Short-chain fermentable carbohydrates

Chapter 2.2
Short-chain fermentable carbohydrates

Sue Shepherd

La Trobe University, Melbourne, Australia

Carbohydrates are classified chemically according to the number of component molecules (degree of polymerisation, DP). The type of bond that exists between the monomers (alpha or beta) is also important as it affects the structure and/or digestibility of the carbohydrate [1]. Carbohydrate terminology has changed over the years, and the current classifications are described in Table 2.2.1 [2]. Sucrose, lactose, maltose, maltodextrin and starches are hydrolysed by gastrointestinal enzymic action to their constituent monosaccharides glucose, galactose and fructose prior to absorption. Classification of carbohydrates according to their digestibility in a normally functioning small intestine is described in Table 2.2.2.

Table 2.2.1 Classification of dietary carbohydrates

Class	Degree of polymerisation	Subgroup	Examples
Sugars	1–2	Monosaccharides	Glucose, fructose, galactose
		Disaccharides	Sucrose, lactose, maltose, trehalose
		Polyols	Sorbitol, mannitol, lactitol, xylitol, erythritol, isomalt, maltitol
Oligosaccharides	3–9	Malto-oligosaccharides(alpha-glucans)	Maltodextrins
		Non-alpha-glucan oligosaccharides	Raffinose, stachyose, fructo-oligosaccharides, galacto-oligosaccharides, polydextrose
Polysaccharides	>10	Starch (alpha-glucan)	Amylose, amylopectin, modified starches
		Non-starch polysaccharides	Cellulose, hemicellulose, pectin, arabinoxylans, beta-glucan, fructans, plant gums and mucilages, hydrocolloids

Table 2.2.2 Dietary carbohydrates classified according to digestibility

Classification	Name	Constituent monosaccharides
Digestible
Monosaccharide	Glucose	n/a
	Fructose	n/a
	Galactose	n/a
Disaccharide	Sucrose	Glucose + fructose
	Lactose	Glucose + galactose
	Maltose	Glucose + glucose
Oligosaccharide	Maltodextrin	Glucose polymer
Polysaccharide	Starch	Glucose polymer
Non-digestible
Polyols^a
Oligosaccharide	Inulin-type fructans (short chain, e.g. fructo-oligosaccharides, oligofructose)	Fructose polymer (glucose terminal end)
	Galacto-oligosaccharides (e.g. raffinose, stachyose)	Galactose polymer (glucose terminal end)
Polysaccharide	Inulin-type fructans (long chain, e.g. inulin)	Fructose polymer (glucose terminal end)
	Arabinoxylans	Xylose with side chains of arabinose
	Resistant starch	Glucose polymer
	Non-starch polysaccharides

^{^a}Considered here as a non-digestible carbohydrate.

2.2.1 Gastrointestinal effects of short-chain fermentable carbohydrates

Short-chain fermentable carbohydrates include fructose, lactose, polyols, fructo-oligosaccharides (FOS)/fructans and galacto-oligosaccharides (GOS). These are small molecules that may be poorly absorbed in the small intestine. These then arrive into the large intestine, which is populated by the gastrointestinal microbiota. Luminal bacteria rapidly ferment these to hydrogen, carbon dioxide and short-chain fatty acids [3]. The major source of gas production in the lumen is via bacterial fermentation, the principal substrate being carbohydrates. How rapidly these are fermented is dictated by the chain length of the carbohydrate; oligosaccharides and sugars are more rapidly fermented compared with polysaccharides such as soluble dietary fibre [4]. Gas, bloating and distension may occur due to rapid fermentation.

Small sugars are highly osmotic. When these are not absorbed in the small intestine, they are delivered into the colonic lumen, together with water due to their osmotic effect. Thus, if sufficient short-chain carbohydrates reach the colon, GI function may be disturbed via effects on motility and/or via the effect of the osmotic load (similar to that utilised by the disaccharide laxative lactulose). Thus, short-chain carbohydrates that are poorly absorbed are reasonable candidates for dietary triggers of luminal distension and irritable bowel syndrome (IBS) symptoms (see Chapter 3.19 Irritable bowel syndrome dietary management).

There have been numerous studies in support of the concept that exceeding the tolerated dose of fructose and lactose causes malabsorption. These unabsorbed sugars and osmotically entrapped water together with electrolytes increase the liquidity of luminal contents and affect GI motility, including speeding up GI transit. This effect is utilised by lactulose, sorbitol and polyethylene glycol in their role as laxatives. Provocation tests with lactose, fructose, FOS or sorbitol cause abdominal symptoms such as bloating, pain, excess wind, nausea and disturbed stool output (diarrhoea and/or constipation) in many people, especially those with IBS [5–7].

