Mannitol, Sorbitol and Xylitol are all sugar alcohols. That means one or more of the Hydrogens connected to a Carbon has been converted to an Hydroxyl (OH) radical. Like spenda, they are close enough to the sugar molecules they are made from, that your taste buds mistake them for sugar.
The body has very limited abilities to metabolize any sugar other than Glucose, and has a veritable of army of enzyme systems to convert just about any sugar into glucose. These enzyme systems are very specific in higher animals (your tastebuds are nowhere near as specific).
An enzyme that redesigns Lactose, won't touch a fructose or sucrose molecule No enzyme system exists in the human body to convert sugar alcohols to glucose. To be absorbed you have two choices.
Small molecules (like Glucose), or lipid soluble molecules.
Sugar alcohols are neither, and are not easily converted to small molecules or made lipid soluble, so they exert osmotic pressure on the gut, causing water to flow in. If the inflow is substantial, it will produce laxative effects similar to epsom salts. If it cannot be absorbed, you cannot obtain any calories from it either.
In the colon, the bacterial utilize much simpler (and much much less efficient) enzyme systems that can break down Sugar alcohols, generally fermenting them, so in addition to giving you the runs, they also produce large volumes of wind from fermentation.
In some parts of the world Sorbitol is sold as a commercial laxative. Prior to development of the PEG solutions like Colyte and Golytely, large volumes of mannitol solution were used for the same purpose.
