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Introduction

The human digestive tract produces α-amylases and brush-border enzymes capable of hydrolysing simple starches and common disaccharides, respectively.  However, a significant variety and quantity of carbohydrates cannot be broken down by humans.  This is because we either lack enzymes able to fully access and recognise many carbohydrate substrates, or absorption is limited because there are no suitable transporter proteins.

Undigestible, and poorly digestible carbohydrates can be divided into three groups:

1. Polysaccharide dietary fibres (DFs) that are soluble;

2. DFs that are insoluble;

3. Miscellaneous small sugars, oligosaccharides and polyols that are enzyme resistant or poorly absorbed.

These three groups are discussed in the next sections.

Dietary Fibre

Dietary fibres are an example of undigestible carbohydrates. These polysaccharides are important in our diet because of their ability absorb water, thus making stools larger, softer and easier to pass. People who suffer from constipation or watery, loose stools usually benefit from an increase of fibre in their diet (Mayo2022). In addition, a high-fibre diet tends to reduce the development of haemorrhoids and diverticular diseases. There is research that suggests fibre marginally reduces blood cholesterol and glucose levels for people suffering from high cholesterol and type-II diabetes, respectively (Mongeau2016).

Dietary fibre is usually defined as the total "non-starch polysaccharides plus lignin" we ingest.  This roughly translates to any polysaccharide (and lignin) part of the plant that is undigestible by our endogenous enzymes.  Resistant starch and oligosaccharides have been added to this definition, which complicates things somewhat, because these carbohydrates, along with some traditional fibre substrates, can be partially fermented by bacteria in the colon.

A list of a number of fibres and the types of glycosidic links they contain are summarised in Table 1.  Included are digestible starches for reference. For a list of foods containing fibre, see Foods with Fibre.

Table 1. Some common polysaccharides and their digestive fates in humans. A number of undigested fibres and oligosaccharides are fermented by colonic bacteria, producing SCFAs, which can be absorbed and used by the body. Full red circle indicates oxygen is above the ring plane, whereas the black & red circles indicates the oxygen is below it. Blue represents nitrogen and green simulates a carboxylate sidechain.

Sugar Linkage Structure	Link	Example	Enzymes	Solubility	Products	Source	Human Digestible	Bacteria Examples
	β-1,4-	cellulose	β-glucosidase,  cellulase, cellobiase	insoluble	glucose	plants	No	Lachnoclostridium phytofermentans; Clostridium chartatabidum; Cellulosimicrobium cellulans; Bacteroides cellulosilyticus; Eubacterium rectale;
	β-1,4-	chitin	β-glucosaminidase, chitinase	insoluble	N-acetyl-glucosamine	fungi, crustaceans	No	Paramuribaculum intestinale; Stenotrophomonas maltophilia; Bacillus licheniformis; Clostridium sporogenes; Clostridium tertium;
	α-1,4-	amylose	α-amylase	varies	glucose	plants	Yes	Bacteroides ovatus; Enterococcus faecium; Bacteroides thetaiotaomicron; Bacteroides caccae; Bacteroides fragilis;
	α-1,6- α-1,4-	amylopectin, glycogen	α-amylase α-(1→6)-glucosidase	insoluble	glucose	plants	Yes
	β-1,4- β-1,3-	β-glucan	glucanase	soluble	glucose	plants: cereals, yeast	No
	β-1,4-	mannan, guar gum	mannanase, mannosidase, galactosidase	insoluble	mannose, galactose (minor)	yeasts	No	Paenibacillus typhae; Lysinibacillus varians; Georgenia satyanarayanai; Brevibacterium samyangense
	α-1,4-	pectin	pectin lyase, pectinase	soluble	glucuronate	plants: apples, citrus	No	Bacillus subtilis; Bacteroides galacturonicus; Bacillus licheniformis; Bacteroides caecimuris; Bacteroides cellulosilyticus;
	β-1,4-	xylan	xylanases	mostly no	xylose, glucuronate (minor)	plant hemicellulose	No	Lachnoclostridium phytofermentans; Bacteroides helcogenes; Bacteroides xylanisolvens; Bacteroides eggerthii; Bacteroides graminisolvens;
	β-2,1-	inulin	inulinases	soluble	fructose, glucose (minor)	plants: fruits, vegetables, grains	No	Bacteroides ovatus; Bacteroides thetaiotaomicron; Bacteroides uniformis; Bacteroides caccae; Bacteroides stercoris;

