Bottom line: When counting carbs, you don't need to count fiber since it doesn't end up raising your blood-glucose levels. You can subtract the grams of fiber from the grams of Total Carbohydrate. In the figure, 30 grams of Total Carbohydrate minus 4 grams of fiber leaves you with 26 grams of digestible carbs.
Dietary fiber is what remains after digestion; it's the indigestible part of the plant. Fiber makes its way all the way through the intestine, pushing things along as it goes, which helps promote regularity in bowel movements. Fiber is important for intestinal health. Fiber is made out of glucose molecules all linked tightly together. It doesn't break down into individual glucose molecules in the same way that starch does.If you inject insulin, you should definitely subtract the fiber if it is going to make a difference to your insulin dose. However, if you have diet-controlled Type 2 diabetes, you really don't need to worry about subtracting the fiber.
When adjusting insulin doses to the amount of carbohydrate eaten, precision is important. Calculating your own doses of insulin allows for flexibility in what you eat, but getting the proper dose of insulin relies on you counting the carbohydrates accurately. Consider the two tortilla labels shown here.
The label on the left shows that the total carbohydrate count is 13 grams and the fiber is a mere 1 gram. If you subtracted the gram of fiber and counted the carbs as 12 grams, you would be very unlikely to change your dose of insulin on that basis. For example, if your doctor recommended that you take 1 unit of rapid-acting insulin for every 12 grams of carb you eat, you would end up taking just 1 unit of insulin regardless of whether you counted this tortilla as 13 grams of carb or as 12 grams of carb.
The insulin-dosing examples used in this book are only to make a point. Individual insulin requirements vary significantly. Do not make changes to your insulin plan without conferring with your doctor.
Next, look at the label on the right. That's a different story. There are 10 grams of carb per tortilla, but 7 of those grams come from fiber. Keep in mind that fiber doesn't digest. Subtract the fiber: 10 grams of carb minus 7 grams of fiber leaves you with just 3 grams of digestible carb. That means only 3 grams will turn into glucose and enter your bloodstream. Anyone calculating insulin-to-carb ratios would want to subtract the fiber in order to calculate the right dose of insulin. If a person took insulin to cover 10 grams but digested only 3 grams, then that person might end up with low blood glucose as a result of taking too much insulin. Eating two high-fiber tortillas amplifies the discrepancy.A food label can say it's a "good" source of fiber if it provides 10 percent of the Daily Value for fiber, or at least 2.5 grams per serving. An excellent source of fiber provides at least 5 grams of fiber per serving, or 20 percent of the Daily Value.