Potatoes And The Variability Of The Glycemic Index
G. Douglas Andersen, DC, DACBSP, CCN
A couple of years ago I wrote an article about the Glycemic index and the factors that can affect it.1 I also compiled a Glycemic index for a chapter I wrote.2 Whenever I hear people discussing Glycemic index the natural response is to see the numbers as absolute. What most Glycemic index charts don't say is how variable true GI's are. Below is a good example of how easy a 'bad' food can become 'good.'
A few months ago I read a very interesting article on the Glycemic index of potatoes. Two Canadian dental students and a professor of nutrition at the University of Toronto Medical School teamed up for a study to see how different varieties of potatoes and different cooking methods could affect the Glycemic index.3 In the first part of the study, the meals compared russet potatoes that were baked, refrigerated at least 24 hours, and then reheated in a microwave with russet potatoes that were baked that day. In the second part they compared russet potatoes that were precooked in a microwave oven, refrigerated at least one day, and then reheated in a microwave oven with russet potatoes that were microwaved that day. They also compared white potatoes that had been boiled, refrigerated, and reheated in a microwave oven with white potatoes that were boiled that day. The results showed there was little difference between baking and microwaving a russet potato just prior to consumption. The precooked russet potatoes by microwave had an 18% lower Glycemic response than the same-day microwaved russet potatoes, although the authors said this was not statistically significant due to the small sample size of subjects (10). The russet potatoes that had been oven baked, refrigerated, and reheated in a microwave had a 30% lower response than those that were oven baked and consumed fresh. Finally, the Glycemic index of white potatoes that were boiled, refrigerated, and reheated did not differ from white potatoes that were freshly boiled.
In a second phase of the study, the authors tested 7 meals that each provided 50 gm of available carbohydrate. The following were compared:
Table 1 - Results
It is clear that both the method of cooking and variety of potato can affect Glycemic index. What was most interesting was when a red potato was boiled, refrigerated, and consumed cold the next day, the Glycemic index plummeted 37% from the upper end of a high Glycemic index food (89) to one point away from a classification of a low Glycemic index food (56)(see Table 2).Table 2
When potatoes are cooked, the starch granules absorb water. This is called gelatinization and tends to change the structure of the starch, making it more susceptible to the digestive enzymes. When the cooked potato starch is cooled, the molecules bond in an irregular fashion, making it more difficult to be hydrolyzed by enzymes. The authors mention that repeating the cooking-cooling cycle will continue to result in a more resistant starch. The more resistant a starch is, the longer it will take the body to break it down, digest, and absorb it.
Please note this was only one small study. But the next time you read a GI chart the published values are far from absolute. In this case of red potatoes, eating them cold the next day makes a huge difference.
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