Prebiotics and Fiber

Dietary fibers: soluble and insoluble

Dietary fibers are complex carbohydrates of plant origin that the human body is unable to digest by its own enzymes. They are classified into two broad categories based on their behavior in an aqueous environment:

• Soluble fibers – dissolve in water to form a viscous gel in the gastrointestinal tract. They slow glucose and fat absorption, contribute to satiety and are fermented by intestinal bacteria. Examples: Inulin, Pectins, Beta-glucan, Guar gum, Psyllium.
• Insoluble fibers-do not dissolve in water, increase the volume of the fecal bolus, accelerate intestinal transit, and reduce the contact time between potential harmful substances and the colonic mucosa. Examples: cellulose, hemicellulose, lignin, fiber from wheat bran.

Most Prebiotics belong to the category of fermentable soluble fibers, although research has also identified structural components of non-glucose origin with prebiotic properties.

The main types of Prebiotics

The most studied Prebiotics used in the formulation of dietary supplements and functional foods are:

• Inulin and FOS (Fructooligosaccharides) -the most common prebiotics in formulation. They are obtained mainly from chicory root. Inulin is a long-chain polymer of fructose units; FOS are shorter-chain oligomers. Both selectively stimulate the growth of Bifidobacterium and Lactobacillus. Native Chicory Inulin is the only prebiotic with an EFSA-approved health claim (see dedicated section).
• GOS (Galactooligosaccharides) — derived from lactose by enzymatic means. Particularly effective in promoting the growth of Bifidobacterium. They occur naturally in human breast milk as components of milk oligosaccharides (HMOs).
• Beta-glucan — soluble fiber found in oats and barley. In addition to its prebiotic properties, it has documented effects on reducing LDL cholesterol and modulating postprandial glycemic response, with approved EFSA claims.
• Pectins — plant cell wall polysaccharides, abundant in apples, citrus fruits and berries. Soluble fibers with gelling properties and documented effects on cholesterol. High and low methoxyl pectins have different functional characteristics.
• Resistant starch (RS) — fraction of starch that escapes digestion in the small intestine and reaches the intact colon, where it is fermented. Main substrate for butyrate production by colonic bacteria.
• XOS (Xyloligosaccharides) — xylose oligomers obtained from lignocellulosic sources such as corncob and wheat straw. High selectivity for Bifidobacterium. Effective at much lower doses than Inulin and FOS (0.4-2 g/day).
• Lactulose – synthetic disaccharide of galactose and fructose. Not digested by human enzymes, it reaches the colon where it is fermented. Only prebiotic with EFSA approved claim to accelerate intestinal transit.

Food sources

Prebiotics are naturally present in many plant-based foods. The richest sources of Inulin and FOS include chicory root, Jerusalem artichoke (topinambur), garlic, onion, leek, asparagus, and unripe banana. Beta-glucan is found mainly in oats and barley. Pectins are abundant in the skin of apples, citrus fruits, and quinces. Resistant Starch is found in legumes, unripe banana, potatoes cooled after cooking, and cooked and cooled rice.

In formulation, the most commonly used sources are standardized chicory extracts for Inulin and FOS, oat fiber for Beta-glucan, and whey products for GOS. The choice of source influences both the technological characteristics of the finished product and the expected intestinal fermentation profile.

Mechanism of action

Prebiotics reach the colon undigested and are selectively fermented by resident beneficial bacteria. This process produces:

• Short-chain fatty acids (SCFA) — mainly butyrate, propionate, and acetate. Butyrate is the main energy source for colonic epithelial cells and plays a role in maintaining the integrity of the intestinal barrier. Propionate is involved in the regulation of hepatic blood glucose. Acetate participates in peripheral metabolism.
• Reduced colic pH — acid production lowers the pH of colon contents, creating a less favorable environment for pathogenic bacteria and promoting the colonization of beneficial bacteria.
• Increased beneficial bacterial biomass — selective fermentation leads to an increase in Bifidobacterium and Lactobacillus populations, resulting in a modulatory effect on the intestinal microbiota as a whole.
• Modulation of intestinal immune response — SCFAs and other fermentation metabolites interact with immune cell receptors in the intestinal mucosa, helping to regulate local inflammation.

