Years ago, smart people determined that a high fiber food item (more than 5 grams of fiber) slows down the blood glucose-raising action of carb enough that it was possible to subtract the grams of fiber from the grams of carb and just take insulin for the remaining number of carbs.
After a while it became clear that we only needed to subtract half the fiber. For example, if you eat a sandwich with two slices of bread and each slice has 4 grams of fiber and 16 grams of carbohydrate, you have a total of 8 grams of fiber and 32 grams of carb. With this formula, you can subtract 4 grams from the 32 and take insulin for 28 grams of carb. This doesn’t seem like a huge difference, and for most people it probably isn’t. That may be why this recommendation is fading out.
But something to keep in mind – especially if you take insulin or other medications that lower blood glucose – if you eat a lot of fiber in one sitting (bean soup, for example), is that the carb in the food may not have as big or as quick an effect as you are expecting.