The glycemic index was invented in 1981 by Dr. Thomas Wolever and Dr. David Jenkins. A food's glycemic index (GI) will tell you how quickly your body will convert the carbohydrates in that food into glucose, which is different than a food's total carb content. The measure ranks food by assigning it a number between 0 and 100.

A glycemic index of 55 or less is considered low, or good. One between 56-69 is considered medium, while a GI of 70 or higher indicates a food you may want to be wary of.

Preventing glucose spikes by choosing low glycemic index foods is especially important if you have diabetes or metabolic syndrome, but anyone who wants to live a healthier lifestyle and prevent future disease would do well to incorporate a food's glycemic index into their dietary choices.

Low-GI diets have been shown to improve insulin resistance. They can also help people with type 2 diabetes lower their glucose, cholesterol and triglyceride levels. Low-GI diets also tend to foster weight loss, since low GI foods help stabilize your hunger and energy levels.

Pure sugar has a score of 100. On the other hand, foods like meat, fish, eggs, olive oil, and butter, which contain no carbs, have the lucky number of 0!

Yet even some seemingly healthy foods like potatoes (up to 98, depending on preparation and whether you eat the skin) or rice crackers (87) have a surprisingly high GI. Better choices are chickpeas (28), and lentils (32).

However, the food with the lower glycemic index isn't always the healthier one. Many fried foods, for example, have relatively low GIs, but their high fat and calorie content means they can do much more damage to your waistline than a plain potato can!

Generally, the more processed a food, the higher its glycemic index. The way you prepare a food also affects its glycemic index, with the number generally going up the longer something is cooked. Some fruits will also acquire a higher glycemic index as they ripen, and consuming more than one serving of a food will naturally increase its GI.

On the other hand, you can bring down the glycemic index of a food by pairing it with another food that is more difficult to digest. To calculate the GI of a meal, you would use the following formula:

Meal GI = [(GI x amount of available carbohydrate)Food A + (GI x amount of available carbohydrate)Food B +…]/ total amount of available carbohydrate

However, glycemic index isn't a perfect prediction because of individual differences in how your body processes carbs, like your age, how active you are, and how fast you digest food.

Another problem with glycemic index is that it assumes we will consume a serving of the food that contains 50 grams of carbohydrates without taking account how likely we are to actually eat that much of the food.

If you're looking for a more precise measurement, you may then want to look at glycemic load, which also takes into account how many carbs are in a typical serving of the food. The formula for that is:

GLFood = (GIFood x amount (g) of available carbohydrateFood per serving)/100

In this system, a high GL is one that's over 20. Intermediate foods have a GL or 11-19, and low foods have a GL of less than 10. This helps correct the value of food like watermelons, which have a GI of 76 but a GL of only 8 for a 1 cup serving, and oranges, which have a GI of 42 but a GL of only 5 for one medium orange.

If you're looking for low GI foods, you can look for this symbol on processed foods; but you'll probably have an easier time finding them in your fruit and vegetable aisle! You can also get a head start on planning a diet with a lower glycemic load by checking out this list of some common foods and their GI values!
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