Glycaemic index, or GI is a measure of the effects of carbohydrates on blood sugar levels. Carbohydrates that break down quickly during digestion and release glucose rapidly into the bloodstream have a high GI; carbohydrates that break down more slowly, releasing glucose more gradually into the bloodstream, have a low GI. GI foods are marked out of 100. One hundred being glucose.
Protein, when eaten with carbohydrates, slows the release of glucose even more thus lowering GI further.