Prebiotics 101

Written by Megan Falk: Experienced health and wellness journalist and editor. Megan is a graduate of Syracuse University’s S.I. Newhouse School of Public Communications, where she earned a bachelor’s degree in Magazine Journalism and a minor in Food Studies. She’s also a certified personal trainer through the American Council on Exercise.
Reviewed by Jennie O’Grady: Senior SciComms Specialist at Seed Health

Prebiotics are a lot like new makeup trends or viral TikTok sounds: You may not know exactly what they are, but you hear about them constantly.

The prebiotics market has nearly tripled over the last decade, and public interest has skyrocketed along with it.¹ According to Google data, there were more searches for prebiotics this January than ever before. Clearly, people are increasingly asking what these compounds are and what they can do for their health—and we’re here with the answers. 

Our team at Seed obsessively tracks the latest microbiome science, so if you’re looking to learn more about prebiotics (beyond a watered-down definition), you’ve arrived in the right place. Here’s your in-depth guide to what prebiotics do, how they work, why they’re not the same as fiber, and how to get your fill through diet and supplementation.

What Are Prebiotics?

Prebiotics were first defined around the same time that Amazon launched—yes, that recently. In 1995, microbiologists Glenn Gibson and Marcel Roberfroid described them as non-digestible food ingredients that improve the health of the host, specifically by stimulating the growth or activity of bacteria in the colon.² 

Over the last 30 years, this definition has been clarified and updated. The International Scientific Association for Probiotics and Prebiotics (ISAPP) now defines a prebiotic as “a substrate that is selectively utilized by host microorganisms conferring a health benefit.”³ Let’s break down this short (but surprisingly complex) definition:

• Substrate: A substrate is a substance that provides nourishment for an organism (in this case, a microbe), helping it to thrive.
• Selectively Utilized: This clarifies that prebiotics must impact a limited group of resident or exogenous (introduced from the outside) microorganisms—not the entire microbial ecosystem. They should only feed beneficial or commensal (“good”) microbes, not harmful bacteria or pathogens.
• Conferring a Health Benefit: The resulting health benefit is what differentiates a prebiotic from any other substrate that can be broken down by microbes. Specifically, a prebiotic needs to sustain, improve, or restore the health of the microbiome’s host (you!). Prebiotics were originally synonymous with the gut/colon, but we now know they can benefit any microbial system. 

Some of the best-known prebiotics—inulin, fructooligosaccharides (FOS), galactooligosaccharides (GOS), and resistant starches—are technically soluble fibers. But non-fiber substances including polyphenols can also be prebiotics.⁴ (We’ll dig into those in detail later.) 

How Prebiotics Work 

Once you consume prebiotics, here’s what happens under the hood:

Step 1. Prebiotics travel through the digestive system.

Prebiotics that target the gut microbiome must make the journey from your mouth to your colon before they can confer their health benefits. 

Some prebiotics are composed of carbs and fibers with complex molecular structures, so they’re not easily broken down by the hydrochloric acid in the stomach or the digestive enzymes in the small intestine.⁵ (By comparison, other food groups like simple sugars and proteins are easily broken down by amylase, protease, and other enzymes in the upper digestive tract.)⁶ 

Step 2: Your microbes mingle with prebiotics. 

After the prebiotics pass into the large intestine, your resident microbes can get to work processing them. Unlike the digestive enzymes in the upper GI tract, some bacteria in the colon can break down the complex carbohydrates found in prebiotics, such as inulin, FOS, and GOS, with the help of bond-breaking enzymes.^5,7

As we mentioned earlier, this is a selective process. Only some microbes break them down, including beneficial bacteria that are commonly used as probiotics like bifidobacteria and lactobacilli.

Other potentially pathogenic bacteria (like: E. coli and C. jejuni ) lack the enzymes required for this metabolic process.^8,9 Instead, these harmful bacteria grow and reproduce by focusing their efforts on easily digestible sugars and amino acids, often found in high-protein, high-sugar, or high-fat foods. 

Step 3: Health benefits follow.

In the process of interacting with the prebiotics, bacteria produce an array of beneficial byproducts. These include compounds like short-chain fatty acids (SCFAs). SCFAs like acetic acid, butyric acid, and propionic acid help maintain a slightly acidic environment in the colon that discourages the growth of harmful bacteria. This is where the health benefits of prebiotics come into focus.^5,10

Some “good” bacteria also produce antimicrobial compounds as they ferment prebiotics, which can slow the growth of “bad” bacteria. For example, certain Lactobacillus species produce lactic acid and bacteriocins, which have antimicrobial effects that can prevent pathogen growth.¹¹ The end result: a stronger, more resilient microbiome. 

