What are Probiotics?

What are Probiotics?

Every day, people send us photos of probiotics, texts with Amazon links, and recommendations from their chiropractor, along with dozens of questions and misconceptions about probiotics. We’re here to clarify the FAQs and debunk some common myths around what probiotics are, what they do, and why they can support your health.

First things first, what exactly are probiotics?

The official definition of ‘probiotics’, authored by a 2002 UN/WHO expert panel (chaired by our Chief Scientist, Dr. Gregor Reid), says:

Live microorganisms which, when administered in adequate amounts, confer a health benefit on the host.

Let’s break that down:

Live microorganisms

This refers to strains of beneficial bacteria that must be alive when they are consumed. ‘Live’ is a critical word here, which means they have to survive throughout manufacturing, transportation (sometimes in hot conditions), and sitting on the shelf—until they reach your mouth.  

Adequate amounts

This is very important, and you’ll generally see quantities enumerated on a label as a certain billion or trillion CFU or AFU. To meet this criteria, each probiotic must be associated with an effective dosage, and this dosage is arrived at through clinical study or studies.

Health benefit

So you’ve taken some live bacteria in the right amounts. But are they actually doing anything for you? To satisfy the definition of a probiotic, the live cultures must demonstrate a health benefit. This means each specific strain (not just the species) must have been clinically studied and shown to be beneficial.


That’s you! (Or your mom, or a dog, or a colony of honeybees, or for whomever the probiotic is intended.)

Can’t I just eat kimchi? Or drink kombucha?

While it’s certainly possible for a fermented food or beverage to meet the scientific criteria for a probiotic, many fall short. Just because something contains live microorganisms, doesn’t mean it satisfies the definition. You might have ingested some bacteria, but do you know which strains? In which quantities? Are they still alive at the time of consumption? Have those strains been studied, in those quantities, to actually do something in your body?

This is not to say that you shouldn’t eat or drink fermented things. Many fermented foods and beverages are extremely nutritious, very tasty additions to your daily diet (though we do suggest keeping an eye out for excessive sugar content—as many commercial products like yogurts and beverages are sweetened with added sugars). The distinction is that they are not necessarily reliable sources of beneficial, effective, and live bacteria in adequate amounts.

My gut is already healthy, so why should I take a probiotic?

This actually goes way beyond digestive health. Your body is complex and interconnected, and the gastrointestinal system sits at the core of it all. It’s connected to and influences everything from gut immune function to cardiovascular and skin health. So, while gut health benefits are often the most immediate, localized, and conspicuous, probiotics can actually have powerful effects across the entire body.

As our mindset shifts from sick care to self care, we’ve become more intentional about our diet, nutrition, exercise, and lifestyle. But we now know that beneficial microbes also offer new tools to preventively and proactively care for our whole selves.

Is more better?

Not necessarily.

You’ve probably seen the term CFU on a probiotic label. This refers to colony-forming units, which basically tells you how many bacteria in the sample are capable of dividing and forming colonies. A bigger number on the bottle does not always mean better results. The best dose, per strain, is the one that has been studied in humans and shown to deliver positive outcomes.

So, how do probiotics work?

We first need to clear up a common misconception: that probiotics have to colonize your gut and alter the composition of your microbiome to be effective. That’s not true:

Probiotics typically don’t take up residence in your gut. Compared to the tens of trillions of microbes already rooted in your intestinal tract, most probiotics don’t contain enough new bacteria to make a significant difference in the composition of your microbiota.

Even if they did, we don’t know enough about the safety of introducing colonizing microbes. Large numbers of newcomers moving in and displacing your existing bacteria could alter the unique balance of your ecosystem within and trigger unintended consequences.

What scientists do know is that, as transient microbes, probiotics travel through your colon, interacting with your immune cells, gut cells, dietary nutrients, and existing bacteria to, directly and indirectly, deliver benefits. Some enhance the gene expressions involved in tight junction signaling, which help protect against intestinal permeability—this means a tight gut barrier. Others trigger neurotransmitters that stimulate muscle contractions for increased motility—think easy, regular poops. Yet other bacteria produce byproducts like short-chain fatty acids, which have been shown—extensively—to be beneficial for metabolic and gut immune health.

My friend offered me some probiotic nuts the other day. What’s the deal with that?

Ha. And you could go buy some ‘probiotic’ shampoo and mattresses, too. Hopefully, by now, you understand that most of the products labeled ‘probiotic’ out there are, unfortunately, not probiotic at all. Remember, to be probiotic, something has to fully satisfy the scientific definition we outlined above.

So, why are all these products allowed to say that they’re probiotic?

It’s a really, really good question.

In the United States, the Food and Drug Administration (FDA) categorizes one class of probiotics as ‘live microbial dietary supplements’. While there are detailed laws and regulations for dietary supplements, the supplement market is under-enforced. As such, the term ‘probiotic’ has been used for marketing and enticement purposes, and much of the science has been lost in translation, or even ignored.

In contrast, throughout most of Europe the term ‘probiotic’ is not allowed to be used on any products, even if the product meets the UN/WHO definition of probiotic (partly due to the perception that a product with the word “-biotic” is like a drug, such as an antibiotic). So while we, at Seed, certainly adhere to FDA regulations, we actually look to even higher global standards like the European Food Safety Authority (EFSA) and Japan’s Foods for Specified Health Uses (FOSHU) in the manufacture and translation of probiotics.

I used an online diagnostics kit and it told me I’m deficient in Bifidobacterium. Should I be taking a probiotic with high concentrations of that bacteria?

Two parts to this answer:

First, let’s talk about strain specificity. Bifidobacterium is what’s called a genus, that is, a ‘group’. Think back to taxonomy mnemonics from middle school biology (King Phillip Came Over For Good Soup = Kingdom Phylum Class Order Family Genus Species) and recall that genus is followed by species. Within the Bifidobacterium genus, we have species like Bifidobacterium breve or Bifidobacterium lactis.

There’s more. Within each species of bacteria, there can also be hundreds or thousands of strains. And just like a golden retriever, a French bulldog, and even a wolf (all of which fall under the Canis lupus species) are incredibly different, strains of bacteria can also be night and day. So, why is strain specificity important when it comes to your probiotic? Here are a couple examples:

Escherichia coli Nissle 1917 is a non-pathogenic bacterial strain of the infamous E. coli family that has actually been extensively studied for its beneficial effects on gastrointestinal disorders. Meanwhile, E. coli O157:H7 (a strain designation) is responsible for hemorrhagic diarrhea, abdominal cramps, and can even lead to kidney failure and death.

As another example: of the thousands of Lactobacillus plantarum strains studied, the strain Lactobacillus plantarum SD-LPLDL-UK (in our Daily Synbiotic) has been shown to support healthy cholesterol metabolism.

So when someone tells you you need more Bifidobacterium or to increase your intake of Lactobacillus rhamnosus, your first response should be ‘Which strains?’.

Your second response should be, ‘That’s not what the science says’. In fact, the National Institutes of Health spearheaded a $173 million Human Microbiome Project, with the goal of characterizing the human microbiome and to determine if changes in microbiome composition could be correlated with health and disease. The project encompassed five years of research and over 200 scientists, and concluded in 2012 that there is no universally healthy microbiome. Their findings also revealed that the metabolic function within our microbiome is more important than what microbial species are present. This means, ‘what your bacteria are doing’ is more important than ‘what they are’.

So while the current ‘diagnostics’ (think stool tests) are certainly fun to do, they’re unable to give us scientifically-substantiated, actionable advice. And they certainly can’t tell you that you need to eat more Bifidobacterium.