Untitled

S44
'Fibre'
17.0 kJ/g
Physiological aspects of energy metabolism
M Elia and JH Cummings
Energy kJ/g
Similar calculation procedures to the above can be used to
establish DE values of polyols. Like NSP, polyols are a
heterogeneous group of substances (Bernier and Pascal,
1990; Japanese Ministry of Health, 1991; Livesey, 1992;
FASEB 1994; Finley and Leveille, 1996) that have different
physiological properties. For example, glycerol, like glucose,
is fully absorbed (digestibility ¼ 1.0), and therefore its DE is
the same as its cEI (18.0 kJ/g). Although other polyols, such
as isomalt, lactitol, maltitol and erythritol, have a cEI of
16.70–17.20 kJ/g, their DE values (11.0–16.61 kJ/g) are 65–
97% of their cEI. In contrast, virtually, all the lactitol in food
escapes digestion and absorption by the small bowel and
finds its way into the large bowel, where about 60% of its cIE
is converted to SCFAs, which are subsequently absorbed by
the colon (cIE, 17.0 kJ/g; DE, estimated to be about 11.1 kJ/g
(DE ¼ 0.65cIE)). With the increasing use of different types of
polyols in foods, it is becoming apparent that their digestibility,
the extent to which they are absorbed by the small bowel and
the extent to which they are subsequently metabolized by
human tissues varies so much (see below) that it might be
appropriate to use individual rather than generic energy values.
A further potential complexity is that polyols may be handled
differently by the gut (resulting in different digestibility and DE
values) when taken in foods compared to drinks. However,
food regulations normally prohibit the use of polyols in drinks.
5.1
3.6
0.6
0.6
7.1
Unfermented fibre
Bacterial matter
H2 and CH4 Fermentation heat
Hine SCFA (1.2 kJ)
Short chain fatty acids
(SCFA)
Faecal energy (8.7 kJ)
Gaseous energy (0.6 kJ)
Metabolisable = Digestible
energy (7.7 kJ)
An exception is the use of erythritol, which is hardly
metabolized once absorbed and hardly fermented when
reaching the colon. An issue for further consideration is
whether low-dose polyols should be used in drinks.
Metabolizable energy
By convention, the metabolizable dietary energy (ME) is measured
at zero nitrogen and energy balance. In these circumstances,
ME is the component of DE that produces heat during oxidative
metabolism. It does not include energy that is lost to urine
(combustible urinary energy (cUE; for example, urea, which is a
partially combusted end product of protein metabolism or
unmetabolized urinary polyols) or body surfaces (combustible
surface energy (cSE; for example, desquamated cells, hair loss,
perspiration), because this energy is not used in metabolism.
Therefore, ME can be defined mathematically as follows:
ME ¼ DE cUE cSE
Since DE ¼ cIE–cGaE, the following also applies:
ME ¼ cIE cGaE cUE cSE
H ine 'fibre'
(1.8 kJ)
Net
metabolisable
energy
(5.9 kJ)
Figure 2 The fate of ‘fibre’ ingested with conventional foods. It is assumed that 70% is fermented, so that 5.1 kJ/g (30% of the combustible
intake energy (cIE) is lost to faeces unchanged. Of the fibre that is fermented, 5% of the cIE energy is lost as gaseous products (H2 and CH4), and
another 5% as heat of fermentation, which contributes to metabolizable energy. The majority (60% of cIE) is converted to short-chain fatty acids,
almost all of which are absorbed and metabolized by human tissues, or lost to faeces (30% of cIE), mainly as bacterial biomass. Of the energy
initially present in total fibre (fermentable and non-fermentable) 51.0% is lost to faeces, 3.5% to gaseous products and the remaining 45.5%
accounts for metabolizable energy.
European Journal of Clinical Nutrition
In normal healthy subjects, loss of surface energy is small
and can be ignored. For many carbohydrates (for example,