Glycemic Index (GI) tells us about the relationship between ingested carbohydrates and blood glucose levels. Glycemic index is a measure of quality of carbohydrate in a food.
Generally it is assumed that simple carbohydrates increase blood glucose levels rapidly, and complex carbohydrates have a slower effect. This may not be true for all foods. Recent work into the Glycemic Index (GI) has changed the understanding of the relationship between ingested carbohydrates and blood glucose levels.
Some carbohydrates are slow releasing, while the others are fast releasing. The glycemic index (GI) is a measure of how fast a carbohydrate triggers to raise the blood sugar. The higher the number, the greater the blood sugar response.
All carbohydrates can be assigned a value, known as glycemic index (GI), compared to glucose (some like to compare to white bread). The glucose is the fastest releasing carbohydrate so it has been assigned a glycemic value of 100.
All complex carbohydrates are not the same. For example, a glucose polymer Maltodextri, is a complex carbohydrate, but it has a GI of ~100 . This means that it rapidly raises blood glucose levels. Similarly all simple carbohydrates are not bad. A good example is that of Fructose, a simple carbohydrate which has GI of ~20. This means that fructose is slow sugar releasing.
The glycemic index is a better method for classifying good and bad carbohydrates as it relies upon actual post-meal blood glucose response.
Note the following points related to Glycemic index of foods.
- A low glycemic index value for a food indicates a slow rise in blood glucose and insulin levels. Conversely, a high GI value indicates that it results in a rapid rise in blood glucose levels and hence a spike in insulin response.
- The glycemic index tells us about the quality of the carbohydrate in the food, not its quantity. The GI of food will remain the same, even if you double the amount of carbohydrate in your meal. This is because the glycemic index is a relative ranking of foods for the same amount of carbohydrate (50 gram). But this will raise your glucose levels to a higher level and take longer to return to baseline. You should therefore note that the GI value does not take the quantity of carbohydrate you eat.
- The GI applies only to foods containing significant amounts of carbohydrate. Most whole grain cereal products, legumes and vegetables, except potatoes have low glycemic index as they contain only a small amounts of carbohydrate.
How is Glycemic Index determined?
Glycemic index of a food is determined by measuring the change in the blood sugar level of one or more people after eating 50 gram of digestible carbohydrate (i.e. total carbohydrate minus fiber) of the food and then consuming a reference food such as glucose or white bread. The experiment is repeated and an average is taken over a set period of time. This gives the food's glycemic index.
Different Glycemic Index for the Same Food
There appears to be a confusion on values of glycemic index. You may have noticed different values of glycemic index for the same food item in the literature. This may be due to the following reason:
As mentioned above, food glycemic index is assessed by taking glucose as a reference. Some scientists take white bread as the reference. Hence the two values differ. Examine the following values:
White Bread Reference
|Potato, white, boiled||80||56|
The actual value is not important. However, if you watch the above values, you will find that each food glycemic index based on white bread as reference to the glucose as reference has a ratio of 1.43 (approx.). So if you know the glycemic index based on one reference, you can find it based on the other reference.
Rice GI = 58 (Glucose based) = 58 x 1.43 = 83 (Bread based)
Low, medium and high Glycemic Index
What is considered as a low glycemic index food or a high glycemic index food. The following table gives an idea.
|Glycemic Index(GI) Value |
with reference to Glucose
|High Glycemic Index||70 or more|
|Medium Glycemic Index||56 to 69|
|Low Glycemic Index||55 or less|
The values of glycemic index and glycemic load for a number of food items for a serving size are given at Glycemic Index List of Foods.