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Do different methods of food prep affect blood glucose levels?

People living with diabetes understand carbohydrates turn into glucose energy and, in turn, raise blood glucose levels. 

Many may also focus on the glycemic index of carbohydrates to help guide the timing of when blood glucose levels may rise after a meal.  However, something less discussed is the type of food preparation these carbohydrates undergo and if food preparation influences blood glucose response to a carbohydrate meal.

A randomized study in 2020 asked the question: “Does food preparation impact the glycemic index of a food?

reheated pasta

In this study, investigators prepared pasta as the tested high-carbohydrate meal, and participants ate a meal of pasta that was either:

    1. freshly cooked and hot
    2. reheated leftovers or
    3. cold leftover pasta

The glucose levels of each participant were measured after each meal by glucometer over a set amount of time.

The results may surprise you!

The blood glucose levels of participants after the hot meal were both the highest in range and in length of time.  The blood sugars after participants ate the reheated pasta and the cooled pasta were not as elevated as hot pasta and they returned faster to normal levels.  Reheated pasta showed the fastest return to normal levels.

Similar results were shown in studies using reheated white rice, lentils, and potatoes. In the previous studies, reheated starches and cooled starches produced less of a glycemic response than fresh, hot meals containing starch.

To understand the outcome of these studies, one must first be aware that carbohydrates are made up of starch, sugar, and fiber.  

The proportion of these components is strongly tied to how quickly one’s glucose level will rise during digestion.  Specifically, when starch is digested, it is absorbed in the small intestine.  However, dietary fiber and something called resistant starch is not absorbed in the small intestine and is not digested effectively at all; resistant starch is excreted out without having much impact on glycemic load at all.

steam over cooking pot

It is during the process of cooling and in turn the reheating process, that a portion of the pasta carbohydrate is turned into resistant starch.  As such, this chemical process creates a pasta that has less impact on glucose levels.  Conversely, the fresh, hot pasta has more swollen structure of the starch and, thus, more readily absorbed and produces the higher glycemic response. 

This is not to say that by eating only resistant starch carbohydrates one will not have a rise in glucose levels.  This study is only showing the comparison among cooking methods, and which may lead to less of an impact on the glucose rise of blood sugars.

Not included in the study was cooking time for other, non-starchy carbohydrates. Cooking methods that are shorter times and lower temperatures often preserve the fibrous nature of a carbohydrate and, thus, cause a lower glycemic response.  Boiling and steaming vegetables can keep the integrity of the fiber.  In contrast, frying vegetables can cause them to lose some of their fiber content which will lead to a more rapid increase in blood glucose levels.

Of course, this is not a way to manage blood sugars alone.  However, having a better understanding of carbohydrates and how they are absorbed and impacting blood sugars will always help with diabetes and blood sugar management. It is simply another aspect of nutrition that may contribute to balancing blood sugars and diabetes management.

Read more about the food preparation study.

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