Title: Alternative Sweeteners

Xylitol 341
tol in a fluoride toothpaste resulted in a 12% reduction in decayed/filled surfaces
after 3 years compared with a fluoride-only toothpaste (64). In addition, there is
evidence that xylitol exerts a plaque-reducing effect (65,66) and that it interferes
with bacterial metabolism particularly in the presence of fluoride and zinc ions
(67). An inhibitory effect on enamel demineralization has been postulated as well
(68,69). There is also evidence that use of a xylitol-containing dentifice can result
in a significant reduction of Streptococcus mutans in saliva (70). Because of its
overall favorable effects on dental health, xylitol has also been applied in other
oral care products, such as in mouth rinses and in artificial saliva (71–74).
V. METABOLISM
In principle, two different metabolic pathways are available for the use of xylitol:
(a) direct metabolism of absorbed xylitol in the mammalian organism, mainly in
the liver, or (b) indirect metabolism by means of fermentative degradation of
unabsorbed xylitol by the intestinal flora.
A. Indirect Metabolic Utilization
All polyols, including xylitol, are slowly absorbed from the digestive tract because
their transport through the intestinal mucosa is not facilitated by a specific
transport system. Therefore, after ingestion of large amounts, only a certain proportion
of the ingested xylitol is absorbed and enters the hepatic metabolic system
through the portal vein blood. A comparatively larger amount of the ingested
xylitol reaches the distal parts of the gut, where extensive fermentation by the
intestinal flora takes place. Besides minor amounts of gas (H 2,CH 4,CO 2), the
end-products of the bacterial metabolism of xylitol are mainly short-chain, volatile
fatty acids, (i.e., acetate, propionate, and butyrate) (75–77). These products
are subsequently absorbed from the gut and enter the mammalian metabolic pathways
(78). Acetate and butyrate are efficiently taken up by the liver and used in
mitochondria for production of acetyl-CoA. Propionate is also almost quantitatively
removed by the liver and yields propionyl-CoA (79–81).
The production of volatile fatty acids (VFA) is a normal process associated
with the consumption of polyols and dietary fibers (cellulose, hemicelluloses,
pectins, gums) for which hydrolyzing enzymes are lacking or poorly efficient in
the small intestine. Under normal conditions, most of the generated VFA are
absorbed from the gut and are further used by established metabolic pathways
in animals and man (77,82). For the energetic use of slowly digestible materials,
this fermentative, indirect route of metabolism plays an important role, and evidence
has been presented that, even under normal dietary conditions (i.e., in the