Those of you who have been reading my blog have probably realized that I am very big about not taking foods at face value. If the product that you are thinking of purchasing has a food label and a list of ingredients, I really can't think of any legitimate reason why you would put that food in your shopping cart (in "real life" or online) without first doing your homework. Of course you also have to know what you are looking for, and one of the main questions I get is about how to make sure you are counting the amount of carbohydrates in that product correctly.
First of all, you need to make sure that you look at what that the serving size of a product is. Fortunately these days most of the serving sizes come in what I call "practical" measures: measuring cups, measuring spoons, x-number of crackers, etc. All the information on that label is for 1 SERVING OF THAT PRODUCT. Now, when I used to teach classes I would always get a bunch of eyeball rolls as far as "Oh, who is only going to eat 10 chips, etc," and I had to remind people--No one is holding a gun to your head to eat that food period, and therefore no one is holding a gun to your head to only eat one serving. That serving is a unit of measurement, nothing more, nothing less.
Opinion: Personally, I would rather see the nutrition info for the whole package on a lot more products, so that way the amount of calories, carbohydrate, etc for that whole bag of organic potato chips (You know who you are!) is staring you right in the face. It might also influence people to think ahead a little bit more, as in "Ok, if I only get 1/4 of this bag out and put the rest away, then I will eat less" rather than "Whoops I ate the whole bag while I was absorbed in the newest internet cat video."
Once you have figured out the serving size, either by using the company's serving size or by looking at the whole package and dividing up into your own serving sizes, you can look at the carbohydrate amount. The "Total Carbohydrate" on the label is the sum total of ALL the different types of carbohydrates found in that product. Some carbohydrates might be in the form of what is commonly called "sugars," those sweet tasting substances composed of carbon/hydrogen/oxygen that form themselves into monosaccharides (the most basic structure of sugar) and disaccharides (to monos combined). Some of the total carbohydrate might consist of what is commonly called "starches," those "pasty tasting" substances formed when a bunch of monosaccharides bond together to form a more complicated structure (polysaccharides). Some carbohydrates come in the form of something commonly called "dietary fiber," which are polysaccharides that you don't digest. And many items advertised as "sugar-free" or "Low-carb" may contain another type of carbohydrate called "polyols" or more commonly "sugar alcohols" which also have a sweeter taste.
Those of you who enjoy chemistry probably want me to talk more about the structure of these, those who aren't as familiar probably have glazed eyes about now. What it boils down to is this--sugars and starches are digested/absorbed by the body and provide calories and blood glucose. Fiber is not digested/absorbed and does not provide calories or affect blood glucose. Sugar alcohols--it depends, more details later. What you want to find is the AMOUNT OF ABSORBABLE CARBOHYDRATE PER SERVING. If certain things aren't digesting, you don't need to worry about the impact on your blood sugar or that they will be providing calories
So let's say you look at a label that has the following:
Total Carbohydrate 20 gm
Sugars 3 gm
Dietary Fiber 5 gm
Sugar Alcohols 4 gm
First we'll find the total carbohydrate-- 20 gm. Then you might look at the sugar--but we know now that sugar is absorbed, so it's included in the 20 gms, so we can just ignore it. Next you look at the fiber--you know that fiber is not aborbed, so you substract it (20 - 5 = 15) and you are now down to 15 grams of absorbable carbohydrate. Now you are ready to look at the sugar alcohols, and here is where the fun begins.
There are actually several different type of sugar alcohols, and each one has slightly different caloric content per gram and a slightly different impact on blood glucose. Typically the amount used per serving is small enough that you can do the same "math" for most of them--you look at the grams of sugar alcohols, DIVIDE the number in HALF, then substract what you get from previous number (4/2 = 2 gm, 15 - 2 = 13 gm).
The one exception to this rule is if the sugar alcohol is erythritol, this one is really not absorbed so you just ignore it. I found this chart gives a good list of the most common sugar alcohols and will also give those of you who want to do more precise calculations the info you need.
1. Find the total carb (20).
2. Ignore "sugars."
3. Subtract the dietary fiber (20 - 5 = 15)
4. Divide the amount of sugar alcohols in half (4/2 = 2)
5. Substract the sugar alcohols from your answer in #3 (15 - 2 = 13)
6. You count 13 grams toward your carbohydrate budget if you eat that food/that serving.
And you know what's really fun? When you find a low carb/sugar free product, do all the math, compare that serving to an equivalent serving of the original product.....And you find there is only a one gram of carbohydrate difference. Sometimes you might find significantly less carbohydrate in the "other product" but you might actually pay double the price per product. At that point you have to start thinking "Is there any other nutritional value I might get from this?" (Often the answer is no, particularly for processed foods). You might also want to ask "Can I make this myself and save a few pennies?" (Often the answer is yes, particularly for items you feel you "have" to have for whatever reason).
Take home message--always read the label and ingredients, and do the math, before you make your decision.