6.2. General and side effects

Dietary administration of xylitol caused a temporary, slight diarrhea resulting from its slow absorption from the intestine (Mäkinen 1994). However, an intestinal adaptation process occurred during the first week, after which no diarrhea was observed. This adaptive phenomenon has been suggested to be caused by induction of increased activity of polyol dehydrogenase (Bässler 1969), or by selection of more suitable intestinal microflora (Krishnan et al. 1980).

The 5, 10 and 20% concentrations of xylitol used in the present studies correspond to a daily intake of approximately one, two and four grams of xylitol, respectively. These are about 3.5, 7 and 14% of the total daily caloric intake of the rats. These might suggest a daily intake of about 20, 40 and 80g of xylitol in middle-aged women, which amounts have been proven to be well tolerated (Mäkinen 1976). However, it should be observed that no direct decisions regarding human metabolism can be drawn from the present studies.

The presence of no significant differences in weight gain (except in study IV in the 20% xylitol supplementation group), in weights of the bones, in food and water intake or in excretion of urine between the control rats and the rats fed xylitol, indicates that there were no major xylitol-induced changes in the development of these animals. The slight reduction in the weight gain along with increasing xylitol concentration of the diet can be attributed to slighty lower availability of energy from ingested xylitol caused by the partial escape of this slowly absorbed polyol from the intestine to the colon (Krishnan et al. 1980), to slightly reduced food intake caused by the reduction in gastric emptying rate (Shafer et al. 1987), and to reduction in fat deposits ascribed to lowered lipid synthesis or increased lipolysis (Hämäläinen & Mäkinen 1983). Furthermore, while xylitol does not cause an insulin-stimulating action, the insulin-caused expedition of fat tissue accumulation is not strengthened (Dwivedi 1977).

In ovariectomized rats, the weight gain was significantly increased as compared to sham-operated controls. This is in accordance with previous studies, and has also been found in studies where pair fed animals were used (Wronski et al. 1989a, Yamazaki & Yamaguchi 1989). This ovarian hormone deficiency-caused obesity has been thought to provide a partial protection against osteopenia in the long bones of the ovariectomized rats (Wronski et al. 1987). This is probably related to the increased body weight, but may also be linked to the increased body fat, which may augment the conversion of adrenal androgens to estrogens (Avioli & Lindsay 1990). 10% dietary xylitol supplementation somewhat diminished the ovariectomy-induced increase of weight gain, suggesting that the xylitol-induced benefical effects on bone metabolism are not likely related to the changes in rat body weights.

Like xylitol, 1M D-mannitol and 1M erythritol also caused a temporary diarrhea. In the case of 1M sorbitol, however, no adequate adaptation process took place, resulting in continuous diarrhea. This was probably also reflected in reduced weight gain, and in associated decrease in bone weight of the rats given sorbitol. This phenomenon is in accordance with previous studies, and is suggested to be a consequence of unaltered turnover rate of sorbitol despite the increased polyol dehydrogenase activity (Bässler 1969). The lower weight gain and the greater food intake in the erythritol group probably resulted from the very low amount of energy available from this polyol (Noda & Oku 1992). D-Mannitol is known to be a strong diuretic agent and an effective osmoregulator in mammalian tissues (Dills 1989). The present results indicate that erythritol also is strongly diuretic in rats, and that enteral administration of erythritol substantially increases water intake. In earlier studies a diuretic effect has also been detected when using xylitol and sorbitol (Hämäläinen & Mäkinen 1986). However, in those studies, a higher dietary polyol concentration (20%) has been used. The above general metabolic differences between the studied polyols should not be ignored when comparing their effects on bone metabolism.