Glycemic Index and Glycemic Load 

 

Carbohydrates are comprised of three groups of molecules that have distinct structures and are thus differently processed in the body. Sugars, also known as simple carbohydrates (glucose, fructose, lactose or sucrose), consist of one or two sugar units, while starches and fiber (known as complex carbohydrates) contain from three to many thousands of units. Sugars and refined starches (white flour and bread, white rice, processed cereal) are rapidly digested and converted to glucose. Unrefined starches (whole grains, brown rice) take longer to process, but eventually they, too, break down to give glucose. Fiber cannot be digested by human enzymes at all and can only be broken down (fermented) by the gut bacteria. While some carbohydrates (refined starches and added sugars) are best avoided, others have significant health benefits (fiber). 

 

In an effort to help people with diabetes chose the “good” carbohydrates, researchers at the University of Toronto proposed glycemic index as a way of ranking the “quality” of carbohydrates.   Glycemic index (GI) is a number used to characterize the rise in blood glucose after ingestion of a food item. Values are reported in reference to glucose, which is assigned the GI of 100. You can think of glycemic index as a figure describing how quickly and easily carbohydrates are digested and broken down into glucose. Because of their rapid digestion, refined starches tend to have higher glycemic index than the unrefined ones. Fiber does not cause the rise in blood glucose (glycemic index = 0), but a particular type of fiber, soluble fiber, can lower the glycemic index of foods ingested alongside.

Glycemic load (GL) is another system that ranks foods in terms of glucose burden.  It combines glycemic index with the quantity of carbohydrates present in a typical serving. Although closely related, the values for GI and GL can be in disagreement, confusing, and difficult to use when making food choices. The infographic illustrates one such example:

As can be seen from the graphic, watermelon has high GI (anything over 70 is considered high, 100 is the maximum). However, a typical serving of watermelon does not contain a lot of carbohydrates, which is why its GL is low (10 and below is considered low, 20 and above is considered high GL). On the other hand, chocolate cake with frosting has low GI (fat reduces GI), but high GL. This is because chocolate cake packs a lot of carbohydrates in a medium sized piece.

 

It is very important to know how to use GI and GL. Choosing food based only on these values can lead to consumption of items that are high in fats and calories, but low in fiber content. Peanuts, for example, have very low GI and GL (7 and 0, respectively), but they are full of fat and calories. A serving size of about 1.8 ounces (50 grams) contains 38% of daily-recommended value for fat and 284 kcal! It is, therefore, clear that overconsumption of peanuts would quickly result in weight gain.

 

To date, low GI and GL diets have not been proven better for weight loss than traditional low-fat diets or even high GI/GL diet. However, GI and GL can be useful for comparing similar foods, for example, when deciding what kind of bread or what kind of potatoes to use. Researchers at the University of Sidney have compiled an extensive list of GIs and GLs for a variety of foods.   However, even without knowing exact GI and GL values you can still make good food choices based on the following guidelines:

 

  • Food processing increases GI. For example, bread made from whole grains has lower GI than white bread because fiber-rich bran, which is present only in whole grains, slows down the digestion considerably and reduces the glucose burden. Instant foods (oatmeal, rice or pasta, for example) have higher GIs than foods that require regular cooking times.

  • Longer cooking times increase GI. For example, al dente pasta has lower GI than overcooked pasta (and it tastes better).

  • Fat and protein lower the GI. If you wish to minimize the effect of a high GI food on your blood glucose, combine it with a lean protein or a bit of healthy fats (avocado, olive oil, nuts).

  • Acids lower the GI. Because of its acid content, sourdough bread has lower GI than white bread. Adding vinegar (acetic acid) or lemon juice (citric acid) to your salad, will lower the GI of food consumed with it. 

Last updated: December 17, 2016

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