Answer: The reason is not totally known, but a strong possibility is because the starch in potatoes, indeed all starch, is made up of long strings of glucose. Since the starch in potatoes is rapidly-digested, the glycemic index of potatoes is almost as high as that of the 100 points for glucose (although there is a wide variation among the various tests, from as low as 58 to as high as 111). Sucrose (table sugar), on the other hand, is a disaccharide (two-sugar) molecule made up of one glucose molecule and one fructose molecule joined together. Fructose is processed differently in our bodies than glucose, and it doesn't affect our blood sugar as much. However, fructose causes problems of its own when we eat too much of it.
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