No number-crunching required.
1. Bigger is better.
Large food particles take longer for the body to break down and absorb, so they move more slowly through your digestive system. So in general, the more intact and less processed a food is, the lower its GI. Think whole rather than refined grains, whole fruit rather than fruit juice, steel-cut oats rather than instant oatmeal and stone-ground rather than plain cornmeal. When buying whole-grain bread choose stone-ground, sprouted or cracked-wheat types; the grain kernels should be visible.