The idea behind the glycemic index is that different foods produce different blood sugar elevations. Studies were undertaken to measure the blood sugar responses to common foods in healthy volunteers. The glycemic index values are useful as guides to food selection, but many factors influence the blood sugar response to eating foods. Combining foods into meals will change the impact of each food on blood sugar. Physical exertion after eating will lower blood sugar peaks.

High fiber vegetables have low glycemic indexes and tend to reduce the glycemic effects of the higher rated foods. The method of cooking also influences the blood sugar effect - cooking rice longer, for example liberates more free sugar from starch and increases the glycemic index. Differences in glycemic response reflects the rate that foods are digested and there are many factors. There is less variability between the GI values of different subjects than there is within the same subject from day to day.

Starchy foods producing low glycemic responses have been identified, including legumes, pasta, barley, bulgur, parboiled rice and whole grain rye breads such as pumpernickel. Incorporation of these foods into diets have been associated with reduced blood glucose, insulin, and lipid levels. Low GI foods increase colonic fermentation,  increase bacterial urea utilization, and increase production and absorption of short chain fatty acids in the colon.

Values from two indices are listed below.  The  index  in the right column below compares a 50 gram dose of the food to 50 grams of glucose with a reference value of 100.  In the  column on the left uses white bread as a reference with the value of 100; glucose then has a value of 138.