Why Do Potatoes Raise Blood Glucose More Than Sugar?

Glycemic Index of Potatoes and Sugar Explained

Basket of potatoes on table
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Question from a Reader: 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: There are a few things to think about in answering this question.

1) Potatoes have more glucose - The first is simply that there is more glucose in potatoes than in sugar.  Are you surprised?  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 80's or low 90's.

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.

2) The glycemic index number can be misleading. The tests for glycemic index only show averages, and the glycemic index number itself is actually an average of those averages.

In the case of potatoes, the different studies used to compute the index have results anywhere from 56 to 111. Each of those studies were run on a number of people with only the average being reported. So the glycemic index number itself might not mean much to a given person.

The upshot is that different people have different responses to different foods. The most important thing is how your own body reacts to a potato, which you can find out with a blood glucose meter.

3) You are on the right track! Especially if your body is on the diabetes spectrum, it does not process sugars well, and high blood sugars take a toll on your body, making it more likely that you will cause more damage in your pancreas, and suffer more of the complications of diabetes.  You are doing the right thing to think about how your diet affects your blood sugar and therefore your health.

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