The Glycemic Index (GI) is a tool that helps measure how quickly the body responds to different types of carbohydrates in foods. It is a relative ranking of how the carbohydrates in these foods impact your blood sugar levels.

How is the GI value calculated?

The GI is calculated by comparing the speed and extent to which a food containing 50g of carbohydrate is broken down into glucose in the blood against a 50g dose of pure glucose.

For example, 50g of pure glucose is benchmarked at 100, and other foods containing 50g carbohydrates are divided into low GI foods (≤ 55), medium GI foods (56-69) and high GI foods (≥ 70), by comparison.

The GI of a food can vary due to many factors. These include how it is cooked and processed, how 'ripe' the food is, moisture content or the amount of protein or fat contained in the food.

Table 1: Common foods and their GI values*

* Note that while one GI value is given to each food in this table, this is an average figure from a number of published studies. The GI of any particular food can vary within and between individuals. Often the degree of variation is very large - especially for foods in the medium and high category.

Not all low GI foods are healthy. However, in general, following a low GI diet which is rich in fresh fruit, vegetables and wholegrains has been suggested to lower the risk of cardiovascular disease, type 2 diabetes and some cancers. Research continues in this area.

The limitation of the GI system is that it compares the glycemic effect of foods containing 50g of carbohydrate, when realistically these foods are often consumed in different amounts. To account for this, the glycemic load can be used.

Glycemic Load

The glycemic load builds on the glycemic index by factoring in the serving size of the food that you eat. It measures both quality and quantity.

You can calculate the glycemic load as follows: 

GL = GI x serving size (g) / 100

The GL is classified as low (10 or less), medium (11-19) and high (20 or more).

Sugar and the glycemic index

Do all sugars have a high GI and result in a 'spike' in blood glucose, or a 'sugar high'? This is a common myth that the science has put to bed. Sucrose or table sugar has a medium GI of 65.

The different types of sugars have different GI values. 

Some sugars actually have a low GI, as well as sugar containing foods like fruit, milk and yoghurt. Also, many starchy foods have a high GI. Rather than a focus on sugar alone, the GI value of all the carbohydrate in the food needs to be considered.

REFERENCES

• Atkinson FS, Foster-Powell K & Brand-Miller JC. (2008). International table of glycaemic index and glycaemic load values: 2008. Diabetes Care, 31(12):2281–2283

• The University of Sydney Glycemic Index. GI Database. https://www.glycemicindex.com/foodSearch.php

• Glycemic Index Foundation. How GI is measured. https://www.gisymbol.com/how-is-gi-measured/ 

• Augustin LSA, Kendall CWC, Jenkins DJA, et al. (2015). Glycemic index, glycemic load and glycemic response: An International Scientific Consensus Summit from the International Carbohydrate Quality Consortium (ICQC).  Nutr Metab Cardiovasc Dis, 25(9):795-815