Sugar alcohols are hydrogenated carbohydrates. They are generally used to give "body" to food products that are desired to be lower in carbohydrates. Carbohydrates have two main purposes in food palatablity: (1)Some carbs give sweetness to foods, and (2)Carbs give food a more pleasant texture. Ever eat straight protein powder? If you managed to do so without choking to death, I'm sure you realize why we need carbs or at least carb alcohols in protein bars.
Chemically speaking, the steric hindrance caused by additional hydroxyl groups causes incomplete uptake of the sugar alcohol within the intestine. Anyone who has ever eaten an entire bag of sugar-free LifeSavers in one night (guilty...) knows the gastric... um... "duress" that is caused in this scenario. The incomplete absorption leaves fractions of the molecule in the gut, and this causes the laxative effect.
Different sugar alcohols convert to glucose at different rates, and hence have different caloric equivalents. The more apt a sugar alcohol is to be expelled without absorption, the more of laxative effect the consumer will likely experience. One exception to this rule is erythritol which, oddly, is fully absorbed into circulation, but it is not enzymatically converted into glucose. It is expelled renally, making it the only (as far as I know) commercially used sugar alcohol that is not excreted via hepatobiliary transport.
~Terumo
