The Basics

Before we get into the specifics about FOS and Inulin we need to understand the concept of Dietary Fiber, as so, dietary fiber is the intake combination of insoluble and soluble fiber foods in our diet.


Insoluble Fiber

Insoluble fiber is mainly found in cereal-based foods; examples include: bread, rice, corn, potato, and some vegetables like cabbage and broccoli 

Cellulose is the best-known substance that acts as insoluble fiber, as its name implies insoluble fiber is NOT fermented by our gut bacteria, instead, what insoluble fiber does is to retain water along the digestive tract, especially the colon, insoluble fiber, therefore, provides larger and softer stools to aid in gut health.

Soluble Fiber

Soluble fiber, on the other hand, is fermented indeed by our gut bacteria, soluble fiber has synonym like prebiotics, this means it helps friendly bacteria to proliferate, the proliferation of this friendly bacteria brings an array of health benefits, not only promotes gut balance but friendly bacteria also regulates hormonal gut secretion, mood, bone health, blood glucose, inflammation, and lipid metabolism.

We can find soluble fiber in a wider variety of foods like tubers, legumes, fruits, cereals, nuts, and vegetables, examples include Yacon, oats, beans, guavas, apples,  almonds and carrots.   


FOS (fructooligosaccharides) and Inulin are the best-known substances to act as soluble fiber - prebiotics.

Now we know that our focus is on the soluble fiber side of the dietary fiber, FOS and Inulin may seem very similar but they are not, by the end of this post we will explain why FOS is scientifically proven to be a better prebiotic than Inulin.


Understanding FOS and Inulin

Both (FOS and Inulin) are chemically categorized as polysaccharides (poly = many and saccharides = sugar) in this context we know get a bigger and simple picture of FOS and Inulin, we can say in simple terms that they are big or long molecules of sugars that can only be digested by our gut bacteria.

Something very important is to not confuse this type of long molecules of sugars with simple or shorter molecules sugars like monosaccharides (one molecule of sugar, like glucose and fructose) and disaccharides (two molecules of sugars, like sucrose), the difference not only is in its chemical structure but also in their metabolism.

Our body can digest simple sugars like monosaccharides and disaccharides because we have specialized enzymes, but a bigger chain of sugar molecules becomes harder to break down and we lack the enzymes to do so, this is were bacteria comes to aid, because bacteria indeed have enzymes to break down big molecules of sugars = polysaccharides.  

In this picture above we can see the difference in the number of sugars molecules, in (1) we can see a Sucrose as a point of reference, Sucrose is a Disaccharide containing two molecules, (G) glucose and (F) fructose, in (2) we can see three examples of FOS being Kestose, Nystose, and Fructosylnystose, the important thing to note is not their name but its structure as they become bigger in length and in molecular weight, with a higher number of fructose molecules each, in (4) we can see a molecule of Inulin being far greater in length, the number of fructose molecules can reach up to 100.

After analyzing the picture the concept of FOS is becoming more clear, the name “Fructooligosccharides” comes from:

  • Fructo = fructose 
  • Oligo = between 10-20 molecules 
  • Saccharides = sugars

This means that FOS are substances of 10-20 molecules of fructose and that Inulin does not classify as an “Oligo” because it posses from 20-100 molecules of fructose, in other words making it the bigger brother of FOS.

Which one is better?

Inulin per se does NOT have more content of FOS like Kestose, Nystose, and Fructosylnystose, what it means is that the Inulin structure is only bigger in size. 

This is the key difference between FOS and Inulin, the size of its structure, because of its longer chain and molecular weight Inulin will be harder to get fermented by friendly bacteria and/or it will take more time to produce bacteria signaling, therefore all the health benefits of friendly bacteria proliferation will be LESS effective than FOS.

In one novel study, scientists measure the BIOACTIVITY of Fermentation of Fructooligosaccharides (FOS) and Inulin by 55 strains of friendly bacteria present in humans, rabbits, rodents, and pigs.

The results were that FOS containing foods exhibits bioactivity in ALL the 55 strains, whereas Inulin failed to be fermented by most of the 55 friendly bacteria strands, this happened because of Inulin molecular weight, friendly bacteria simply couldn't break it down.

In the picture above we can see all the mayor friendly bacteria strands, the longer the bar in each one reflects a higher “proliferation” effect.

  • Glucose (used as reference) = white bar
  • FOS = gray bar
  • Inulin = black bar


In fact, Inulin is soluble fiber, so that it can be taunted as a prebiotic, but its FOS molecular composition and its chemical that gives it the edge, friendly bacteria proliferate faster, and all the health benefits that come from that are more efficient; therefore the bioavailability of FOS as a prebiotic is superior than Inulin.

In addition, Inulin is mainly extracted from a plant called Chicory (Cichorium intybus) because this plant is made up to 40% of its total mass of Inulin, pharmacological studies on Chicory are highlighting the potential toxicological effects due to secondary metabolites, and considering the plant as a known source of allergic problems, therefore a contraindication for hypersensitivity should be mention when advised to take Inulin.


  • A. Judzentiene and J.Budiene, “Volatile constituents from aerial parts and roots of Cichorium intybus L. (chicory) grown in Lithuania,” Chemija, vol. 19, pp. 25–28, 2008.
  • Rossi M, Corradini C, Amaretti A, Nicolini M, Pompei A, Zanoni S, et al. Fermentation of fructooligosaccharides and inulin by bifidobacteria: a comparative study of pure and fecal cultures. Appl Environ Microbiol. 2005.
  • European Medicines Agency, “Assessment report on Cichorium intybus L., radix,” EMA/HMPC/113041/2010, 2013.
  • Sabater-Molina, M.; Larqué, E.; Torrella, F.; Zamora, S. Dietary fructooligosaccharides and potential benefits on health. J. Physiol. Biochem. 2009.
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This article/this website is very informative.


We are channel partners of 100% pure FOS distribution in India .There were some myths about Inulin. But the Article is so good and self explanatory . Thanks to the writer.

Ajay chopra

Thank you very much for this very informative article. I used FOS in the past and then inulin, thinking of all the benefits its suppose to have and coming from chirory thinking it a healthy vegetable. My ‘eyes’ have been wide opened by your article and I will be trying to find pure 100% FOS, althought mostly I see it together with the inulin. Thanks again from Olga in UK


I got a very clear and precise understanding from this article. Thanks a lot!!!


Since the much of the world mention FOS when they are talking about inulin how can we ensure that we are purchasing true FOS, and not inulin ?

Robert Crombie

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