Gut Microbiota and Prebiotics

Prebiotics act directly on the resident Gut Microbiota, unlike Probiotics that introduce live microorganisms from outside. This makes them particularly attractive for long-term microbiota modulation strategies: instead of administering bacteria, they provide the necessary substrate for those already in the body to thrive.

The Symbiotic concept — the combination of Probiotics and Prebiotics in the same formulation-is based on this complementarity. The combination can be either complementary (the prebiotic feeds the resident microbiota in general) or synergistic (the prebiotic is specifically selected to feed the probiotic strain introduced with the formulation).

Probiotics, Prebiotics and Postbiotics: the distinctions

Prebiotics fit into the family of biotics along with Probiotics and Postbiotics. While Probiotics are live microorganisms and Postbiotics are the bioactive compounds produced by microbial fermentation — short-chain fatty acids, peptides, enzymes, vitamins — Prebiotics are the substrate that makes the whole process possible. Understanding the distinctions between these three categories is the starting point for informed formulation in the gut health segment.

EFSA Claims Approved for Prebiotics and Fiber

Unlike Probiotics, some fibers with prebiotic properties have health claims approved by EFSA under EC Regulation 1924/2006. The relevant distinction is that the approved claims refer to specific compounds, not to the generic category of “Prebiotics.”

• Native Chicory Inulin — approved claim: “Inulin improves bowel function,” attributed to ability to increase stool frequency. Effective dose: at least 12 g/day.
• Beta-glucans from Oats — approved claims for maintaining normal blood cholesterol levels (3 g/day) and reducing postprandial glycemic response (1.2 g per 30 g of available carbohydrate in the meal).
• Beta-glucans from Barley — approved claim for maintaining normal blood cholesterol levels (3 g/day).
• Pectins — approved claim for maintaining normal blood cholesterol levels (6 g/day).
• Lactulose — approved claim: accelerates intestinal transit.
• Oat, Barley and Rye fibers — approved claims for contribution to increased fecal mass and regular intestinal transit.

To date, the generic term «prebiotic» is not authorized as a health claim by EFSA, similarly to the term «probiotic». In Italy, the Ministry of Health Guidelines on Probiotics and Prebiotics (revised March 2018) include specific provisions for the use of Prebiotics in dietary supplements.

The forms available for formulation

Prebiotics and Fibers are among the most technologically versatile functional components. The most widely used forms in formulation are:

• Powders and Granules — the most widely used form. They allow high dosages, good solubility (for soluble fiber) and easy incorporation into sachets, stick packs and powder formulations. Inulin powder also has interesting technological properties as a fat replacer in some food matrices.
• Capsules and Tablets — suitable for smaller doses typical of XOS and symbiotic formulations with Probiotics. They facilitate compliance for those who do not like the slightly sweet taste of Inulin and FOS.
• Liquid Formulations — GOS and FOS find application in functional beverages and liquid supplements because of their good solubility and neutral or slightly sweet taste. They require attention to stability in acidic environments.
• Additive Foods — yogurts, bars, breads and baked goods fortified with prebiotic fiber represent the main mode of intake in the general population. The choice of fiber depends on compatibility with the food matrix and process conditions.

Excessive intake of Prebiotics, particularly Inulin and FOS, may cause gastrointestinal effects such as bloating, flatulence and abdominal cramping, especially in the early stages of supplementation or in individuals with sensitive intestines. It is advisable to start with small doses and increase gradually. Those with irritable bowel syndrome or inflammatory bowel disease should consult their physician before supplementing with high doses of fermentable fiber.

The information in this sheet is for informational purposes only and is not a substitute for the advice of a health care professional. You should always consult your physician or pharmacist before beginning any supplementation.