The things you listed (leaf lettuce, high fiber cereal, fruit) are all
carbohydrates, meaning they all end up in the blood in exactly the same form: glucose. To your pancreas, insulin, or your blood, it really doesn't matter wether you ate a potato, cereals, a cake, pasta, 5 spoons of sugar, 10 apples, spinach, carrots, or drank beer. The only difference is the taste on your tongue, and in the stomach. After that they all get converted to same substance - blood glucose.