Glycemic index definition: What is the GI?
What is the Glycemic Index (GI)?
The glycemic index is a tool that measures the potential of a carbohydrate-containing food to raise blood glucose levels.
The glycemic index considers the available carbohydrate content of the meal to measure how much and how quickly the blood sugar levels rise and fall after consumption.
The glycemic index measures the quality of carbohydrates but not the quantity.
In other words, the GI measures the ability of the available carbohydrate in carbohydrate-rich foods to increase blood glucose on a gram-for-gram basis (1).
The glycemic index value falls in the range of 0 to 100. A food containing no carbohydrates has a glycemic index of 0, while pure glucose has a glycemic index value of 100.
The glycemic index is measured by using the available carbohydrates in the food. Available carbohydrates are the carbs that are digested and absorbed in the intestines, contributing to the elevation of blood glucose. These are free sugars and complex carbohydrates but not dietary fiber. In this context, “available carbohydrates” can be interchangeable with the term “net carbs.”
How is the Glycemic Index Calculated?
The glycemic index value is measured by using a portion of the given food that provides 50 or 25g of available carbohydrates and comparing it to a control food containing the same amount of carbohydrates. The control food is usually white wheat bread or pure glucose.
Blood samples are taken before and after the food is consumed, at periodic intervals over two hours. Blood glucose levels at different times are then presented as a curve.
The glycemic index is measured by calculating the incremental area under the curve (iAUC):
GI = iAUC of the test food/ iAUC of the control food x 100.
Glycemic Index Classification
The glycemic index values are classified into three groups:
- low (a GI value of less than or equal to 55),
- medium (a GI value of 56 to 69),
- high (a GI value above or equal to 70) (2).
Consuming high GI foods leads to a rapid increase in blood glucose, followed by a rapid rise in blood insulin. This, in turn, leads to a rapid fall in blood glucose levels.
Low GI foods, on the other hand, break down slower, leading to a more gradual increase in blood glucose levels, which stay at this level for longer. Consequently, low GI foods also result in lower insulin levels.
On our page, you can find a complete list of the glycemic index values of over 350 foods.
What is the Glycemic Load (GL)?
The glycemic load is directly correlated to the glycemic index; however, the glycemic load also takes into account the contained available carbohydrates per serving size:
GL = GI x available carbohydrates (g) per serving/ 100
While the GI provides an idea of the quality of carbohydrates, the GL also talks about quantity.
The glycemic load is practical as foods with the same glycemic index values can differ in average serving sizes and consequently in provided carbohydrate amounts.
For example, Mars bars and honey both have average glycemic index values of around 60. However, based on our calculations, the GL of honey is low, equal to 10, while a Mars bar has a high GL of 20.
The glycemic load is also classified into three categories:
- a glycemic load of over 20 is considered high,
- a GL falling in the range of 11 to 19 is moderate,
- a GL value equal to or below 10 is low.
The average glycemic load of the food can differ depending on the glycemic index and the chosen portion as serving size.
What is the Insulin Index?
As mentioned above, foods with no carbohydrates have a glycemic index value of 0. This means consuming these foods doesn’t immediately raise blood glucose levels. However, this does not mean these foods do not affect blood insulin levels.
The insulin index of foods demonstrates how much a food increases the insulin level in the blood in the first two-hour period after consumption. The insulin index is measured by using a portion of the given food, along with a reference food containing equal amounts of calories (3).
If interested, you can visit the insulin index chart page to learn more about the insulin index and find insulin index values of over 140 foods.
Glycemic Index Values of Mixed Meals
The glycemic index value of a mixed meal is not measured but calculated:
Mean GI = [(GI x available carbohydrates (g)) of the food A] x [(GI x available carbohydrates (g) of the food B] / total available carbohydrates (g).
Some studies find that calculating the glycemic index of meals this way can lead to a gross overestimation (4). This is because macronutrients such as fats and protein can have various effects on the glycemic response of the meal.
However, other research has found that by accounting for the effects of fat and protein, the calculated glycemic index of a mixed meal can be adjusted appropriately to present accurate values (5).
To learn more about how different macronutrients and cooking methods affect a meal’s glycemic response, you can visit the What affects the glycemic index and response page.