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 negative effect on blood glucose levels. Potatoes fall into the moderately high to high ranges on the glycemic index, 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 starches, which are composed of long chains of glucose, notes the Linus Pauling Institute.
Effects
To determine the glycemic of a food requires tests on human volunteers to see how it affects their glucose levels. The Linus Pauling Institute's website explains that the volunteers receive 50 g of a test food, such as a carbohydrate like a potato, and a control food, either pure glucose or white bread containing the same amount of carbohydrate, on a later day. Volunteers' blood is drawn before eating and for several hours after.
Significance
A baked potato has a glycemic index of 76 relative to glucose and 108 relative to white bread, the institute states. This indicates that the blood glucose increases after eating a baked potato at 76 percent of the amount it would respond when eating the same amount of carbohydrates in pure glucose and 108 percent of the identical amount in white bread. The lower a food's glycemic index, the less it affects insulin levels and blood sugar, Harvard Health Publications notes.
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 representing 100 on the index, boiled white potatoes averaged 50 and baked russet potatoes and instant mashed potatoes averaged 85, while sweet potatoes averaged 61. Potato chips, oft-criticized for processing that adds fats and sodium, actually have a lower glycemic index than other types of potatoes, at an average of 54.
Expert Insight
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 as 89.
Considerations
If you want to minimize the glycemic index of foods in your diet, precook potatoes and consume them cold or reheated, recommends nutritionists at the School of Biological and Molecular Sciences at Oxford Brookes University in England who tested eight varieties of potato eaten in the United Kingdom. They found that potatoes with waxy textures produced medium glycemic index values, while those with floury consistencies had high GI values.
