It's all about blood glucose. The whole idea of a low carb diet is that we are eating in a way to keep our blood glucose from spiking up. And it turns out that some ingredients that, while classified by the FDA as carbohydrates, don't cause as much of a blood sugar rise as pure starch or sugar. Unfortunately, however, it's not so straightforward as the manufacturers would like as to think. Some of the ingredients are better than others, and probably all of them vary according to the individual. So when you see that "Net Carb" label, it should be a sign that you'd best get out your magnifying glass and read the REST of the label very carefully. Here are some of the things you might see:
FiberFiber is the most straightforward. The idea of subtracting fiber from the total carbohydrate when figuring out the carb count of a food came from the authors of the Protein Power books over 10 years ago, and it makes good sense. By definition, fiber isn't digested in the small intestine and so isn't broken down into glucose and absorbed into the blood.
This is true for any natural fiber that is eaten as part of a plant. But what about manufactured ingredients that have the chemical structure of fiber? I think the jury is still out on some of these ingredients. When I see "oligofructose syrup" I have to wonder if this ingredient acts the same way in the body as oligofructose which naturally occurs in a plant, even though these molecules can be regarded as fiber in most ways.
A Note on Glycemic Index and Glycemic ImpactOne of the difficulties with determining "net carbs" is that even with regular foods, it is difficult, if not impossible, to predict the blood glucose reaction of any one person to a food. If you look at this list of the glycemic indexes of foods you will see that there is a great amount of variation for any one food. This is also going to be true for many of the ingredients that are not counted as "net carbs" on some labels.
This is especially so because when there is testing of these ingredients they are almost always on "healthy" subjects who do not have a problem with processing glucose. The Atkins company has stated that diabetics may respond differently to their products. I would ask, "What about pre-diabetics? What about people who don't yet have prediabetes, but have insulin levels which are higher than normal?" There is a whole range of non-diabetics who are somewhere on the Road to Diabetes, and I just don't think we know how they are likely to react to these ingredients. Since these are the people most likely to respond well to low carb diets, these questions are important to consider.
Sugar AlcoholsSugar alcohols (mostly with names ending in "tol," such as sorbitol, maltitol, and erythritol) are sweet substances which have a highly variable impact on blood glucose depending on which one. If I was going to advise people to avoid a single ingredient in "low carb" products, it would be maltitol. The other sugar alcohols range from a little to a lot better than maltitol, with erythritol being the best. There is a chart at the bottom of this article about sugar alcohols which compares the different types.
Sugar alcohols must be included in "total carbohydrates" on the label, and if used in sugar-free foods, also have their own line on the label so you can see how much of the total carb count is from sugar alcohols.
GlycerineGlycerine (or glycerin) is an interesting and sort of mysterious molecule. The glycerol molecule (another name for glycerine) is the backbone of the triglyceride molecule - the storage form of fat in our bodies (3 fatty acidsa are "hooked on" to each glycerol molecule). It is not a carbohydrate, but our bodies can use it to make glucose. Although there is not as much research on the blood sugar impact of glycerine as I'd like to see, the signs are pointing towards it having quite a low impact in most people.
PolydextrosePolydextrose sounds very suspicious, doesn't it? "Dextrose" is sugar, plain and simple. Polydextrose is a sweet manufactured product from dextrose which supposedly acts like fiber (although it is not counted as fiber on food labels). I've only been able to find one study which looked at its impact on blood glucose. In that study, done in China, it did not raise blood sugar.
Oligofructose and InulinThese substances are in a class of carbohydrate (oligosaccharides) in between sugars and starches. They have a lower impact on blood glucose than sugars, because most of oligosaccharides makes it through the small intestine without being digested, at least in most people. Interestingly, by the time they get to the colon, oligosaccharides have positive effects. Read more about oligosaccharides here. However, when removed from the original plant and used as an ingredient, the individual glycemic response is more variable, and some people find that these ingredients do raise blood sugar significantly.
Have questions about other "low impact carb" ingredients? Ask about them in our Forum, and I will research them.