What Are Prebiotics and Why Are They in So Many Supplements?

Prebiotics are one of the most strategically important components of modern gut health supplements and also one of the most misunderstood. They're often mentioned alongside probiotics without much explanation of what they actually are, how they differ, and why they matter independently. Here's a thorough account.

The Definition: What Makes Something a Prebiotic

The formal scientific definition of a prebiotic has evolved over the decades. The current consensus definition (from a 2017 consensus statement by the International Scientific Association for Probiotics and Prebiotics) defines a prebiotic as "a substrate that is selectively utilised by host microorganisms conferring a health benefit."

This definition has three key components:

1. Not digested by the host: Prebiotics resist digestion in the stomach and small intestine, arriving intact in the colon
2. Selectively utilised by beneficial microorganisms: Not all gut bacteria benefit equally prebiotics selectively feed specific beneficial species
3. Confers a health benefit: The microbial change must translate to a measurable health outcome, not just a change in bacterial populations

This is why not all dietary fibre qualifies as a prebiotic under the strict definition only fibres that selectively promote beneficial bacteria in ways that produce documented health effects are technically prebiotics.

The Established Prebiotic Classes

Fructooligosaccharides (FOS) and Inulin

The most extensively researched prebiotic class. Inulin is a polysaccharide (long chain) found naturally in chicory root, Jerusalem artichoke, leek, onion, and garlic. FOS are shorter chains of the same fructose-based structure. Both selectively feed Bifidobacterium and Lactobacillus species among the most beneficial gut bacteria associated with immune health, SCFA production, and reduced intestinal permeability.

The limitation: inulin and FOS are high-FODMAP, meaning they can cause bloating, gas, and digestive discomfort in people with IBS or gut sensitivity, particularly at higher doses. This is why products containing chicory root (a primary inulin source) aren't appropriate for everyone.

Galactooligosaccharides (GOS)

Produced from lactose (though the final product contains no lactose), GOS are the primary prebiotic in human breast milk which is why infant formula often includes them. They selectively feed Bifidobacterium infantis and are among the most important prebiotics for early immune programming. Adults also benefit: GOS at 1g per day has shown reductions in self-reported anxiety and cortisol reactivity to stress in clinical trials linking the gut-brain axis to prebiotic research.

Resistant Starch

Starch that resists digestion in the small intestine and arrives intact in the colon. Four types exist: RS1 (physically enclosed in plant cells), RS2 (raw potato starch, unripe banana), RS3 (retrograded starch formed when cooked starch is cooled why cold rice or pasta has different properties than hot), and RS4 (chemically modified). Resistant starch selectively feeds butyrate-producing bacteria including Faecalibacterium prausnitzii and Roseburia species making it particularly relevant to gut barrier health and inflammatory conditions.

Beta-Glucan

A soluble fibre found in oats and barley. The most evidence-backed prebiotic for cholesterol reduction (where it has pharmaceutical-grade evidence and FDA-approved heart health claims). Also supports gut microbiome diversity and immune function. Beta-glucan is one of the few prebiotic fibres with consistent positive effects in cardiovascular health research alongside its gut health role.

Partially Hydrolysed Guar Gum (PHGG)

A relatively gentle prebiotic that has minimal FODMAP concerns better tolerated in gut-sensitive populations than inulin or FOS. Research shows benefits for IBS symptoms including constipation and diarrhoea, and consistent prebiotic effects on Bifidobacterium populations.

Polyphenols as Prebiotics

A newer and expanding area of prebiotic research: plant polyphenols (from berries, green tea, cocoa, and other plant sources) selectively modulate gut microbiome composition in ways that meet the formal prebiotic definition. They increase Akkermansia muciniphila abundance (associated with improved gut barrier and metabolic health), feed Bifidobacterium, and inhibit some pathogenic species. This provides another mechanistic rationale for polyphenol-rich plant foods and greens supplements beyond their direct antioxidant and anti-inflammatory effects.

How Prebiotics Work: The SCFA Mechanism

When beneficial gut bacteria ferment prebiotic fibre, they produce short-chain fatty acids (SCFAs) primarily acetate, propionate, and butyrate. These metabolites are among the most important compounds produced in the gut:

• Butyrate: The primary fuel source for colonocytes (intestinal lining cells); supports gut barrier integrity; has anti-inflammatory and potential anti-cancer properties; signals appetite-regulating hormones including GLP-1
• Propionate: Transported to the liver for gluconeogenesis regulation; signals GLP-1 secretion; associated with reduced appetite and body weight regulation
• Acetate: Systemic anti-inflammatory effects; substrate for propionate and butyrate production by cross-feeding bacteria

Reduced SCFA production from insufficient prebiotic fibre intake is associated with increased intestinal permeability ("leaky gut"), higher systemic inflammation, impaired immune regulation, and worse metabolic outcomes. This is the core mechanistic link between low plant fibre intake and chronic disease risk.

Why They're in Supplements

The average Australian adult consumes approximately 20-25g of dietary fibre daily below the recommended 25-38g. More importantly, much of that fibre comes from a narrow range of plant foods that may not include the most effective prebiotic substrates. A greens supplement with prebiotic fibre closes the gap more efficiently than relying on dietary pattern change alone.

For people on restrictive diets, with limited access to plant food diversity, or managing conditions that reduce food intake (like GLP-1 therapy), prebiotic supplementation is particularly valuable.

GRNS includes prebiotic fibre sourced from gut-friendly plant sources chosen to provide meaningful prebiotic benefits without the high-FODMAP concerns that make some prebiotic sources problematic for gut-sensitive users.

Frequently Asked Questions

What's the difference between prebiotics and probiotics?
Probiotics are live beneficial microorganisms you add bacteria directly to the gut. Prebiotics are the food that feeds and selectively grows the beneficial bacteria already living in your gut. Both have value; they work best in combination (sometimes called a "synbiotic" combination). Prebiotics are arguably the more fundamental intervention because they support the entire existing microbiome rather than adding specific strains.

Can I get enough prebiotics from food?
Yes, if you eat 30+ different plant foods per week with an emphasis on high-fibre vegetables, legumes, whole grains, and a variety of fruits. For most people's actual dietary patterns, supplemental prebiotic fibre significantly increases the prebiotic substrate available to gut bacteria. Both approaches are better than either alone.

Are there any side effects from prebiotic supplements?
The most common are digestive: gas, bloating, and altered bowel frequency particularly when first starting, or with high-FODMAP prebiotics (inulin, FOS) in sensitive individuals. Starting with a lower dose and building up gradually minimises these effects. Low-FODMAP prebiotic options (PHGG, resistant starch) are better tolerated by people with IBS.