6. Jäger R, Mohr AE, Carpenter KC, Kerksick CM, Purpura M, Moussa A, Townsend JR, Lamprecht M, West NP, Black K, Gleeson M, Pyne DB, Wells SD, Arent SM, Smith-Ryan AE, Kreider RB, Campbell BI, Bannock L, Scheiman J, Wissent CJ, Pane M, Kalman DS, Pugh JN, Ter Haar JA, Antonio J. International Society of Sports Nutrition Position Stand: Probiotics. J Int Soc Sports Nutr. 2019 Dec 21;16(1):62.
  7. World Health Organization. Guidelines for the Evaluation of Probiotics in Food. Report of a Joint FAO/WHO Working Group on Drafting Guidelines for the Evaluation of Probiotics in Food. 2002.
  8. Gibson GR, Roberfroid MB. Dietary modulation of the human colonic microbiota: introducing the concept of prebiotics. J Nutr. 1995 Jun;125(6):1401-12.
  9. Gilbert JA, Blaser MJ, Caporaso JG, Jansson JK, Lynch SV, Knight R. Current understanding of the human microbiome. Nat Med. 2018;24(4):392-400.
  10. Wastyk, H., Fragiadakis, G., Perelman, D., Dahan, D., Merrill, B., Yu, F., Topf, M., Gonzalez, C., Robinson, J., Elias, J., Sonnenburg, E., Gardner, C., & Sonnenburg, J. (n.d.). Gut Microbiota-Targeted Diets Modulate…Status. bioRxiv.

Meet Our Experts

This article features advice and has been reviewed by members of our Science Team.

Science Thumb — Arianne

Arianne Vance, MPH, Research Scientist

Arianne Vance is a Senior Scientist at Ritual. She earned her MPH in Epidemiology from UCLA. Her graduate research focused on maternal and child health, with an emphasis on breastfeeding and maternal mental health. She is passionate about sharing her love of science by presenting cutting-edge research in an accessible and engaging way.

Science Thumb — Arianne

Arianne Vance, MPH, Research Scientist

Arianne Vance is a Senior Scientist at Ritual. She earned her MPH in Epidemiology from UCLA. Her graduate research focused on maternal and child health, with an emphasis on breastfeeding and maternal mental health. She is passionate about sharing her love of science by presenting cutting-edge research in an accessible and engaging way.


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