Prebiotics Vs. Fiber

Despite sometimes being used interchangeably, fiber and prebiotics are not the same thing. While most prebiotics are dietary fibers, not all dietary fibers are prebiotics.

Prebiotics cannot be digested by humans and are primarily used by a limited group of “good” gut microbes (read: selective utilization).

Dietary fibers can’t be digested by humans either. Some are fermented and selectively used by gut microbes—but this isn’t a requirement to be classified as fiber. 

Soluble fibers (the type that can dissolve in water) function as prebiotics more often than their insoluble counterparts, since they are more easily fermented by bacteria in the colon.¹² 

P.S. Polyphenols—naturally occurring plant-based compounds, including phenolic acids, flavonoids, stilbenes, and lignans—are also a type of prebiotic.⁷ Instead of being fermented by bacteria, polyphenols are further broken down by bacteria into beneficial metabolites like urolithins. Urolithins have a wide range of benefits on mitochondrial function, inflammation, and muscle health and have even been shown to extend lifespan in animal models.^13,14

How to Get Prebiotics in Your Diet

Walking through the aisles of a grocery store, you’ll likely spot some packaged foods like sodas, bars, and shakes with added prebiotic substances. Ingredients like garlic, onion, honey, asparagus, apples, and certain grains also naturally contain prebiotics (though you’re not going to see this on their labels, due to FDA regulations.)³ 

While you certainly can get prebiotics through your diet, the concentration of prebiotics in various food sources tends to be relatively low.¹⁵ Dietary prebiotics are also highly variable, making it impossible to count them gram-by-gram. You’re better off monitoring your overall fiber intake instead.

Women should aim to consume 25 grams per day and men should get 38 grams, per the National Institute of Medicine. (Womp: The average American is falling short of this and only gets 15 grams).^16,17 

Generally speaking, the more fruits and vegetables on your plate, the more fiber you’ll consume. According to findings from a 2018 study called the American Gut Project, people who ate 30 or more types of plants throughout the week had a significantly more diverse gut microbiome than folks who ate fewer than ten.¹⁸

To increase the diversity of your diet, add some of these lesser-loved fiber- and prebiotic-rich foods to your cart:

• Cocoa: Rich in flavanols (a type of polyphenol), which have prebiotic effects¹⁹
• Seaweed: Contains polyphenols and polyunsaturated fatty acids that potentially act as prebiotics²⁰
• Unripe bananas: High in resistant starch, which acts as a prebiotic.²¹ Boil them, add them to oatmeal or stews, or blend them into a smoothie to make them more palatable.
• Dandelion greens: High in the prebiotic inulin²²
• Cooked, then cooled potatoes: Contain a resistant starch that functions as a prebiotic²³
• Onions, garlic, Jerusalem artichokes, and chicory root: Contains prebiotics oligofructose and inulin (albeit in low amounts)²⁴

P.S. Noshing on prebiotic-rich foods isn’t the only way to support your gut microbiome. Your gut functions best when you eat a wide variety of fruits, vegetables, legumes, nuts, and grains; prioritize foods with omega-3 fatty acids like salmon, avocados, and olive oil; and limit microbiome disruptors like alcohol and sugar that can cause bacterial imbalances.^18,25,26,27 

Prebiotic Supplements, Explained

While the best way to get fiber is through the diet, the best way to get verifiable prebiotic doses is through supplementation. 

To return to the definition of prebiotics, they must sustain, improve, or restore the health of the microbiome’s host. Unlike dietary sources, supplements contain consistent doses of prebiotic substrates (that are often orders of magnitude higher than those you’d get from food). They can also be clinically studied for their impacts on a host. 

According to ISAPP, it takes at least 3 grams of a prebiotic in order to confer a health benefit. Most prebiotic supplements will fulfill this using some combination of GOS, FOS, oligofructose (OF), chicory fiber, and/or inulin.⁴

Synbiotic supplements combine prebiotics with probiotics in order to deliver beneficial bacteria and the nutrients your microbiota needs to thrive.

This combo is meant to enhance the survival, colonization, and activity of probiotics, and research suggests that synbiotics may be more effective than probiotics alone. They’ve been shown to enhance the production of SCFAs, relieve digestive discomfort, and enhance gut immune function (though it’s hard to tell if these benefits are attributable to prebiotics, probiotics, or the synergy between the two).*^28,29,30 This is why you’ll find some form of prebiotic in every one of Seed’s formulations.

