Fructooligosaccharides (FOS) are short chain di-, tri- and polysaccharides that are composed of a glucose unit bonded to between 1 and 4 β(1-2) linked fructose units. A number of plant foods contain FOS (e.g. onion, leek, asparagus, garlic and artichoke) but they are also produced on a commercial scale synthetically from sucrose using the fungal transfructosylating enzyme fructofuranosidase or via partial hydrolysis with endoglycosidases from chicory inulin. Because of its sweet taste and reduced energy value compared to sucrose, FOS is often used to sweeten foods, but it is also available as a dietary supplement. Because there are no enzymes present in the small intestine capable of hydrolysing the β(1-2) bonds it is unlikely that digestion occurs, as evidenced from rat studies. More likely, FOS are fermented in the colon by colonic microflora, resulting in the production of short-chain fatty acids (SCFA).
The metabolic fate of FOS in humans has been investigated by researchers1, who fed 6 healthy volunteers (aged 20-27 years) either a control diet or a diet containing 20.1g of FOS per day for 11 days. The control diet was designed to contain as few foods with β(1-2) fructans as possible. Following ingestion, FOS appeared in the distal ileum after 30min and was recovered for the following 8 hours via intestinal aspiration. Around 89% of the ingested dose of FOS was detected in the distal ileum, suggesting that FOS is poorly digested and absorbed. The amount of FOS recovered from urine was 24mg, which corresponded to 0.04% of the total ingested dose of FOS, and no FOS was detected in the faeces. Although ingestion of FOS can be associated with gastrointestinal symptoms, no side effects were reported by the subjects.
The researchers estimated the mean energy value of the FOS as 2.3kcal, based on the energy produced from the SCFA propionate, acetate and lactate. Most of the FOS reaching the colon had not been hydrolysed indicating minimal involvement of the small intestine in digestion. Approximately 1% of the FOS that disappeared from the small intestine, appeared in the urine suggesting that at least some absorption of intact FOS occurs from the small intestine. Some hydrolysis of FOS may have occurred in the very distal part of the ileum, where low levels of colonic microflora exist. In addition, the low pH of the stomach has been shown to be capable of fructan bond hydrolysis in vitro. The fact that no FOS was detected in stools suggests that it is completely fermented in the colon by microflora, which results in the formation of SCFA, gas production, bacterial growth and heat production.
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