How accurately are foods listed on the glycemic index chart?

Foods listed on the glycemic index chart are accurate but don't take into account many variables that affect glycemic index levels, according to the American Diabetes Association. For instance, GI value does not take portion size into account, and GI value changes depending on what other foods are eaten.

The longer a fruit or vegetable has been stored or is ripe raises its GI value, according to the American Diabetes Association. The length of time a food is cooked also raises its GI value. The specific variety of a food and how it is processed also change its GI value. An example of this is white rice, which has a higher glycemic index than brown rice.