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Sugar alcohols are absorbed very slowly and incompletely from the intestines. This means that only part of the molecule is available for metabolism; thus they provide fewer carbs and calories than sugar. The part that is absorbed is converted into energy by a process that requires little -- if any -- insulin. This makes these products generally safe for diabetics, who need to control their insulin carefully.
However, how to assess the carb content of sugar alcohols is a subject of debate. Typically, total carb value is listed on the product label, but many low-carb product labels use a calculation that eliminates the carb value of sugar alcohols, resulting in a claim of "no [or very low] net carbs."
This advertising effort is probably overly optimistic -- sugar alcohols do have some carb effect. Experts differ, but I recommend counting the carbs from these alcohols at about half their total true carb value.
Sugar alcohols in significant quantities may knock you out of ketosis, the desirable fat-burning goal of a low-carb diet, so I recommend avoiding them during the early phase, when your body is settling into a nice fat-burning metabolism.
There are two other issues of note with the sugar alcohols. First, the unabsorbed portion that sticks around in the intestines causes most people varying degrees of intestinal upset (e.g., diarrhea, gas). This is dosedependent, so a small amount of sugar alcohols may be fine, but a larger portion could be a problem.
Second, using these "trick" sugars can keep sugar cravings alive. It's your decision whether the benefits of these sweet substitutes are worth their drawbacks.