6.4. Bone structure

Bone trabecular volume was significantly greater in all xylitol-fed groups than in the controls, the volume increasing along with increasing xylitol content. Furthermore, 10% dietary xylitol supplementation prevented significantly the accelerated trabecular bone loss in ovariectomized rats.

This bone conserving action of dietary xylitol in experimental osteoporosis is different from those provided by calcium supplements, which have not been able to show reduced loss of bone trabeculae (Recker et al. 1977, Riis et al. 1987, Breslau 1994). As estrogen deficiency is accompanied with malabsorption of calcium attributed to impaired vitamin D response, dietary administration of pure accessory calcium would probably not lead to increased bone calcium levels. However, as mentioned above, dietary xylitol increases calcium absorption independently of vitamin D action (Hämäläinen et al. 1985). Thus, dietary xylitol is probably able to provide sufficient calcium levels also during estrogen deficiency.

The xylitol-induced increase of bone trabeculation is, most likely, partly a consequence of diminished bone resorption, but it probably also reflects an anabolic effect of xylitol. This was suggested in a previous study (Svanberg & Knuuttila 1993), where the increase of cortical bone mineral content after dietary xylitol administration was greater in newly synthetized bone than in formerly formed bone when compared to the controls. The anabolic effect of intravenously and parenterally administered xylitol has been detected in many studies (Lang 1971, Wilkinson 1972, Förster 1974). A positive nitrogen balance, which is related to increased bone mineral content (Morris et al. 1992), has also been detected during intravenous and parenteral xylitol nutrition (Georgieff et al. 1985, Almdal et al. 1993).

Increased NADH/NAD ratio, occuring also during the metabolism of xylitol, has been linked with the initiation of bone mineralization (Shapiro et al. 1982). Furthermore, the reduced redox state is associated with increased collagen synthesis and decreased collagenase activity (Hernández-Munôz et al. 1994). Unfortunatelly, no dynamic bone histomorphometry or other determinations of bone formation were done in the present studies.

Dietary xylitol did not cause quantitative changes of hydroxyproline, pyridinoline or deoxypyridinoline in healthy rats. The volume of these collagenous elements was decreased in ovariectomized rats, but largely maintained if 10% xylitol was included in their diet. This suggests a dietary xylitol-associated normalizing effect on the rate of bone turnover in these rats. The relative volumes of collagenous elements remained unchanged in all groups. This is in accordance with previous findings (Robins & Duncan 1987), where no significant differences in crosslink concentrations, or their ratios were found between ovariectomized and sham-operated rats, when the crosslink concentrations were expressed as the number of residues per collagen molecule. Thus, no profound selective changes in the structure of bone collagen were suggested either during xylitol feeding, or following ovariectomy.