 

Fibre can be divided into two broad groups (Mongeau2016).  The component of dietary fibre that is soluble in water is defined as soluble fibre. It is found in oats, peas, beans, apples, citrus fruits, carrots, barley and psyllium (Mayo2022).  The portion not soluble is logically called insoluble fibre. Whole-wheat flour, wheat bran, nuts, beans and vegetables, such as cauliflower, green beans and potatoes, are good sources of this form (Mayo2022).

1. Soluble Fibre

Soluble fibre tends to form a gel-like substance when dissolved in water. It slows the process of stomach emptying and gives a longer sense of 'fullness'. Naturally occurring fibre of this type can come from several chemical classes, including those containing inulin, psyllium, pectin, oat β-glucan and guar gum.  Artificially modified soluble fibres, such as wheat dextrin and polydextrose, are often used to enrich processed foods with dietary fibre.

Inulin, a fructooligosaccharide (FOS) - and more specifically - a glucosyl (fructosyl)n fructose (n=3-60, Roberfroid1997, Raninen2011), is a linear polymer of up to 60 fructose units terminated with a glucose residue (Figure 1). It is classified as a soluble, non-viscous, readily fermentable, non-digestible oligosaccharide (McRorie2013).  Many colonic bacteria, such as Bacteroides ovatus, Bacteroides thetaiotaomicron, Bacteroides uniformis, Bacteroides caccae, and Bacteroides stercoris, are able to ferment this carbohydrate. Inulin is found in a variety of foods, including onions, wheat, bananas, artichokes, leeks, beet root and chicory root. 

 

The biopolymer is used to fortify foods with fibre and is thought to be of benefit to a healthy colonic microbiome, such as increasing levels of Bifidobacterium spp (Raninen2011). However, its effect on total cholesterol, LDL & HDL, and postprandial glycemia is marginal at best (Raninen2011).

Figure 1. Inulin is a heteropolymer made up primarily of fructose molecules (up to 60 residues) joined via a 1,6-glycosidic link but capped with a glucose residue.

 

Arabinoxylans are hemicellulose structures found in the primary and secondary cell walls of cereal grains and consist of a polymer of xylose with arabinose sidechains (Figure 2).  

Psyllium mucilage, which is characterised as a soluble, viscous non-fermenting fibre, is isolated from the husks of Plantago ovata seeds (Figure 3) and is perhaps the best known example of this class (McRorie2013). 

Because psyllium absorbs and holds water, it is used as a food thickener and has been consumed to relieve the symptoms of constipation.  Extended use of this fibre is also believed to reduce blood cholesterol in people with dyslipidemia, reduce blood glucose levels of patients suffering from type II diabetes, and lead to weight loss.

 

Figure 3. Psyllium is sourced from Plantago ovata, a plant native the Mediterranean (a). 
Husks (b) are light coloured from this species, but black from Plantago psyllium (from Draksiene2019).

Virtually all psyllium is excreted since few bacteria can act on this polysaccharide. Despite some research suggesting this fibre has prebiotic potential (Elli2008, Pollet2012), in vitro fermentation experiments performed in these studies exceed the usual transit time expected for psyllium to be eliminated from the large intestine. Extended use of this fibre has been shown to alter the microbiome populations of subjects experiencing chronic constipation, with a reduction in Actinomyces bacteria (e.g. Bifidobacterium spp.) and an increase in some SCFA-producing species (Jalanka2019). However, it is not clear whether this change is because of physical changes (increased stool size and water content) or any fermentative properties of psyllium.

 

Polyuronides are formed as copolymers of uronic acids (e.g. glucoronic, mannuronic and galacturonic acids) and monosaccharides (Figure 4).  Pectic polyuronides (pectin) are important soluble components of fruits and vegetables, and are described as a soluble, non-viscous, readily fermentable fibres (McRorie2013).

Figure 4. Generalised subunit of a pectic polyuronide (pectin).

Pectin is found in citrus, apples, rose hips and seeds.  Some important colonic bacteria, such as Bacillus subtilis, Bacteroides galacturonicus, Bacillus licheniformis, Bacteroides caecimuris, and Bacteroides cellulosilyticus, can attack pectin.