2.2.2 Short-chain fermentable carbohydrates (FODMAPs)

Fructose, lactose, polyols, fructans and GOS are widely distributed in foods and have collectively been termed FODMAPs [8,9] (Table 2.2.3). FODMAP is an acronym referring to fermentable oligosaccharides (fructans, galacto-oligosaccharides), disaccharides (lactose), monosaccharides (free fructose) and polyols.

Table 2.2.3 Details of FODMAPs

FODMAP	Structure / composition	Alternative names	Gastrointestinal absorption	Significant sources
Fructans	Oligo- and polysaccharides of fructose with a glucose terminal end. Most have beta(1 → 2) bond but may have beta(2 → 6) bond	Fructo-oligosaccharides (FOS) or oligofructose (OF) Degree of polymerisation (DP) <10Inulins: DP >10	Human small intestine doesnot produce a hydrolase, therefore is incapable ofbreaking the beta(1 → 2)bond	Major dietary sources of fructans are wheat and onions Inulin is also increasingly being used as a food additive, being promoted as a prebiotic
Galacto-oligosaccharides (GOS)	Oligosaccharides with a beta-fructosidic bond and an alpha-galactosidic bond	Raffinose consists of one fructose, one glucose and one galactose molecule Stachyose contains one glucose, one fructose and two galactose molecules	Humans do not producegalactosidases, therefore are incapable of hydrolysing the galactosidic linkages of stachyose and raffinose to their monosaccharide constituents	Legumes (e.g. baked beans, red kidney beans, borlotti beans, soy beans), lentils, chickpeas
Polyols and polydextrose		Sugar alcohols including sorbitol, mannitol, xylitol, erythritol, isomalt, maltitol	Polyols are poorly absorbed in the small intestine via ‘pores’Polydextrose behaves in a similar way to polyols	Naturally occurring, e.g. sorbitol and mannitol Food additives: sorbitol, xylitol, mannitol, maltitol
Fructose	Six-carbon monosaccharide (hexose)		Absorbed via two mechanisms, glucose-independent facilitated transport and glucose-dependent fructose co-transport Well absorbed when ingested in equimolar combination with glucose (e.g. as sucrose)	Present in three forms in food: as free fructose (fruits, honey) as a constituent of the disaccharide sucrose, or as an oligosaccharide form of fructans
Lactose	Disaccharide: constituent monosaccharides areglucose and galactose		Absorption is dependent upon the brush border enzyme lactase, which hydrolyses lactose Lactase deficiency is common, affecting approximately 10% of Caucasians and >90% of Chinese	Present in cow, goat and sheep milk Tolerance of dietary sources of lactose will vary according to production of lactase Sources are milk, ice cream, yoghurt, low-fat soft cheeses

Subclassification of FODMAPs

FODMAPs can be subclassified into two categories.

FODMAPs that are poorly absorbed in some people: fructose, lactose, polyols – as the capacity to absorb these varies between individuals, the symptoms will also vary. Breath testing may be useful to determine if there is successful absorption.
FODMAPs that are not absorbed in anyone: fructans and GOS – these are always fermented in the colon, and a certain amount will result in symptoms in susceptible individuals.

Fructose

Fructose is a monosaccharide found in three main forms in the diet: as free fructose (present, for instance, in fruits and honey); as a constituent of the disaccharide sucrose; or as fructans, a polymer of fructose usually in oligosaccharide form (discussed later in this section) [10].

Intestinal absorption of free fructose The majority of fructose absorption occurs via two transporters in the brush border epithelium of the small intestine [11].

GLUT5 – a facultative transporter (i.e. it depends upon a concentration gradient for movement of fructose across the membrane), which is specific for fructose. This is a glucose-independent pathway of fructose absorption.
GLUT2 – a low-affinity, facultative transporter that will carry glucose, fructose and galactose. In this pathway, glucose facilitates the uptake of fructose. This is a glucose-dependent pathway for fructose absorption.

Fructose malabsorption People with fructose malabsorption (which differs from hereditary fructose intolerance) have impairment in the glucose-independent pathway (GLUT5), whilst the glucose-dependent (GLUT2) pathway remains functioning. The proportion of people who can completely absorb a load of fructose depends upon the load given; approximately 60% of people can completely absorb 35 g of fructose but this is reduced to 20% when 50 g is given [12,13].

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