Glycemic Index

The glycemic index of a food is an estimate of the effect that the food will have on blood sugar levels. The system of ranking foods was originally developed for diabetic patients looking to get a handle on their blood sugar levels, but has become more widely applied in recent years and is the basis for a number of popular diets.

The glycemic index (GI) compares the blood sugar effect of one gram of a target food to one gram of some standard. The standard’s effect on blood sugar levels is well known already. Most often this standard is pure table sugar (glucose), and it is given a ranking of 100 on the GI scale. Other scales give white bread a value of 100, so users can base their choices off of whether the food in question spikes their blood sugar more (GI greater than 100) or less (less than 100) than white bread. Since most people are more familiar with eating white bread than eating plain table sugar, this scale makes dietary choices easier.

Generally, the literature GI value of a food is a good approximation of how the food will affect your blood sugar. Some confounding factors can alter this effect in some foods and some individuals so the scale is not 100% reliable. For example, as some fruits and vegetables ripen or sit around, their sugar content changes, so the GI of a green banana is different than the GI of a mostly brown banana, while GI charts only have one GI value for a banana. The blood sugar in your body is affected by everything you’ve taken in within the past few hours, not simply the last thing you swallowed. Alcohol can lower blood sugar levels for a couple hours, and if you eat two foods with different GI values in the same meal, the meal’s GI will generally fall somewhere in the middle of the two.

Charts and tables that denote the GI of foods usually divide them into high, medium, and low GI foods. Foods that have a value of 70 and above (using glucose as a standard) are considered high GI. These foods include white bread, white rice, sugar, sugar-sweetened breakfast cereals, and some potatoes. Foods between 56 and 69 on the GI scale are medium GI foods. Pita bread, some potatoes, raisins, cranberry juice, pumpernickel bread, ice cream, and bananas all fall into the medium GI category. Low GI foods fall under 55 on the scale. These are foods like beans, most whole grains, most fruits, and most vegetables. Some of these foods are predictable, easy to guess even if you haven’t read a GI chart. Sweeter, more refined grains spike your blood sugar more than less refined fibrous ones. Vegetables do less than starchy potatoes. But some foods are surprises. Who would have guessed that ice cream is a medium-GI food? Or that Peanut M&Ms rank somewhere in the 30s on the GI scale? Clearly, the GI scale is not all you need to build a healthy diet.

Eating a low-GI diet is associated with a number of health benefits. Especially in animal research, high-GI diets have been linked with obesity. In a number of studies, low-GI diets were associated with decreased risk for age-related diabetes and coronary disease. High-GI diets have been at times associated with cardiovascular disease and cancer. Most health and disease prevention groups recommend incorporating GI guidelines into your diet with some caution, since the indices of some foods make it seem as though users could eat an unlimited amount and stay healthy (beef, chicken, and ham all have literature GI values around 0).

The glycemic index presents a healthy diet in a new kind of way, going one step beyond simple calories and low fat to present a diet that paints a picture of what actually goes on inside the body. Again, caution may be advisable in the case of glycemic index diets. It is still true that eating a ton of low-GI foods will make you gain weight, but likely not as much as eating the same amount of high-GI foods would. The South Beach Diet and the aptly named Glycemic Index Diet are two commercially successful diets that implement the glycemic index to promote weight loss successfully in many cases.


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