Simulations of fermentation by the colonic microbiota
in vitro have shown that exposure of this microbiota to xylitol leads to a rapid disappearance of the xylitol, as determined by enzymatic colorimetry, indicating that it is readily fermented by the simulated intestinal microbiota. Gas chromatographic analysis of the simulated colonic digesta showed an increased formation of butyric acid compared to the non-supplemented control simulations [
26]. Strains from the genus
Anaerostipes have been observed by 16S rRNA denaturing gradient gel electrophoresis (DGGE) analyses to be associated with the increased production of butyric acid in fecal cultures [
27]. Production of butyric acid is considered beneficial for colonic health as it is the preferred energy source for colonocytes and is thought to be associated with a reduced risk for colorectal cancer [
28]. Furthermore, butyric acid promotes the generation of regulatory T-cells that promote immune system balance [
29]. In rats (at least 5 animals/group), early fecal microscopy studies indicated that 20% of dietary xylitol caused a shift from fecal Gram-negative to Gram-positive bacteria after six weeks compared to animals fed an unsupplemented diet; the magnitude of this change was, however, not reported. Similar observations were made in humans; six volunteers, after an overnight fast, consumed in a cross-over design randomly a single 30 g dose of xylitol or a single 30 g dose of glucose (control) in 200 mL water. Fecal microscopy indicated an increase in Gram-positive bacteria from 20%–30% to 50%–55% for glucose and xylitol, respectively, and a concomitant decrease in Gram-negative bacteria was observed. Furthermore, a reduction in the fecal level of yeasts was reported, from Log10 9.2–9.4 colony forming units (CFU)/g feces during the control phase to Log10 7.2–7.5 CFU/g feces after xylitol consumption [
30]. The type of yeast that was reduced was not reported, but
in vitro studies have reported that xylitol can suppress the growth of
Candida with a minimal inhibitory concentration of 200 mg/mL and a 99.95% reduction in colony-forming units at 400 mg/mL [
31]. Recent mouse studies (5 animals/group) have reported that consumption of xylitol (40 or 194 mg/kg body weight/day) for 15 weeks was associated with an increase in the genus
Prevotella, the phyla Eubacteria and Firmicutes and a reduction in the phylum Bacteroidetes by DGGE analysis [
32]. Others have made similar observations, terminal restriction fragment length polymorphism (TRFLP) analysis indicated reduced levels of
Bacteroides and
Clostridium cluster XIVa and increased levels of
Prevotella in mice (7 animals/group) fed 5% xylitol for 28 days as compared to animals fed unsupplemented chow [
33]. In studies with cyclophosphamide-immune suppressed mice, 5%–10% xylitol (12 animals) was observed to lead to significantly lower fecal counts of
Candida albicans (7.58 vs. 5.22 Log10 CFU/g, control and xylitol respectively) and significantly less and fewer cases of
C. albicans invasion of the gastric wall as compared to animals not fed xylitol (10 animals); 80% vs. 10% of animals, control and xylitol respectively [
34]. Furthermore, urinary HPLC analysis indicated an increased metabolism of daidzein to equol when mouse diet (7 animals/group) was supplemented with 0.05% daidzein (control) or 0.05% daidzein and 5% xylitol for 28 days [
33]; this may contribute improved bone health.
Even though organisms may not be able to metabolize and grow on xylitol, there may still be an opportunity for synergy with xylitol and probiotic bacteria, as was shown with the combination of
Lactobacillus plantarum Inducia in combination with 5% xylitol which was reported to completely stop spore germination of
Clostridioides (formerly
Clostridium)
difficile,
in vitro after 48 h. In addition, prefeeding with a single dose of 0.2 g xylitol improved the survival of hamsters in a
C. difficile challenge model (5 out of 9 survived in the xylitol test against 2 out of 15 in the unsupplemented group). Fecal colonization with
C. difficile quantified by real-time PCR was lower in the xylitol group, 3.5 vs. 4.9 Log10 gene copy number/g in the control group. Real-time PCR
Lactobacillus fecal counts, however, were highest in the xylitol group, 6.6 vs. 4.6 Log10 gene copy number/g in the control group [
39].