Prebiotics

What is a prebiotic?

The 2017 ISAPP expert panel defined a prebiotic as “a substrate that is selectively utilized by host microorganisms conferring a health benefit.” This definition extends the concept to non-carbohydrate substances, to body sites other than the digestive tract and to non-food products; the requirement of a selective, microbiota-mediated mechanism is retained, and a beneficial health effect must be documented. ^[1] This requirement matters: that a fiber is fermented in the gut does not automatically make it a prebiotic.

A prebiotic is not a living microorganism; this is the difference from a probiotic. A mixture of the two can be a synbiotic, according to ISAPP’s 2020 definition. ^[2]

Which foods contain prebiotics?

According to Slavin’s review, prebiotics occur naturally in foods such as leeks, asparagus, chicory, Jerusalem artichoke, garlic, onion, wheat, oats and soybeans. Inulin and oligofructose, lactulose and resistant starch meet all aspects of the prebiotic definition according to the review; galacto-oligosaccharides (GOS), polydextrose, wheat dextrin, acacia gum, psyllium, bananas, whole-grain wheat and whole-grain corn also show a prebiotic effect. ^[3]

Intake from foods is in small amounts. According to a calculation from the 1994–1996 US dietary survey, average daily intake is 2.6 g of inulin and 2.5 g of oligofructose; about 70% of this is provided by wheat and 25% by onion. ^[6] This data belongs to US diets; equivalent intake data for Türkiye has not been used on this page.

How do prebiotics act in the body?

Prebiotics reach the large intestine undigested in the small intestine and are fermented there by bacteria. The fermentation products are mainly short-chain fatty acids (SCFAs) such as acetate, propionate and butyrate, and lactate. Butyrate is the preferred energy source for colon cells; SCFAs lower colon pH and may increase absorption of minerals such as calcium. Fermentation also increases bacterial mass and stool mass. SCFAs are water-soluble and pass into the blood; the brain, muscle and tissues use acetate systemically, while propionate is cleared in the liver; SCFAs are thought to contribute about 7–8% of the host’s daily energy requirement. ^[3] The review also states that butyrate may be protective against colon diseases, but that data on this are conflicting. ^[3]

Are prebiotics good for constipation?

Two sources should be read side by side. A meta-analysis examining the effect of inulin in chronic constipation (5 RCTs, 252 people) found a significant favorable effect on stool frequency (effect size 0.69; 95% CI 0.04–1.34), stool consistency (Bristol scale, 1.07; 95% CI 0.70–1.45) and bowel transit time (−0.57; 95% CI −0.99 to −0.15); the risk of hard stools also decreased (RR 0.42; 95% CI 0.26–0.70). Pain and bloating did not improve. One of the authors of the meta-analysis is affiliated with a dairy company. ^[4] Slavin’s review, on the other hand, states that although inulin is extensively fermented, its effect on stool weight is small, increasing stool weight by less than 1 g per gram of fiber consumed; wheat bran is referred to as the “gold standard” for stool bulk. ^[3]

Prebiotic trials in chronic idiopathic constipation are few; a systematic review states that no firm conclusion can be drawn on this. ^[8]

Do prebiotics increase calcium absorption?

In a double-blind, crossover study of 31 healthy girls aged 10–13, participants consumed beverages containing 0, 5 or 10 g/day of GOS in three-week periods. The calcium absorption fraction was 0.393 in the control, 0.444 with 5 g GOS and 0.419 with 10 g GOS; both doses were significantly higher than the control, but there was no dose-response relationship. Fecal bifidobacteria also increased with GOS. The authors concluded that 5 g/day of GOS increased calcium absorption and that the effect may be mediated through bifidobacteria. ^[5] This finding rests on a single, small study in adolescent girls; bone outcomes were not measured.

Prebiotics in irritable bowel syndrome: the tolerance problem

Fermentable substances can cause gas and bloating. Slavin’s review reports that some prebiotics (inulin) impair digestive tolerance, especially when consumed in large amounts, that fructo-oligosaccharides can cause complaints even at low doses such as 10 g, whereas polydextrose and resistant starch were consumed without symptoms up to 50 g. ^[3] The 2021 ACG guideline suggests a time-limited trial of a diet low in fermentable oligosaccharides, disaccharides, monosaccharides and polyols (FODMAP) to improve global symptoms in IBS (conditional recommendation, very low certainty evidence). ^[7] Trials of prebiotics and synbiotics in IBS are few, and no firm conclusion could be drawn. ^[8]

Prebiotic evidence at a glance

Are prebiotics taken from food or supplements?

According to Slavin’s review, the most commonly consumed foods are low in fiber; one serving usually contains 1–3 g of fiber, and whole-grain cereals, legumes and dried fruits contain more. Fiber intake in the USA is about half of the recommended amount (about 15 g/day; label regulation is based on 25 g/day). ^[3] This is US data; equivalent intake data for Türkiye has not been used on this page. The tolerance level of prebiotic substances varies by substance and amount (inulin and FOS have been tolerated at lower amounts, polydextrose and resistant starch at higher amounts). ^[3]

What is a synbiotic?

The 2020 ISAPP consensus defines a synbiotic as “a mixture comprising live microorganisms and substrate(s) selectively utilized by host microorganisms that confers a health benefit on the host.” The panel concluded that regarding a synbiotic simply as a mixture of a probiotic and a prebiotic may limit innovation. A “complementary” synbiotic, whose components are not designed to work together, must consist of a probiotic plus a prebiotic; this requirement does not apply to a “synergistic” synbiotic, in which the substrate is designed to be selectively utilized by the co-administered microorganism. ^[2]

Prebiotic supplements and health claims in Türkiye

The prebiotic component claim on TİTCK’s health claims list is: “Prebiotic components support the development and survival in the gut of probiotic microorganisms that help regulate the digestive system and support the immune system.” The condition for the claim was that the product contain at least 1.25 g and at most 3.75 g of prebiotic component per serving and that the label state that at least 5 g/day must be consumed for the effect to be seen. ^[10] By its September 27, 2024 Makam Olur (official approval), TİTCK suspended this claim on the grounds that there was insufficient scientific evidence to support it and that there was no official dietary recommendation on “adequate intake” or “recommended daily amount” for prebiotics in healthy individuals; existing products may use the claim only on their labels until the decision is finalized. ^[9]

This guide does not recommend personal doses or products; in people with digestive disease, increases in fiber and prebiotics are evaluated together with a physician or dietitian.