The Glycemic Index of Potatoes

While a whole potato would seem on the surface to be a reasonably healthy choice compared with processed foods, spuds are a dud when it comes to their effect on your blood glucose level. Potatoes fall into the moderately high to high ranges on the glycemic index scale, which measures the effects of foods on blood glucose. Potatoes actually rank closer to table sugar, which consists of glucose and fructose, than to other starchy foods that contain long and complex chains of glucose, according to the Linus Pauling Institute.

Measuring Glycemic Index

Determining the glycemic index of a food requires tests on human volunteers to see how the food affects their blood glucose levels. The Linus Pauling Institute’s website explains that volunteers receive 50 grams of a test food such as a potato, and a control food such as pure glucose that contains exactly the same amount of carbohydrate, on a later day. Volunteers’ blood is drawn before eating and for several hours after eating, and blood glucose is measured at regular intervals.

Significance

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The Linus Pauling Institute reports that a medium-sized baked Russet potato has a glycemic index of 76. This means that the change in blood glucose over time after eating a baked potato is 76 percent of the change in blood glucose that occurs after eating the same amount of carbohydrate as pure glucose. Foods with lower glycemic index values tend to produce lower blood insulin levels, in addition to lower blood sugar levels, according to Harvard Health Publications.

Potato Types

The glycemic index varies depending on the type of potato -- Harvard Health Publications reports in its listing of the glycemic index of more than 100 foods. Using glucose to represent 100 on the glycemic index scale, boiled white potatoes have a glycemic index of 82, instant mashed potatoes have a value of 85, and sweet potatoes a value of 70. Potato chips, often criticized for processing that adds fats and sodium, actually have a lower glycemic index than other forms of potatoes -- at a value of 51.

Expert Insight

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Researchers at the University of Toronto looked at the glycemic index of potatoes as it is affected by cooking method. In a 2005 study reported in the "Journal of the American Dietetic Association," they found that boiled red potatoes consumed cold fell in the intermediate range at 56, while roasted California white potatoes at 72 and baked Russet potatoes at 77 fell in the moderately high range. The high range included instant mashed potatoes at 88 and boiled red potatoes at 89.

Considerations

If you want to lower the glycemic index of the potatoes in your diet, precook the potatoes and consume them cold or reheated, recommends nutritionists at Oxford Brookes University in England who tested eight varieties of potatoes commonly eaten in the United Kingdom. They found that potatoes with a waxy texture had medium glycemic index values, while those with a floury consistency had higher glycemic index values.