glycemic index is a ranking of how quickly food is metabolized into glucose when digested. it compares available carbohydrates gram-for-gram in foods to provide a numerical, evidence-based index of postprandial (post-meal) blood sugar level. the concept was invented by david j. jenkins and colleagues in 1981 at the university of toronto. the glycemic load (gl) of food is a number which estimates how much a food will raise a person's blood glucose level.