Your body converts protein into sugar...example: 10g of protein would be like 4g-5g of carbohydrates spread out over 2 or more hours. Any food that has nutritional value will have some impact on BG. Today many PWD's consider protein when calculating a bolus.
The corner stone of the old ADA diet was low fat and it only contained about 20% protein so the clinical world decided it was not necessary to consider protein when calculating a bolus. Today many PWD's diet may contain 60% protein or even more so a bolus calculation will have a lot of error if protein is not covered with some insulin. It is also hard to deal with large amounts of protein if you are not pumping bolus insulin.