Meet Your Prebiotic Dream Team

• Our DS-01^® Daily Synbiotic is formulated with 24 clinically studied probiotic strains, plus a polyphenol-based prebiotic. This non-fermenting prebiotic compound is derived from the fruit and skin of Indian pomegranate and is standardized to deliver efficacious polyphenols like Punicalagins and Ellagitannins. (To reach the prebiotic content included in a single serving of our DS-01®, you’d have to eat 38 pomegranates!). The synbiotic formula has been clinically shown to support comfortable and regular bowel movements in people who experience occasional digestive discomforts.*³¹ 
• Our PDS-08^® Pediatric Daily Synbiotic contains nine probiotic strains and a dual-phase prebiotic extracted from chicory root. The included inulin and FOS stimulate the growth of beneficial bifidobacteria and play a helping hand in producing beneficial SFCAs.*
• Our VS-01™ Vaginal Synbiotic contains three proprietary L. crispatus probiotic strains, as well as a complex of nutrients the vaginal microbiome (VMB) needs to survive and thrive. This combination helps establish an optimal, resilient VMB and regulate vaginal pH.* 

The Potential of Prebiotics

One of the (many) fascinating areas of prebiotic research is how these substrates can be leveraged beyond the gut. Here’s the latest on their potential role in other biological biomes:

• The vaginal microbiome is primarily dominated by Lactobacillus species, which create an acidic environment that fends off conditions like yeast infections and bacterial vaginosis.³² Prebiotics such as FOS and lactulose feed the lactobacilli to maintain an inhospitable environment for harmful bacteria and yeast.^32,33 (PSA, you can find the former in our VS-01™ Vaginal Synbiotic.)
• Your oral microbiome may also benefit from prebiotics. Feeding your mouth’s “good” microbes via prebiotics like xylitol and inulin may prevent the growth of harmful bacteria that cause plaque and gum disease, as well as stimulate saliva production that will neutralize teeth-eroding acid.³⁴ Stay on the lookout for prebiotics in products like toothpaste and mouthwash. 
• Finally, prebiotics are increasingly being harnessed for the skin microbiome. Studies suggest prebiotics may curb the development and severity of atopic dermatitis and eczema in infants, potentially by moderating bacterial growth and interactions with the gut’s immune system.^35,36

FAQs

What’s the difference between probiotics and prebiotics? 

Probiotics are live microorganisms that benefit host health when taken in adequate amounts.³⁷ Prebiotics are non-living substrates (like fibers, polyphenols, and other compounds) that selectively feed those beneficial microorganisms. 

If probiotics are the seeds you plant in your garden, prebiotics are the water you give those seeds to help them grow and thrive. 

EXPLORE FURTHER: Your Definitive Guide to All the ‘-Biotics’

What about probiotics vs. postbiotics? 

Postbiotics are bacteria that have been deliberately killed, but still lend a health benefit to the host.³⁸ They can act similarly to a probiotic, but they’re much easier to produce (since you don’t need to worry about them making it to the colon alive).

To continue with the garden analogy, postbiotics are the mulch you spread on top of your plants to stifle weeds and slowly feed the soil as it breaks down. 

When should I take prebiotic supplements?

You can take a prebiotic supplement anytime, morning or night. The most important thing is to do it consistently. (You wouldn’t water your flowers one time and leave them out in the blistering sun for weeks, would you?)

How many prebiotics do I really need per day?

To support your resident microbes, ISAPP recommends consuming at least 3 grams of prebiotics per day. You can do this by eating a diversity of fiber-rich foods and using a prebiotic supplement to fill in any gaps. 

How can I up my prebiotic intake?

Prioritize foods rich in soluble fiber, such as apples, bananas, oats, and beans.¹⁷ Since you’d have to consume a high volume of these foods to see their prebiotic benefits, taking a supplement is the best way to get verifiable prebiotic benefits.

What should I look for in a prebiotic supplement?

There’s a chance “prebiotic” won’t be front-and-center on your supplement’s label. Instead, zoom in on the ingredient list, searching for keywords like GOS, FOS, OF, chicory fiber, or inulin. Polyphenols can also clue you into a product’s prebiotic offerings, as is the case with our DS-01^® Daily Synbiotic.

The Key Insight 

The next time the question “What do prebiotics do?” comes up in your group chat, you’ll know the answer: They feed the good bacteria in your gut. When consumed in adequate amounts through diet or supplementation, prebiotics kick off a chain of events that ultimately enhances your microbiome and whole-body health. Next up on your TBR list: Probiotics 101.