Title: Alternative Sweeteners

Xylitol 343
The recognition of xylitol as a normal endogenous metabolite in humans
originates from observations in patients with a genetic abnormality, essential pentosuria.
These persons excrete considerable quantities of l-xylulose in the urine.
When incubated in tissue preparations, l-xylulose was found to be metabolized
to xylitol by a specific NADPH-linked enzyme, l-xylulose-reductase. This enzyme
has subsequently been demonstrated to be deficient in essential pentosuria.
Because pentosuric patients excrete 2–15 g of l-xylulose per day, the daily production
of xylitol was estimated to be of a similar order of magnitude (5,90,91).
C. Estimation of the Caloric Value of Xylitol
If the caloric value of polyols is to be estimated on the basis of their metabolic
fate, precise knowledge about the different steps of their digestion and metabolic
use is required. In particular, it is, for example, essential to know (a) how much
of an ingested dose is absorbed directly from the gut, (b) by which pathways the
absorbed portion is metabolized in the human organism, (c) which proportion of
an ingested dose is fermented by the gut microflora, (d) to what extent the resulting
fermentation products are absorbed and metabolically used by the host,
and (e) how much of an ingested dose leaves the intestinal tract unchanged with
the feces or urine. Although the experimental database on xylitol does not allow
one to answer all these questions with sufficient precision, it is possible to obtain
reasonable estimates by extrapolation from existing data and studies on other
polyols that are also incompletely absorbed and are subject to the same fermentative
degradation in the gut.
On the basis of a thorough assessment of numerous in vitro and in vivo
experiments with xylitol and other polyols, it has been estimated that approximately
one fourth of an ingested xylitol dose is absorbed from the gastrointestinal
tract (92). This portion of xylitol, which is effectively metabolized by means
of the glucuronate-pentose phosphate shunt, is energetically fully available and
provides about 4 kcal/g. The nonabsorbed three-fourths of the ingested load,
however, are almost completely fermented by the intestinal flora. Combining a
78% retention of energy in bacterial fermentation products and a growth yield
of 20 g bacteria per 100 g substrate, it has been estimated that about 42% of the
energy provided with unabsorbed xylitol is consumed by bacterial metabolism
and growth, whereas about 58% of the energy becomes available to the host after
absorption of the fermentation end-products (VFA) (93). This value is well in
line with an estimated 50% energy salvage proposed by a Dutch expert group
(94).
On the basis of these estimates, a metabolizable energy value of about 2.8–
2.9 kcal/g may be calculated for xylitol (92). This value is in line with the results
of an in vivo study in which xylitol was found to be about 60% as effective as
glucose in promoting growth (95).