In man, ingested xylitol and sorbitol are absorbed through the gut wall at virtually the same rate, and appreciably more slowly than D-glucose and D-fructose. Both polyols are absorbed passively. In most healthy subjects, an adaptive increase in the activity levels of an enzyme (a non-specific polyol dehydrogenase) greatly increases the rate of xylitol absorption in a few days. This is not the case with sorbitol. In unadapted subjects xylitol doses of about 0.5 g per kg body weight may result in transient soft stools (osmotic diarrhea). Xylitol is slowly absorbed from the digestive tract owing to the absence in the intestinal mucosa of a specific transport system for xylitol. Consequently, about one third of the ingested xylitol (when large single doses are taken in) is absorbed, subsequently entering the hepatic metabolic system. The other two thirds of the ingested xylitol will reach the distal parts of the intestinal tract where xylitol will be broken down by gut bacteria. The end products are mainly short-chain fatty acids, most of which will normally be absorbed and utilized by the body. When very small quantities of xylitol are consumed (as in one piece of chewing gum), it is possible that proportionally larger amounts are directly absorbed.
After appropriate adaptation, xylitol has been administered to human subjects in amounts of 200 g and higher per day without diarrhea occurring. In practice, usually not more than 50-70 g daily, spread evenly throughout the day, should be given. Dentally effective quantities may vary between about 1 and 20 g per day, preferably between 6 to 12 g. Owing to the slow absorption of xylitol, it has sometimes been characterized as "glucose with delay", a property that can be advantageous in certain clinical situations. Premature infants possess full capacity to metabolize xylitol.
Xylitol supplies large amounts of liver glycogen, or primarily D-glucose. Xylitol is oxidized to carbon dioxide and water by the normal, physiologic pathway of carbohydrate breakdown. About 85% of the xylitol turnover in the body takes place in the liver. About 10 % is metabolized extrahepatically in the kidneys, and the small remainder is used up by blood cells, the adrenal cortex, lung, testes, brain, fat tissue, etc. These figures are similar regardless of the way of administration, i.e. whether oral or by the intravenous route. There is a small difference between endogenous ("natural") xylitol and that which is supplied from outside, for example, when a xylitol-containing diet is consumed. Endogenous xylitol is the physiologic intermediate product from D-xylulose and L-xylulose (these are the keto-sugars corresponding to xylitol). This reaction takes place in the mitochondria catalyzed by enzymes which are specific for xylitol. By contrast, exogenous (ingested) xylitol is slowly absorbed, and eventually enters the portal circulation and the liver where it is dehydrogenated in the cytoplasm of the liver cells by the above mentioned non-specific polyol dehydrogenase enzyme which can also act on sorbitol. This enzyme is a key enzyme in xylitol metabolism and largely determines the metabolic rate of xylitol. When xylitol is given for a few days, an adaptation takes place: the enzyme's levels are increased so that the metabolic capacity of a subject who is accustomed to xylitol, is appreciably augmented.