It should be noted that polyuronides can depolymerise at elevated temperatures and the process of cooking may change the characteristics of these dietary fibres (Mongeau2016). Although pectins share a similar α-1,4-glycosidic structure with amylose, there are no human enzymes that recognise the uronic acid moiety in this polysaccharide.

 

Cereal β-glucans, from oats, wheat, barley and rye are linear β-1,3- and β-1,4-glycosidic linked polymers of glucose (Figure 5). Oat, barley, and wheat contain between 2–6, 3–9, and 0.5–1 grams of β-glucan per 100 grams of cereal kernel, respectively (Livesey2014).

Figure 5. Beta-glucan glucose polymer substructure.  The 1,3-linked glycosidic residues prevent hydrolysis by human enzymes.

Oat β-glucan, which is found in the endosperm of the kernel, is classified as a water soluble, viscous and readily fermentable fibre (McRorie2013). Although it is not meaningfully digestible by human enzymes, there are numerous species of colonic bacteria that can rapidly and efficiently ferment this carbohydrate to SCFAs.

 

 

Galactomannan polysaccharides are polmers of mannose with galactose sidechains (Figure 6). Guar gum, an example of this group, is derived from guar beans (Cyamopsis tetragonoloba).  

Figure 6. Guar gum is a galactomannan polysaccharide with a β-1,4-mannose polymer backbone and 1,6-linked galactose sidechains.  Human enzymes are unable to digest this soluble fibre.

The raw, milled and unhydrolysed bean is a free-flowing powder and is described as a soluble, viscous and readily fermentable dietary fibre (McRorie2013).  The powder is used as a thickener, stabiliser, as a gluten-free replacement for wheat flower in baking (Fenster2014), and to increase the fibre content of foods. While humans cannot digest this dietary fibre, colonic bacteria (e.g. Ruminococcus albus, Salyers1977) are able to rapidly and completely break it down (Livesey2014).

 

 

Human milk oligosaccharides (HMO) are a family of heterogeneous unconjugated glycans that are abundant in human milk (Smilowitz2014, Bode2012). HMOs are indigestible and are transferred unchanged to the colon of the infant, where colonic bacteria - particularly important bifidobacteria (e.g. Bifidobacterium longum subsp. infantis, Bifidobacterium longum subsp. longum, Bifidobacterium breve and Bifidobacterium bifidum) - metabolise these oligosaccharides and are encouraged to grow (Smilowitz2014, Bode2012). For a broader discussion of HMOs see Human milk oligosaccharides.

 

Wheat dextrins are artificially created when wheat starch is first heated at high temperatures and then treated with amylase to hydrolyse accessible amylose polysaccharides (Slavin2009).  The product of this process is a resistant starch containing mixed glycosidic linkages formed as a result of the heating process (Figure 7).

Figure 7. Representation of a wheat dextrin substructure.  The glucose residues have a mixture of glycosidic linkages as a result of heating wheat starch to high temperatures.

This dextrin has been classified as a soluble, non-viscous fermentable fibre (McRorie2013) and it is widely used to bolster the fibre content of processed foods. The health benefits of wheat dextrin are equivocal, however (McRorie2013).

 

 

 

 

 

Polydextrose is a synthetic glucose polymer made by heating glucose, sorbitol and citric acid. The fibre has a mixture of 1,2-, 1,3-, 1,4- and 1,6-glycosidic bonds, much the same as wheat dextrins, except polydextrose usually has a terminal sorbitol residue.  It is a water soluble, non-viscous, readily fermentable polysaccharide (McRorie2013), and is used as a sugar replacement and dietary fibre.  However, even relatively low doses can cause flatulence and high doses can cause severe diarrhoea.

 

2. Insoluble Fibre

The presence of insoluble dietary fibre in our diet is important because it helps move consumed food (digesta) through our digestive system.  It also absorbs water increasing bulk and softening the faeces, making it easier for stools to pass. Cellulose and chitin are polysaccharide examples, and lignin a non-carbohydrate-based insoluble fibre.

