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Why do potatoes raise blood glucose more than sugar?

Glycemic Index of Potatoes and Sugar Explained

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Updated December 08, 2008

Potatoes

Various Types of Potatoes

Photo: James Baigriev/Getty Images
Question: I've been reading about the glycemic index, and trying to choose foods which will have less impact on my blood sugar. I really don't understand why a food like potatoes (glycemic index 90) would be so much higher than white sugar (glycemic index 59). If I'm understanding this right, the glycemic index is based on the same amount of carbohydrate in the different foods. Can you explain this to me?

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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