After all, we don’t think of regular white potatoes as sweet! It turns out that the starch in potatoes, indeed almost 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 glucose alone. The glycemic index of glucose is 100 points where potatoes are usually listed as being in the high 80s or low 90s.
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.
So it turns out that an ounce of carbohydrate from potatoes has twice the glucose as sugar. When you think of it that way, of course, potatoes would raise blood glucose more.