The Glycemic Index diet is based on how foods containing carbohydrates affect blood sugar levels and insulin release.
The plan is billed as helpful for people with type 2 diabetes, and may also be a good bet for dropping extra weight.
Originally developed in the 1980s as a tool for diabetics to manage spikes in blood sugar (1), glycemic index (GI) ratings go from 1 to 100; foods measured at 55 or less are considered low glycemic; values from 56 to 69 are in the medium range; 70 to 100 are high glycemic foods.
Eating foods with a high glycemic rating is more likely to cause blood sugar spikes in type 2 diabetes patients, which raises the risk of ongoing high blood sugar readings. (2)
Biology and Nutrient Composition
Carbohydrates can have sugars, starches and fiber; when you ingest carbs, the body breaks starches and sugars into glucose, which fuels the cells. Fiber is passes through the body, and is not digested.
The pancreas releases two main hormones to regulate blood sugar. Insulin moves glucose from the blood into the cells for use as energy. When blood sugar levels fall, glucagon triggers the release of stored glucose in the liver so the body doesn’t run out of fuel.
Foods with high fiber content are usually processed and utilized more slowly. Theoretically, the more even and measured the pace for energy release, the more potential positive effects are on tap for controlling three vital aspects of health maintenance: blood sugar control, appetite control and weight control. (3)
Getting familiar with the GI values of various foods for guidance in food choices is an alternative approach to regulating carb intake. The strategy is intended to help in selecting foods containing carbohydrates that will be assimilated at a slower pace, while avoiding foods that jack up blood sugar. (4)
The bottom line is GI values can be a useful tool for choosing “good carbs” or “bad carbs.”
Glycemic Rating System
The GI only applies to foods with carbohydrate content. Proteins (like beef, chicken, fish and pork) aren’t a significant part of the picture, and neither are fats, although eating fats and protein with carbohydrates can affect the response of blood sugar levels to carbohydrates. (5)
Some foods, like nuts and seeds, contain both fats and carbs, but carb counts in this group are low enough that they are usually considered fatty foods.
Just to give you an idea, a baked russet potato has a glycemic rating of 111. Since it’s actually over the top of the charts, this food is likely to spike blood sugar. On the other end of the spectrum, chick peas (also called garbanzo beans, which are the basis for a hearty hummus dip) have a glycemic rating of 6. (6) Other legumes also have relatively low GI values.
The difference between a GI rating and a glycemic load (GL) rating is associated with serving sizes. Watermelon has a GI value of 80, which falls in the “high” category. But you’d have to eat a huge amount of watermelon to get the amount of digestible carbohydrates it would take to jack up your blood sugar, because watermelon is mostly water.
Researchers apply a formula that can give you a more accurate idea of how you will respond to a particular food by addressing the potion size issue. The amount of food eaten determines its effect on blood sugar levels. If you eat 120 grams (a little more than 4 ounces) of watermelon, its GL tops out at 5, which is low. (7)
To find out how the food you have on your plate will actually affect blood sugar, multiply its GI value by the number of carbs in a serving; then divide by 100. Low readings are between 1 and 10, medium runs from 11 to 19, and high readings are 20 and up.
For example, one cake doughnut has 23 carbs and a GI value of 76. Multiply 23 by 76 and divide by 100 for a GL rating of 17.48. This value is close to the top end of the moderate GL bracket.
A whole wheat English muffin has a GI of 45; a white wheat English muffin clocks in at 77, so it’s easy to see a single serving of the refined flour product will have a greater impact on blood sugar levels.
Information on glycemic ratings of foods is readily available online from many sources. The University of Sydney operates a GI testing facility and you can enter common foods in the search engine provided by the university website to get values. (8)
Studies and Clinical Trials
As reported in the “Journal of American Medicine Association,” the OmniCarb Randomized Clinical Trial could find no solid evidence that eating foods with low GI values improved cholesterol levels, systolic blood pressure or insulin sensitivity. (9)
The National Institutes of Health recently reviewed a number of clinical trials to assess the effects of basing nutritional choices on GI ratings. The report noted that evidence suggested some low GI diets had good results for weight-loss, but tagged the trend “non-significant,” and stated more studies are needed to determine long-term effects of such diets. (10)
The Glycemic Index diet is included in a list of diets helpful in managing diabetes; other diets with favorable reviews in this meta-analysis were the Mediterranean diet, low-carb diets, and high-protein diets. (11)
In a study of the GI and GL ratings of foods and their relationship to diet and disease, the large variation of responses by subjects indicated other nutritional factors need to be correlated in order to reach reliable conclusions. (12)
Long-term evaluations of the GI diet’s effectiveness are not yet available, but an 18-month trial tracking weight changes in Brazilian women following the diet did not show favorable results for keeping weight off. (13)
If you plan to use the Glycemic Index diet, remember that the way you combine menu choices can moderate the effect of foods likely to spike blood sugar; consider pairing high GI foods with fats, proteins, or foods with low GI values. (14)
The most compelling convenience factor of the GI diet may be that counting calories and keeping a close eye on portion control aren’t necessary. Considering GI or GL ratings while planning meals can be complex, so keep that smart phone handy.