Cellulose is a non-branching β-1,4-glycosidic polymer of glucose (Figure 8). Chains of up two several thousand glucose units are made by plants and these polymers are able to tightly pack into rigid secondary structures using extensive hydrogen bonding.  

Figure 8. Subunit of cellulose, a β-1,4-glycosidic linked glucose polymer. The β-1,4-glycosidic link is unrecognised by human glucosidases and is undigestible as a result.

While cellulose represents a significant proportion of the mass of all plant life, there is significantly less of this material in food plants. 

Humans cannot digest cellulose, whereas a few colonic bacteria are able to (e.g. Lachnoclostridium phytofermentans, Clostridium chartatabidum, Cellulosimicrobium cellulans, Bacteroides cellulosilyticus, and Eubacterium rectale). The level of bacterial degradation of cellulose is not expected to be high in the colon during digesta transit time because the microcrystalline nature of the fibres would limit bacterial enzyme access (Livesey2014).

 

Chitin is a polymer of N-acetylglucosamine units. It has a similar molecular configuration to cellulose (β-1,4-glycosidic links, Figure 9) and is used by molluscs, insects and fungi as a structural component and for protection (e.g. exoskeletons of crabs).  On average, chitin-glucan fibres make up about a quarter of fungal biomass dry weight, and mushroom cell walls constitute between 8-43% chitin (Jones2020).

Figure 9. Chitin is a linear, non-branching polymer of N-acetylglucosamine subunits connected via β-1,4-glycosidic links.  Humans cannot digest this fibre.

Chitin is generally undigestible by human enzymes, whereas a handful of important colonic bacteria, such as Paramuribaculum intestinale, Stenotrophomonas maltophilia, Bacillus licheniformis, Clostridium sporogenes, and Clostridium tertium, are able to attack the polymer.  Whether any breakdown of chitin by bacteria during its transit time through the digestive system is of any consequence is unclear.  Like cellulose, much of the chitin secondary structure is compact and resistant to enzymatic decomposition.

Although lignin is not a carbohydrate, it is believed to play an important role as an insoluble dietary fibre and is often associated with other polysaccharides, such as cellulose. It is a complex polymer of cross-linked heterogeneous phenoxypropyl monomers (Figure 10) and is deposited in the cell walls of structural tissues of a maturing plant to provide rigidity and protection. This biopolymer is chemically and biologically resistant to breakdown.  As such it passes through the digestive tract essentially unchanged, although lignin does slow the fermentation of dietary fibre in the colon (Mongeau2016).  Like cellulose, the proportion of lignin in our diet tends to be small because the plant food we consume contains rather young, unlignified plant tissue.

Figure 10. Lignin is a complex mixed polymer of phenoxy propyl subunits.  The dense structure is undigestible by human enzymes and passes through the gut largely unchanged.

3. Mono-, Di- & (small) Oligosaccharides, and Polyols

In addition to dietary fibres, there are a number of small carbohydrate fragments (some oligo-, di- and monosaccharides; some polyols) that are poorly absorbed or completely undigestible by humans.  Most of these, such as cellobiose (disaccharide), erythritol (polyol) and raffinose (oligosaccharide), are transferred to the large intestine where they are fermented by colonic bacteria.

Disaccharides - Undigestible

Cellobiose is generated by enzymatic decomposition of cellulose by some bacteria and fungi.

Figure 11. Cellobiose is a disaccharide with two glucose units joined with a β-1,4 bridge.

It is a disaccharide of glucose (Figure 11) and is not efficiently digested by humans because lactase - a brush border enzyme - hydrolyses this sugar slowly (Lau1987).  Well known colonic bacteria, such as Bacteroides ovatus, Bacteroides thetaiotaomicron, Bacteroides uniformis, Bacteroides caccae, and Bacteroides fragilis, however, can readily consume this sugar.

Melibiose is a disaccharide of galactose and glucose attached via an α-1,6-glycosidic link (Figure 12), which is not digestible by humans because we lack an α-galactosidase enzyme.  

Figure 12. Melibiose is a disaccharide of galactose and glucose attached via an α-1,6-glycosidic link.

The sugar occurs naturally and is found in honey, olives and Jerusalem artichokes.  Bacteria, such as Bacillus pumilus, Absiella tortuosum, Anaerostipes caccae, Acinetobacter calcoaceticus, and Bacillus licheniformis, are able to metabolise melibiose.

