Carbohydrates and Health

2.3 Carbohydrates can be classified based on their digestion and absorption in the
small intestine rather than their chemical characteristics. Digestible carbohydrates
are absorbed and digested in the small intestine; non-digestible carbohydrates are
resistant to hydrolysis in the small intestine and reach the human large intestine
where they are at least partially fermented by the commensal bacteria present in
the colon. The term ‘dietary fibre’ refers to either some or all of the constituents
of non-digestible carbohydrates and may also include other quantitatively minor
components, e.g. lignin, that are associated with non-digestible carbohydrates in
plant cell walls (see paragraphs 2.29 to 2.36 for a consideration of different dietary
fibre definitions).
Chemical classification
Sugars
2.4 The term ‘sugars’ conventionally describes mono- and di-saccharides. Pentoses are
five-carbon sugars that occur in polymeric forms and are collectively known as
pentosans. Xylose and arabinose are the constituents of pentosans present in the
non-starch polysaccharides hemicelluloses and pectin (McNaught, 1997). The three
principal hexoses (six-carbon sugars) are glucose, fructose and galactose, which
are the building blocks of naturally occurring di-, oligo- and poly-saccharides.
The hexose mannose is present in some plant polysaccharides collectively
termed mannans and is present in hemicelluloses. Glucose is widely distributed
in small amounts in fruits, plant juices and honey. Fructose is found in the free
state alongside glucose in ripening fruits and honey. Sucrose is the predominant
disaccharide occurring in the free form and is composed of the monosaccharides
glucose and fructose. Galactose occurs in milk, in chemical combination with
glucose as lactose, while the disaccharide maltose, derived from starch hydrolysis,
comprises two glucose molecules (Cummings & Stephen, 2007).
Polyols
2.5 Polyols include hydrogenated mono- and disaccharides (‘sugar alcohols’) used
as sugar replacers, as well as hydrogenated oligo- and polysaccharides, e.g.
hydrogenated starch hydrolysate and polyglycitol, respectively (Livesey, 2003).
They are found naturally in some fruits and are made commercially by using
aldose reductase to convert the aldehyde group of the glucose molecule to the
alcohol (Cummings & Stephen, 2007). There is wide variation in the absorption
of different polyols, ranging from almost complete absorption of erythritol, to
partial absorption of sorbitol and almost complete lack of absorption of lactitol
(Livesey, 2003). The metabolism of erythritol is, however, minimal and being poorly
reabsorbed via the kidneys it is excreted in the urine. There is also a wide range in
the colonic fermentation of polyols ranging from almost complete fermentation
of lactitol, to almost complete lack of fermentation of erythritol (Livesey, 2003).
This variation in availability for energy production leads to estimated caloric values
ranging from almost 0 to about 3 kcal/g.
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