When I first started Atkins, I discovered sugar alcohols (AKA 'polyols') and thought they were a wonder-substance. Atkins says they're ok, they taste like sugar, and contribute no net carbs! However, after eating some "0 net carb" candy made with sugar alcohols, I noticed that I instantly craved more and that I would get knocked out of ketosis.
I did some research, and it turns out that:
(1) They *do not* have zero effect on one's blood sugar. (i.e., they're not necessarily low on the Glycemic Index, which is a rating from 0 to 100 of the extent to which foods cause a spike in blood sugar.) Surges in blood sugar levels and the resultant excess insulin release are what cause you to gain weight.
(2) They have calories. *Even* if, theoretically, they had absolutely no effect on blood sugar or insulin levels (a Glycemic Index of 0), the calories you get from them would be used for energy instead of your own body fat. I mean, some pasta doesn't have a huge effect on blood sugar, but it isn't allowed... so I don't know why sugar alcohols are.
(3) They affect everyone differently. Some people are able to absorb more calories from sugar alcohols than others. Some people experience a blood sugar spike after eating them, others don't. Some people find that they cause more carb cravings, others don't. They stall ketosis in some people, they don't in others.
Interestingly, the original editions of the Atkins' Diet Revolution books say that sugar alcohols are prohibited. It's only in the later editions of the book that Dr. Atkins says that they are ok. Strangely enough, this corresponds to the time when Atkins food products began to show up... And, yes, some of them contain sugar alcohols. Seems fishy to me...
I've been generalizing, but in terms of induction, it's a good idea not to provide your body with sources of energy that can be used instead of your own body fat, regardless of whether they are low on the Glycemic Index (GI). And sugar alcohols aren't even necessarily low on the GI. If you do want a treat occasionally, despite the calories, it *is* better to pick something that's low on the GI. There are some sugar alcohols that are better than others in terms of calories and GI value. I choose to *occasionally* have treats with these types of sugar alcohols, despite that I am continuing induction. For reference: Maltitol, has 3 calories per gram, and a 53 on the GI. Erythritol, on the other hand, has .2 calories per gram, and a 0 on the GI. Glycerin, which isn't quite a sugar alcohol, but is a common low-carb food additive, has 4.32 calories per gram, and I'm not sure what GI value.
Some people recommend including total grams of sugar alcohols in the net carb count. Others recommend counting half of the total grams of sugar alcohol. Discovering this freaked me out -- I wasn't having under 20 net carbs, like I thought I was.
Basically, be aware that sugar alcohols aren't all they're cracked up to be, and keep in mind that extra carb calories, even if they're low on the GI, will hinder burning off body fat. Also, be sure to read nutrition labels to figure out which types of sugar alcohols are used. They're even used in some shakes and bars.
http://www.mendosa.com/netcarbs.htm has excellent information on polyols, and includes a chart of the caloric content and GI of some common sugar alcohols.
I hope I haven't told you a lot of things that you already know, or confused the issue even more. I tend to blather on and on a wee bit. If I didn't explain clearly, or if you have more questions, feel free to ask.
-e.