 

 

Gentiobiose, a disaccharide of two glucose units attached via a β-1,6-glycosidic link (Figure 13), is found naturally in foods, such as fresh and fermenting cucumbers (Ucar2020), and ripening tomato fruit (Dumville2003).

Figure 13. Gentiobiose is a disaccharide of glucose joined via a β-1,6-glycosidic link.

Since we lack a β-glucosidase enzyme capable of hydrolysing gentiobiose, no digestion occurs in the small intestine until it reaches the large bowel, where the likes of  Enterococcus faecalis, Bifidobacterium pseudocatenulatum, Bifidobacterium adolescentis, Bacteroides caccae, and Enterobacter cloacae, can ferment or oxidise it.

 

Lactulose is an artificially produced disaccharide composed of galactose and fructose joined with a  β-1,4-glycosidic link (Figure 14).

Figure 14. Lactulose is an artificially produced disaccharide of galactose and fructose.

The sugar is produced commercially through isomerisation of lactose or by heating milk (Luzzana2003) and is often used to treat constipation and other medical conditions. Lactulose is not absorbed nor digested by human enzymes; colonic bacteria, such as Bacteroides cellulosilyticus, Subdoligranulum variabile, Klebsiella pneumoniae, and Cronobacter sakazakii, on the other hand, are capable of metabolising the sugar.  If not present, lactulose is excreted.

 

Disaccharides - Partially Digestible

Maltitol is an artificially synthesised, reduced disaccharide made up of glucose and sorbitol (Figure 15).  

Figure 15. Maltitol is made by reduction of maltose.

The compound is used as a sugar replacement and is slowly digested within the small intestine.  Regular and significant consumption of maltitol (>50 grams) can cause diarrhoea due to the osmotic effect of excess sorbitol, and flatulence as a result of colonic bacteria consuming the sorbitol (RuskoneFourmestraux2003) or any undigested maltitol (e.g.  Citrobacter koseri; Enterobacter asburiae; Enterobacter cloacae; Klebsiella pneumoniae; Cronobacter sakazakii).

 

Trisaccharides and Small Oligosaccharides

Raffinose is a trisaccharide composed of galactose, glucose and fructose (Figure 16).

Figure 16. Raffinose is a trisaccharide composed of galactose, glucose and fructose.

The sugar can be found in brassica vegetables, such as Brussels sprouts, cabbage and broccoli, as well as asparagus and whole grains. While humans cannot digest raffinose, because we lack the α-galactosidase enzyme, colonic bacteria, such as Bacteroides ovatus, Bacteroides thetaiotaomicron, Bacteroides uniformis, Erysipelatoclostridium ramosum, and Bacteroides fragilis, are able to ferment it effectively.

 

 

 

Melezitose is a rare trisaccharide comprised of two glucose units attached to a fructose residue (Figure 17).

Figure 17. Melezitose is a rare trisaccharide comprised of two glucose units attached to a fructose residue.

The sugar is produced by sap-eating insects and then transferred to honey by foraging bees. Humans cannot digest melezitose, but common anaerobic bacteria, like Bacteroides ovatus, Bacteroides thetaiotaomicron, Bacteroides caccae, Bifidobacterium longum subsp. longum, and Eubacterium rectale, are able to ferment it.

 

 

Soy bean α-galactosides (soy oligosaccharides) are a family of oligosaccharides made up of small chains of galactose (1-4) attached to sucrose, with raffinose being the simplest member (Figure 18, Choct2010). Humans are unable to digest these sugars because we lack an α-galactosidase enzyme. Soy milk contains small amounts of raffinose and stachyose, which are readily fermented by colonic bacteria and cause bloating and diarrhoea in some people. Soy has been shown to increase mutualist bacteria such as Bifidobacterium longum subsp. longum (Hayakawa1990).

Figure 18. Family of soy oligosaccharides. Raffinose is the simplest member with one galactose residue attached to a sucrose molecule (from Choct2010).

 

Polyols - Partially Digestible/Absorbed

Erythritol is a naturally occuring four-carbon polyol (Figure 19) found in grapes, mushrooms, pears, watermelon and fermented foods.

Figure 19. Erythritol is a naturally occuring four-carbon polyol.

