What is fructo-oligosaccharide (FOS), and where is it found?

Fructo-oligosaccharide, usually shortened to FOS, is a short chain of fructose sugars. In the form made from table sugar, one glucose unit is joined to 2, 3 or 4 fructose units. Inulin is the same kind of fructose chain, only longer, and the two are often grouped together as inulin-type fructans. Humans lack the gut enzymes that break the bonds between the fructose units, so FOS is not digested the way sugar is. Onion, garlic, banana, wheat and chicory root contain these chains, mostly in small amounts. FOS tastes about 30 to 35% as sweet as sugar. It has no Thai Reference Daily Intake (Thai RDI); the Thai RDI for dietary fibre is 25 g a day.

In short:

• What it is: a short chain of fructose units, usually with a glucose at one end
• Chain length: FOS is the short form; inulin is the long form of the same chain
• Foods: onion, garlic, shallot, leek, banana, wheat, chicory root, Jerusalem artichoke
• How it is made: from sucrose with enzymes, or by shortening inulin from chicory root
• Sweetness: about 30 to 35% of sugar
• Daily amount: no Thai RDI for FOS; Thai RDI for dietary fibre 25 g
• Comfort: increasing fibre too quickly can cause gas and bloating; fructans can cause stomach symptoms in some people

FOS is often mentioned together with probiotics. What the terms probiotic and prebiotic mean is explained in What is the difference between probiotics and prebiotics?

What is FOS made of?

A chain of fructose units

Fructose is the sugar found in fruit and honey. When several fructose units are linked in a chain, the result is called a fructan. A 2023 review from the University of Reading in the United Kingdom describes inulin-type fructans as fructose units joined by a particular bond, called β-(2-1), with chains of different lengths. The number of units in a chain is called the degree of polymerisation (DP).

When FOS is made from sucrose (table sugar), the products are named by how many fructose units are attached to the glucose:

Source: Jackson and colleagues, University of Reading, 2023; PubChem for 1-kestose and nystose.

Why it is not digested like sugar

The Reading review explains that humans lack the enzymes on the lining of the small intestine that break the β-(2-1) bonds between fructose units. FOS and inulin therefore pass through without being digested the way sugar is, and the review gives their energy as about 1 to 1.5 kcal per gram.

How is FOS different from inulin?

FOS and inulin are the same kind of chain. The difference is length. The Reading review puts the cut-off at about 10 units: below it are the short forms (FOS and oligofructose), above it the long forms (inulin).

Source: Jackson and colleagues, 2023.

The review also says that the chain length in plants depends on the plant, the time of year and how long it is stored after harvest. Wheat, bananas and onions have short chains (fewer than 10 units), Jerusalem artichokes medium chains (fewer than 40) and globe artichokes and chicory root long chains (fewer than 100). Neither form is better than the other; they behave differently in food.

Which foods contain FOS?

Most fruits and vegetables contain only small amounts of these chains. The Reading review notes that chicory root is the exception, with up to 70% inulin on a dry weight basis, which is why commercial inulin is mainly made from it. The studies below measured different things (total fructans, single FOS molecules, or inulin as FOS) in different units, so the numbers cannot be compared with one another directly.

Onion, garlic and other vegetables

Researchers at Monash University in Australia measured total fructans in 60 vegetables and 43 fruits in 2007. Garlic, artichoke, shallots, leek bulb and onions had the most, at 1.2 to 17.4 g per 100 g fresh weight. Fruits such as longan, white peach, persimmon and melon had low but measurable amounts. Total fructans include both short and long chains, so this is not FOS alone.

Thai bananas

A 2022 study from Naresuan University and Rajamangala University of Technology Lanna Tak followed two varieties of kluai namwa, the banana used for sun-dried banana (kluai tak) in Phitsanulok. Inulin as FOS rose as the fruit ripened, from 0.05 to 0.12 g per 100 g dry mass in Mali Ong and from 0 to 0.75 g in Nuanchan. Four commercial sun-dried banana products had 0.11 to 0.24 g per 100 g dry mass. The figures are per dry mass, not per fresh banana.

A 2008 study from the University of São Paulo found 1-kestose, the smallest FOS, in all eight Brazilian banana cultivars it tested when fully ripe, at 297 to 1,600 µg per g of dry matter. Nystose was found only in one cultivar. The authors noted that FOS amounts in bananas reported by different studies vary with the cultivar, the ripeness and the method of analysis.

Chicory root and Jerusalem artichoke

A 2008 study from the National Institute for Nutrition and Food Safety, China CDC found 6.61 g of 1-kestose and 6.51 g of nystose per 100 g dry weight in chicory root, the highest of the vegetable tissues it tested. In Thailand, researchers at Suranaree University of Technology and Khon Kaen University found the most inulin in Jerusalem artichoke tubers harvested 50 days after flowering, at 734.9 g per kg dry weight. This is mostly long-chain inulin rather than FOS.

How is FOS made?

From table sugar

The Reading review explains that short-chain FOS can be made by treating sucrose with enzymes that move fructose units onto it, producing 1-kestose, nystose and 1F-fructofuranosylnystose. A 2023 study from Naresuan University made short-chain FOS in the same way from sugarcane syrup, using the two cultivars most widely grown in Thailand, Khon Kaen 3 and Suphanburi 50, and two commercial enzyme preparations. The amount of short-chain FOS rose as the sucrose in the mixture was raised up to 40%, and it levelled off after about 4 hours of reaction.

From chicory root

Inulin is extracted from chicory root in three main steps, according to the Reading review: extraction with hot water, purification and spray drying. If shorter chains are wanted, the inulin is partly broken down with enzymes, which gives oligofructose.

In food products

Because short chains are sweet and dissolve easily, food makers use them to replace part of the sugar, while long-chain inulin is used to replace part of the fat. The review lists cakes, muffins, bread, breakfast cereals, cheese, ice cream, yoghurt and fruit juices among the foods that have been made with them.

Do cooking, storage and processing change FOS?

The Reading review and a Thai study report these measurements. They describe what happens to the chains, not how anyone should cook.

Is there a recommended daily amount of FOS?

Thai RDI

FOS has no Thai RDI of its own. The Thai RDI for dietary fibre is 25 g a day.

United States

In 2000, the US Food and Drug Administration closed a notice on FOS with no questions. The notice covered use in foods in general, excluding meat, poultry and infant formula, at up to 20 g a day for the general population and up to 4.2 g a day for infants under one year.

Who should be careful with FOS?

People who notice gas or bloating

MedlinePlus, from the US National Library of Medicine, advises adding dietary fibre to the diet slowly, because increasing it too quickly can lead to gas, bloating and cramps. The Monash University researchers also note that fructans can cause gastrointestinal symptoms in some people.

People with a regular health concern who are under a doctor's care

People whose doctor or dietitian has given them advice about what to eat should follow that advice and ask before adding FOS.

Pregnant or breastfeeding women and people who take regular medication

If you are pregnant or breastfeeding, or take medication regularly, check with a doctor or pharmacist before taking supplements.

Dietary supplements are not intended to prevent, treat or cure any disease.