Answer: This is a great question, and it applies to understanding the carb counts listed in all nutritional information.
There are three important issues that go into answering Bruce’s question:
- Fiber. Bruce did not mention fiber in his question. Fiber is a carbohydrate, but since it isn’t digested, most low carb diets say it doesn’t “count” in the total carbohydrate you eat. It won’t impact blood glucose (except perhaps in a positive way by slowing the impact of the other carbs ingested). Whenever I give nutritional information at the end of a recipe, I cover this
issue by saying how much effective carbohydrate (that is, total
minus fiber) in a serving and then I give the fiber count. So in the case of the
focaccia, I say that there is a .8 gram *effective* carbohydrate plus
5 grams of fiber in a serving. Again, while this fiber is carbohydrate, it provides no calories or "useable" carbs.
In emailing with Bruce, it turns out that the package says that each 2 T serving has 4 grams of fiber, which makes the effective carb count 1 gram per serving. But sometimes packages vary – how could this be?
- All nutritional information is an estimate. Every food varies in composition from one to another. The particular variety of plant or animal, where it grew, the weather, the fertilizer – many factors go into the final product. Ask a wine aficionado about this, and they will tell you how true this is for grapes, to the point where the flavor varies based on where the grapes grew. It is just as true for every other fresh food and the products made from it. So, we can never know exactly how many carbs or calories or vitamins are in any particular strawberry, unless we analyze that particular strawberry. Since nutritional labels are derived from a certain batch of food, they will reflect this variation. The USDA database (as well as the Canadian and other databases) takes averages from many batches of the food to come up with their numbers. Sometimes, as they get more data, the numbers change somewhat. This happened in the most recent update of the database (version 18) which shows different carb counts for flax seed than version 17 did. This explains some of the variation in current online databases.
- The serving size on the label can have a big effect, especially if the amount used is much more than a serving size. The problem is that rounding error is multiplied. So, for example, if a label says that 1 tablespoon of a food has one gram of carbohydrate, that could be anything from .51 grams to 1.49 grams. That’s not a big deal if you are eating one serving. But there are 16 tablespoons in a cup, so the error could be as much as 8 grams in either direction if you are using that much in a recipe.
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