The glycemic index and glycemic load are ways to measure how quickly a carbohydrate food is broken down by the body into glucose (blood sugar).
The glycemic index (GI) is a scale from 0 to 100. The higher a food is on the scale, the more quickly the food is broken down into glucose. Above 70 is a high GI value, 55 to 70 is a medium GI value, and under 55 is a low GI value.
Glycemic load (GL) is a combination of the GI value of a food and the amount of food consumed. The higher a food's glycemic load, the more quickly the food is broken down into glucose. A GL of 20 or more is high, a GL of 11 to 19 is medium, and a GL of 10 or less is low.
When a carbohydrate food is quickly broken down into glucose, it causes a rapid rise in blood sugar, which is then followed by a drop in blood sugar. This resulting low blood sugar causes hunger and decreased energy.
A rapid rise in blood sugar also causes the body to produce a high level of insulin, which is the hormone that moves the blood sugar from the bloodstream into the cells of the body. High levels of insulin can lead to increased fat storage, and can also increase the risk for type 2 diabetes, heart disease and some types of cancers.
In general, complex carbohydrates are broken down into blood sugar more slowly than simple carbohydrates, although this is not always the case. Potatoes and carrots are complex carbohydrates that are broken down into blood sugar quickly because they have high GI values. Apples are simple carbohydrates that are broken down into blood sugar slowly because they have a low GI value.
Many popular diet programs base their food choices entirely on the glycemic index, stating that foods with high GI values are fattening and foods with low GI values are not. But this is an oversimplification, as nutrition is more complicated than that.
The glycemic index also has the following limitations:
GI values can have wide variations.
GI values are generally averages of several tests, and individual test results can vary quite a bit. For example, baked Russet potatoes have been tested with a GI value as low as 56 and as high as 111. The GI value of certain fruits has been shown to increase as the fruit ripens. This amount of variation adds a great deal of uncertainty to GI values.
GI values are affected by how a food is prepared.
Generally, any significant food processing, such as grinding or cooking, will elevate GI values for certain foods because it makes those foods easier to digest. Even subtle differences in preparation, such as boiling pasta for 15 minutes instead of 10, affect GI values.
GI values are affected when foods are combined.
Tests for GI values are usually done on individual foods. The addition of other foods that contain protein, fat or fiber will generally reduce the GI value of a meal.
Glycemic response varies by individual.
The rate at which different people digest carbohydrates varies, so there are individual differences in glycemic response from person to person. A person's glycemic response can also vary from one time of day to another.
In conclusion, the glycemic index and glycemic load can be helpful in determining what foods to eat, but they should not be the only criteria used for choosing foods.