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Glycaemic Index

Glycaemic Index

There are a number of popular diets based on the glycaemic index (GI), such as the Zone and Slow-Carb diet as well as using the GI of foods as a means to identify carbohydrate containing foods which may influence blood sugar levels. This fact file explores both the GI and glycaemic load (GL), why it can be useful and the limitations.

What is the GI?

The GI is a ranking of carbohydrate containing foods, by number, based on their effect on blood glucose (sugar) levels. Glucose or white bread is used as the standard reference of 100 and other foods are ranked against this according to how quickly or slowly the affect of specific foods have on blood sugars after eating them. Foods that are ‘slow-releasing’ have a lower ranking (low GI) and can raise blood sugar less than a ‘quick releasing’ food. However the GI of a food only tells you how quickly or slowly it raises blood glucose when the food is eaten on its own.

The three categories of GI values are listed below with some examples:

Low 1-55Lentils, apples, porridge
Medium 56-69Couscous, eggs, apricots
High 70 and higherSweetened fruit juice, waffles, most processed meats

Why is the GI useful?

The purpose of GI was developed as a guide for people with diabetes aiming to help control blood sugar levels. The GI is now additionally used to help with weight loss, as a heart friendly diet and to control appetite by increasing the feeling of fullness with eating low GI foods, but the evidence for helping with these are mixed, especially if the limitations of the GI are not considered.

Glycaemic Load

The glycaemic load (GL) helps to address one of the limitations of the GI which incorporates a typical portion of the food rather than using the standard test level weight of which the GI is based on. Using the GL can potentially be a more useful tool as it reflects the quantity of specific foods you are likely to eat. For example couscous is categorised as a medium GI food, whilst watermelon as a high GI food, however if typical portion sizes are considered, both of these foods are categorised as low GL foods.

The GL values are categorised as follows:

  • Low GL: 1 to 10
  • Medium GL: 11 to 19
  • High GL: 20 or more

Limitations of GI

In reality, foods tend to be eaten in combination, especially at mealtimes. Although the concept of GI can still be applied, combining foods of different ranking will alter the overall GI value of the meal, this will also apply to GL. Cooking and changing the structure of foods, for example, solid to liquid will also change the GI value.

In addition, it is important to think about how balanced your meals are as the GI may provide a ranking, but does not necessarily encourage or help guide healthier choices as there is no nutritional information, such that not all foods that have a high GI are necessarily bad for you and those with a low GI are not all necessarily good for you, this is partly because fat within the food can lower the GI rating, so for example, crisps or ice-cream can have a medium to low rating because of the high fat content. There are also many healthy foods that are not in the database.


The GI can be used as a guide in combination with a balanced diet to help achieve healthy eating, however it is important to consider the limitations and other factors such as exercise, portion sizes and alcohol intake for a healthier lifestyle.

This fact file is intended for adults as a general guide only and not a substitute for professional advice or a diagnosis. If you are on certain medication or suffer from a medical condition, seek individual advice from your health care professional. Date produced January 2016.

The Healthy Employee
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