While it can partially absorbed, it is excreted unchanged in the urine. Due to osmotic effects, excess consumption can lead to diarrhoea. Few colonic bacteria (e.g. Streptococcus pneumoniae, Anaerostipes caccae, Blautia massiliensis, Blautia stercoris, and Eubacterium limosum) are able to utilise or metabolise this polyol.  Erythritol has also found use as a sugar substitute.

 

Mannitol comes from reduction of mannose (Figure 20). It is widespread in vegetables (sweet potato, cauliflower) and fruit (watermelon, peaches).

Figure 20.  Mannitol is a six-carbon polyol derived from mannose.

Mannitol is partially absorbed by the small intestine but remains essentially unmetabolised prior to excretion in urine. Numerous anaerobic colonic bacteria, such as Bacteroides ovatus, Anaerobutyricum hallii, Bacteroides xylanisolvens, Bifidobacterium adolescentis, and Bifidobacterium breve, on the other hand, are able to readily ferment mannitol. It is also used as a sweetner (sucrose replacement).

Sorbitol is malabsorbed like most polyols, and has been used as a laxative (Figure 21).

Figure 21. Sorbitol is a six-carbon polyol.

It is widespread in vegetables and fruit (apples, nectarines, pears and cherries). Numerous anaerobic colonic bacteria can ferment this carbohydrate (e.g. Enterococcus faecalis, Enterocloster bolteae, Blautia obeum, Bifidobacterium breve, and Enterobacter cloacae).

 

Xylitol, a 6-carbon polyol (Figure 22), can be absorbed, but slower than the digestion of sucrose. High levels of consumption can cause diarrhoea.

Figure 22. Xylitol is a six-carbon polyol.

The absorbed xylitol is metabolised by the liver resulting in about 50% of the carbohydrate being used for energy generation (Livesey2014). Small amounts of the polyol are found in plums, strawberries, cauliflower, and pumpkin, and several common facultatively anaerobic colonic bacteria, such as Enterococcus avium, Bacillus clausii, Staphylococcus aureus subsp. aureus, Enterobacter hormaechei subsp. hormaechei, and Eubacterium rectale, can utilise this polyol.

 

Monosaccharides

Rhamnose is a hexose monosaccharide isolated from Buckthorn and in a bound form in other plants (Figure 23). After subjecting some of these plant foods to digestion-like conditions, free rhamnose was detected in blackcurrant and carrot (Parkar2021).

Figure 23. Rhamnose is a rare monosaccharide found in a few plant foods.

Rhamnose is poorly absorbed and has been found excreted in the urine (Jenkins1994). While humans don't utilise this sugar well, colonic bacteria, such as Bacteroides ovatus, Bacteroides thetaiotaomicron, Escherichia coli, Parabacteroides distasonis, and Phocaeicola vulgatus; are able to metabolise any bound or unabsorbed monosaccharide.

 

Mannose is a hexose reducing sugar (Figure 24) that is widespread in fruits and vegetables.

Figure 24. D-mannose is a reducing sugar found in many fruits and vegetables.

After subjecting some plant foods to digestion-like conditions, mannose was detected in many foods, such as apples, pumpkins and kiwifruit (Parkar2021). Humans can easily absorb this sugar, but is almost completely excreted in the urine. Important colonic bacteria, such as Bacteroides ovatus, Bacteroides thetaiotaomicron, Bacteroides uniformis, Collinsella aerofaciens, and Bacteroides fragilis, readily metabolise mannose.

 

L-Arabinose is a pentose reducing sugar (Figure 25) most commonly found as a constituent of hemicellulose and pectin polymers.

Figure 25. L-Arabinose is a pentose sugar used by some plants to create hemicellulose and pectin polymers.

After subjecting some plant foods to digestion-like conditions, L-arabinose was detected in apple, carrot and sweetcorn (Parkar2021). The sugar is poorly absorbed. Any absorption by the gut is likely degraded by the liver while a portion is known to be excreted in urine. L-Arabinose is known to inhibit human sucrase (KrogMikkelsen2011). Many well known colonic bacteria, such as Enterococcus faecium, Bacteroides uniformis, Bifidobacterium adolescentis, Bacteroides caccae, and Bifidobacterium longum subsp. longum, are able to metabolise the sugar.