What does Glycemic Index measure?

Photo by Gabrielle Henderson

Ever wondered what does Glycemic Index measure? The glycemic index (GI) is the value given to carbohydrate-containing foods according to how quickly the body will break them down and release glucose into the blood. A high GI food (GI value greater than 70) will break down quickly, causing a spike in BGLs and energy levels that drop off quickly, often leaving you feeling low on energy and also hungry.

Low GI foods (GI value less than 55) are preferable as the body takes longer to break these foods down and therefore releases glucose into your blood over a longer period of time. This helps manage your BGLs by avoiding spikes and provides you with sustained energy and feelings of being full across a longer period of time.


How GI is estimated?

For many years it was assumed that complex carbohydrates would be broken down much more slowly than simple sugars, and so the blood glucose would rise more slowly after eating starches than after eating table sugar or fruit juice or jam. It was also assumed that carbohydrates that contained a lot of fiber (like brown rice or whole-wheat bread) would be absorbed more slowly than white rice and white bread. A series of research studies on this topic in the 1970s and 1980s showed that these assumptions may not always be true.

The researchers fed people 50 grams of different kinds of carbohydrate and measured how quickly their blood glucose went up, how high it got, and how quickly it came back down again. They found that eating 50 grams of table sugar and 50 grams of white bread gave an almost identical profile. In both cases, the blood glucose went up very quickly, had a high peak, and came down again quickly after the food had all been absorbed. They called that response a high glycemic index and gave it a value of 100.

Then they compared the response that people had to other foods and to white bread. If something was absorbed even faster and had a higher peak than white bread or table sugar, then it would have a glycemic index of over 100. But if something was absorbed more slowly and with a lower peak, it would have a glycemic index of under 100. To their surprise, they found that whole wheat bread and brown rice were absorbed just about as fast as white bread (with glycemic indices of 96–99); but white rice had a lower glycemic index (83), and white spaghetti was even lower (66). The popular breakfast cereal cornflakes had an even higher glycemic index measure than white bread (119), whereas ice cream had a very low glycemic index (49)!

GI foods list

This does not mean that you should eat ice cream for breakfast rather than cornflakes, however. It does show us that how quickly we absorb our food is more complicated than we used to think. Because it is sometimes surprising what effect certain foods will have on your blood glucose, it is a good idea to check your blood glucose before you eat a new type of food, and then test again one or two hours afterward to see how high your blood glucose goes and how long it stays up. This is particularly important for foods that you like to eat fairly often.

When it comes to recipes and prepared meals, the method and duration of cooking, as well as the storing and temperature of the food, can impact the GI value by altering the carbohydrate composition. So always be sure to follow the instructions, as overcooking some foods, such as pasta or potatoes, can increase the GI value, while cooking pasta al dente or cooling potatoes prior to eating can reduce the GI value.

The GI value of a food or meal is influenced by several other factors. For example, the higher the fiber, protein or fat content and/or acidity of a food or meal the slower the digestion rate and/or emptying of the stomach, causing a slower release of glucose into the bloodstream and therefore a lower GI rating. It can, therefore, be quite tricky to determine the GI of a food or meal that has not been properly tested or analyzed.

When in the supermarket, look out for packaged foods that carry the low GI symbol as an easy way of finding nutritious low GI foods. These foods have been tested and proven to be low GI, plus they meet the strict nutrient criteria set by the Glycemic Index Foundation.

Of course not all high GI carbohydrates are necessarily unhealthy to eat, just as not all low GI carbohydrates are healthy to eat on a regular basis. It is always important to consider the overall nutritional value of the food – that is, is it low in fat, especially saturated fat; is it high in fiber and whole grains; is it low in sodium; and does it contain essential vitamins and minerals?

GI food set


Aim to include at least one healthy low GI food at each meal or snack. By combining a low GI food with a high GI food you will achieve an overall medium glycaemic effect. And remember to choose portions that meet your individual needs, as too much of any carbohydrate, regardless of its Glycemic Index measure, as it can cause blood glucose levels (BGLs) to rise too high.