A surprising way to lower the glycemic index of foods...

Once a staple on American dinner tables, the white potato has been shunned as a high-carb food for the last couple of years. Farmers are even introducing a new "low-carb" potato to the marketplace this year.

If you don't want to eat genetically modified foods, you might reconsider the temperature at which you eat it. That's because cooling a potato reduces its effect on blood sugar levels, according to research published in the journal Nutrition Research [1] .

The study involved nine healthy men, aged 17 to 27, who were tested on three separate mornings. The meals were hot boiled potatoes (83.6 °C, or 182.5 °F), boiled potatoes cooled to 26 degrees C (78.8 °F), or white bread.

Average blood sugar levels were significantly higher 30 minutes after the hot potato meal than after the cool potato meal. The glycemic index of hot potatoes was found to be higher than for cool potatoes: 121 versus 77.

In a similar study, Swedish researchers recruited 13 healthy subjects to test the following four types of meal: freshly boiled potatoes, boiled and cold-stored potatoes (8 °C for 24 hours), boiled and cold-stored potatoes treated with a vinaigrette dressing (olive oil, white vinegar at 6% acetic acid; 8 °C for 24 hours), and white wheat bread used for reference purposes [2].  

All meals contained 50-grams available carbohydrate and were served at breakfast time after an overnight fast. Capillary blood samples were analysed for glucose and serum insulin levels.

The cold storage of the potatoes increased their resistant starch content from 3.3 to 5.2%. Compared with freshly boiled potatoes, the glycemic and insulin index of the boiled cold stored potatoes which had been treated with the vinaigrette, were significantly reduced by 43% and 31%, respectively.

Cold storage on its own reduced the insulin index of the potatoes by 28% compared with the corresponding value for freshly boiled potatoes.

It's possible that cooling cooked foods causes a shift in the structure of their starches that changes the rate at which glucose is absorbed.

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References
1. Nadine Najjar, Nada Adra and Nahla Hwalla. (2004). Glycemic and insulinemic responses to hot vs cooled potato in males with varied insulin sensitivity. Nutrition Research, 24, 993-1004
2. Leeman, E Östman and I Björck (2005). Vinegar dressing and cold storage of potatoes lowers postprandial glycaemic and insulinaemic responses in healthy subjects. European Journal of Clinical Nutrition, 59, 1266-1271