What determines a food’s glycemic index?

Gelatinization – The more starch granules are swollen and burst during cooking the higher the GI. For this reason baked potatoes have a high glycemic index (85) whereas pasta is unique in that the internal starch granules are entrapped and not as swollen leading to lower glycemic values (30-60). Pasta cooked al dente (slightly uncooked core) has an even lower glycemic value because of this.

Amylose to Amylopectin ratio - The Amylose to amylopectin ratio similarly determines a food’s glycemic index by the amount of gelatinization that occurs. Foods with higher amylase (which less readily gelatinize and digest) have lower glycemic values than foods with higher amylopectin (which more readily gelatinize and digest). For example basmati rice with higher amounts of amylase, has a lower glycemic value (58). Conversely, Japanese style white rice with lower amounts of amylase, has a higher glycemic value (83).

Processing - Grinding wheat into smaller particles increasing the surface area for digestive enzymes. The more finely milled the higher the glycemic index. In this example the degree of processing determines the relative glycemic index of wheat (whole wheat < cracked wheat < course four < white flour).

Fiber – This depends on whether the fiber acts as a protective barrier or not. For instance most wheat bread (73) has the same glycemic index as white bread (70) because the fiber is not intact and does not play a protective role. When fiber plays a protective role to digestive processes such as is the case with stone ground (cracked kernel) wheat bread the glycemic value is lower (53).