From http://wilstar.com/lowcarb/print-sugaralcohols.htm
" Sugar alcohols are chemically alcohols, but are derived from sugar molecules. They include sorbitol, mannitol, xylitol, lactitol, maltitol, and others. They vary in their sweetness, ranging from about half as sweet as sugar to about as sweet.

Sugar alcohols, or polyols, may be used in place of sugar by most people on a low-carbohydrate diet or who have diabetes. Polyols are slowly and incompletely absorbed from the small intestine into the blood. Absorbed polyols are converted to energy by processes that require little or no insulin. Some of the polyol that is not absorbed into the blood is broken down into fatty acids in the large intestine.

Since they are incompletely absorbed by the intestine, over-consumption may produce a laxative effect in some people. They are often used in "sugar free" candies and syrups. They have about half to three-fourths as many calories as sugar."

These are exactly the kind of sugars we want to avoid, because they are designed, literally, to stay in our gut and not be absorbed. We can't digest them, but they are beloved by the bugs that live in our guts.