What exactly is the Glycemic Index?

In 1981, Professor of nutrition Dr David Jenkins was looking at how different carbohydrate-rich foods affected blood sugar levels in people with diabetes and discovered that, contrary to popular belief, many starchy foods affected blood sugar levels quite dramatically, while some sugary foods had little effect.

From his research, he developed a scale called the Glycemic index, which quite simply ranked foods based on the effect they had on blood sugar levels.